supercov_engine/

workspace.rs

//! Isolated project snapshots and crash-recoverable stable build cache.

use std::{
    collections::{BTreeMap, BTreeSet},
    fs::{self, OpenOptions},
    io::{self, Write},
    path::{Component, Path, PathBuf},
    sync::atomic::{AtomicU64, Ordering},
    time::{SystemTime, UNIX_EPOCH},
};

use serde::Serialize;
use sha2::{Digest, Sha256};

use crate::lifecycle::{LifecycleError, ProjectLock, atomic_rename, remove_stored_tree_deferred};

const WORKSPACE_MARKER: &str = ".supercov-workspace-store";
const WORKSPACE_MARKER_CONTENTS: &[u8] = b"Supercov instrumented workspace. Safe to delete.\n";
const ROOT_EXCLUSIONS: &[&str] = &[
    ".cache",
    ".git",
    ".supercov",
    ".mcdc-pool",
    "node_modules",
    "build",
    "dist",
    ".next",
    ".nuxt",
    ".output",
    "coverage",
    "playwright-report",
    "test-results",
    "target",
];
const NESTED_EXCLUSIONS: &[&str] = &[".supercov", ".mcdc-pool"];
static UNIQUE: AtomicU64 = AtomicU64::new(0);

#[derive(Debug)]
pub enum WorkspaceError {
    Io { path: PathBuf, source: io::Error },
    UnsafePath(PathBuf),
    UnsupportedEntry(PathBuf),
    MissingLock,
    Lifecycle(LifecycleError),
    UnsupportedPlatform(&'static str),
}

impl std::fmt::Display for WorkspaceError {
    fn fmt(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::Io { path, source } => write!(formatter, "{}: {source}", path.display()),
            Self::UnsafePath(path) => {
                write!(formatter, "unsafe workspace path: {}", path.display())
            }
            Self::UnsupportedEntry(path) => {
                write!(
                    formatter,
                    "unsupported filesystem entry in isolated project: {}",
                    path.display()
                )
            }
            Self::MissingLock => write!(
                formatter,
                "isolated workspace preparation requires the active project lock"
            ),
            Self::Lifecycle(error) => write!(formatter, "{error}"),
            Self::UnsupportedPlatform(reason) => {
                write!(formatter, "unsupported workspace platform: {reason}")
            }
        }
    }
}

impl std::error::Error for WorkspaceError {}

impl From<LifecycleError> for WorkspaceError {
    fn from(value: LifecycleError) -> Self {
        Self::Lifecycle(value)
    }
}

/// Leftover tool state -- a Chrome test profile linking into /tmp was the
/// field case -- must not abort measurement. The mirror omits the entry:
/// nothing outside the project is ever followed, and a test that truly needs
/// the link fails visibly inside the workspace with this line as the cause.
fn omit_escaping_link(path: &Path, target: &Path) {
    eprintln!(
        "[supercov] omitting symlink outside the isolated project: {} -> {}",
        path.display(),
        target.display()
    );
}

/// `fs::canonicalize` on Windows returns a verbatim path -- `\\?\C:\...` or
/// `\\?\UNC\server\share\...` -- and that prefix does not survive contact with
/// anything else: Node's pathToFileURL reads `?` as a UNC host, `Path::starts_with`
/// treats `\\?\C:\` and `C:\` as different prefixes so containment checks
/// misjudge every link, and the prefix shows up in every diagnostic. Canonical
/// paths are simplified back to the ordinary form at the one place they are
/// made, so nothing downstream ever sees the verbatim spelling.
pub(crate) fn canonicalize_simplified<P: AsRef<Path>>(path: P) -> io::Result<PathBuf> {
    fs::canonicalize(path).map(simplified)
}

#[cfg(windows)]
pub(crate) fn simplified(path: PathBuf) -> PathBuf {
    match path.to_str().and_then(strip_verbatim) {
        Some(plain) => PathBuf::from(plain),
        None => path,
    }
}

#[cfg(not(windows))]
pub(crate) fn simplified(path: PathBuf) -> PathBuf {
    path
}

/// The pure string half, kept host-independent so it can be tested anywhere:
/// `\\?\C:\a` -> `C:\a`, `\\?\UNC\srv\share\a` -> `\\srv\share\a`, and `None`
/// for a path that carries no verbatim prefix.
#[cfg_attr(not(windows), allow(dead_code))]
pub(crate) fn strip_verbatim(path: &str) -> Option<String> {
    let rest = path.strip_prefix(r"\\?\")?;
    if let Some(unc) = rest.strip_prefix(r"UNC\") {
        return Some(format!(r"\\{unc}"));
    }
    let mut chars = rest.chars();
    let drive = chars.next()?;
    if drive.is_ascii_alphabetic() && chars.next() == Some(':') {
        return Some(rest.to_owned());
    }
    None
}

/// The `file://` URL for an absolute path, the way Node's `pathToFileURL`
/// derives it. Node takes a bare `--import=C:\...` for a URL with scheme `c:`
/// and refuses it -- ERR_UNSUPPORTED_ESM_URL_SCHEME -- while `/abs/path` has no
/// colon and passes, which is why every POSIX run worked and every Windows run
/// died in the first Node child. Windows paths reach here already simplified
/// (no `\\?\`); `\` becomes `/`, a drive path becomes `file:///C:/...`, a UNC
/// path `file://server/share/...`, and every byte outside the URL-safe set is
/// percent-encoded so a space or a `#` in a temp directory cannot break it.
pub(crate) fn file_url(path: &Path) -> String {
    let text = path.to_string_lossy();
    #[cfg(windows)]
    let text = text.replace('\\', "/");
    file_url_from_slash_path(&text)
}

/// The pure half: `slash_path` uses `/` separators on every host.
pub(crate) fn file_url_from_slash_path(slash_path: &str) -> String {
    let mut url = String::from("file://");
    let mut rest = slash_path;
    if let Some(unc) = rest.strip_prefix("//") {
        // `//server/share/dir` -> `file://server/share/dir`: the host is the server.
        let (host, tail) = unc.split_once('/').unwrap_or((unc, ""));
        url.push_str(host);
        rest = tail;
        url.push('/');
    } else if !rest.starts_with('/') {
        // A drive path such as `C:/dir`.
        url.push('/');
    }
    for byte in rest.bytes() {
        let keep =
            byte.is_ascii_alphanumeric() || matches!(byte, b'-' | b'.' | b'_' | b'~' | b'/' | b':');
        if keep {
            url.push(byte as char);
        } else {
            url.push_str(&format!("%{byte:02X}"));
        }
    }
    url
}

fn io_error(path: &Path, source: io::Error) -> WorkspaceError {
    WorkspaceError::Io {
        path: path.to_owned(),
        source,
    }
}

fn unique() -> String {
    let nanos = SystemTime::now()
        .duration_since(UNIX_EPOCH)
        .unwrap_or_default()
        .as_nanos();
    format!(
        "{}-{nanos}-{}",
        std::process::id(),
        UNIQUE.fetch_add(1, Ordering::Relaxed)
    )
}

fn project_name(root: &Path) -> Result<&std::ffi::OsStr, WorkspaceError> {
    root.file_name()
        .ok_or_else(|| WorkspaceError::UnsafePath(root.into()))
}

pub fn workspace_container(root: &Path) -> PathBuf {
    // Everything Supercov keeps in a project lives under .supercov: one
    // directory to gitignore, one directory to delete. Users interact with
    // their real files and the CLI; command outputs sync back after every
    // run, so nothing in the container is ever theirs to fetch.
    let preferred = root.join(".supercov/workspaces");
    if fs::symlink_metadata(&preferred).is_err() || owned_workspace_path(&preferred) {
        return preferred;
    }
    let digest = format!("{:x}", Sha256::digest(root.as_os_str().as_encoded_bytes()));
    for sequence in 0..1_024usize {
        let name = if sequence == 0 {
            format!(".supercov/workspaces-{}", &digest[..16])
        } else {
            format!(".supercov/workspaces-{}-{sequence}", &digest[..16])
        };
        let candidate = root.join(name);
        if fs::symlink_metadata(&candidate).is_err() || owned_workspace_path(&candidate) {
            return candidate;
        }
    }
    root.join(format!(".supercov/workspaces-{}-overflow", &digest[..16]))
}

/// A Cargo configuration in one of Supercov's own directories above the
/// isolated copy of `root`. Cargo reads `.cargo/config.toml` from every
/// ancestor of where it builds, so one there would change how the copy
/// builds -- `[env]`, a runner, an alias -- while the project's own `cargo
/// test` never sees it. The project's own `.cargo`, and those above the
/// project, apply to both and are not Supercov's to judge.
pub fn planted_cargo_configuration(root: &Path, workspace: &Path) -> Option<PathBuf> {
    if !workspace.starts_with(root) {
        return None;
    }
    workspace
        .ancestors()
        .skip(1)
        .take_while(|directory| *directory != root)
        .map(|directory| directory.join(".cargo"))
        .find(|path| fs::symlink_metadata(path).is_ok())
}

/// The directory, in the container, that holds a copy of the project:
/// `workspace` for the first, the one a run alone takes, and `workspace-2`
/// for the one a run takes while the first is in use.
fn copy_directory_name(copy: usize) -> String {
    if copy <= 1 {
        "workspace".into()
    } else {
        format!("workspace-{copy}")
    }
}

/// Everything in the container that belongs to one copy: the copy, and the
/// view of the author's source kept beside it.
pub(crate) fn copy_directories(container: &Path, copy: usize) -> [PathBuf; 2] {
    let name = copy_directory_name(copy);
    [
        container.join(&name),
        container.join(name.replacen("workspace", "source", 1)),
    ]
}

/// Where the copy a run holds is.
pub fn cached_workspace_path(root: &Path, lock: &ProjectLock) -> Result<PathBuf, WorkspaceError> {
    copy_path(root, lock.copy())
}

/// Where one of the project's copies is, by its number.
pub fn copy_path(root: &Path, copy: usize) -> Result<PathBuf, WorkspaceError> {
    Ok(workspace_container(root)
        .join(copy_directory_name(copy))
        .join(project_name(root)?))
}

pub fn isolated_workspace_path(root: &Path, run_id: &str) -> Result<PathBuf, WorkspaceError> {
    if run_id.is_empty()
        || run_id == "."
        || run_id == ".."
        || run_id
            .chars()
            .any(|character| matches!(character, '/' | '\\' | '\0') || character.is_control())
    {
        return Err(WorkspaceError::UnsafePath(PathBuf::from(run_id)));
    }
    Ok(workspace_container(root)
        .join("work")
        .join(run_id)
        .join(project_name(root)?))
}

pub(crate) fn owned_workspace_path(path: &Path) -> bool {
    fs::symlink_metadata(path).is_ok_and(|metadata| metadata.file_type().is_dir())
        && fs::symlink_metadata(path.join(WORKSPACE_MARKER))
            .is_ok_and(|metadata| metadata.file_type().is_file())
        && fs::read(path.join(WORKSPACE_MARKER))
            .is_ok_and(|contents| contents == WORKSPACE_MARKER_CONTENTS)
}

fn ensure_container(root: &Path) -> Result<PathBuf, WorkspaceError> {
    let container = workspace_container(root);
    let mut created = false;
    match fs::symlink_metadata(&container) {
        Ok(metadata) if !metadata.file_type().is_dir() => {
            return Err(WorkspaceError::UnsafePath(container));
        }
        Ok(_) => {}
        Err(error) if error.kind() == io::ErrorKind::NotFound => {
            let store = container.parent().expect("container parent");
            fs::create_dir_all(store).map_err(|source| io_error(store, source))?;
            // One directory covers everything Supercov writes; make Git
            // ignore it without the user touching their own .gitignore.
            let store_ignore = root.join(".supercov/.gitignore");
            if fs::symlink_metadata(&store_ignore).is_err() {
                let _ = fs::write(&store_ignore, b"*\n");
            }
            // Made complete and then named, in one step. A run starting at
            // the same moment took a container that existed and did not yet
            // hold its marker for someone else's directory, kept its copy in
            // another, and then could not find it.
            let made = store.join(format!(".workspaces-{}", unique()));
            fs::create_dir(&made).map_err(|source| io_error(&made, source))?;
            fs::write(made.join(WORKSPACE_MARKER), WORKSPACE_MARKER_CONTENTS)
                .map_err(|source| io_error(&made, source))?;
            match fs::rename(&made, &container) {
                Ok(()) => created = true,
                // Another run named its own first.
                Err(_) => {
                    let _ = fs::remove_dir_all(&made);
                }
            }
        }
        Err(source) => return Err(io_error(&container, source)),
    }
    let result = (|| {
        if !created && !owned_workspace_path(&container) {
            return Err(WorkspaceError::UnsafePath(container.clone()));
        }
        for (name, contents) in [
            (".gitignore", b"*\n".as_slice()),
            (WORKSPACE_MARKER, WORKSPACE_MARKER_CONTENTS),
        ] {
            let path = container.join(name);
            match OpenOptions::new().write(true).create_new(true).open(&path) {
                Ok(mut file) => file
                    .write_all(contents)
                    .and_then(|_| file.sync_all())
                    .map_err(|source| io_error(&path, source))?,
                Err(error) if error.kind() == io::ErrorKind::AlreadyExists => {
                    if !fs::symlink_metadata(&path)
                        .is_ok_and(|metadata| metadata.file_type().is_file())
                    {
                        return Err(WorkspaceError::UnsafePath(path));
                    }
                    if name == WORKSPACE_MARKER
                        && !fs::read(&path)
                            .is_ok_and(|contents| contents == WORKSPACE_MARKER_CONTENTS)
                    {
                        return Err(WorkspaceError::UnsafePath(path));
                    }
                }
                Err(source) => return Err(io_error(&path, source)),
            }
        }
        Ok(container.clone())
    })();
    if result.is_err() && created {
        let _ = fs::remove_dir_all(&container);
    }
    result
}

fn lexical_normalize(path: &Path) -> Option<PathBuf> {
    let mut normalized = PathBuf::new();
    for component in path.components() {
        match component {
            Component::Prefix(prefix) => normalized.push(prefix.as_os_str()),
            Component::RootDir => normalized.push(component.as_os_str()),
            Component::CurDir => {}
            Component::ParentDir => {
                if !normalized.pop() {
                    return None;
                }
            }
            Component::Normal(value) => normalized.push(value),
        }
    }
    Some(normalized)
}

fn inside(root: &Path, path: &Path) -> bool {
    path == root
        || path.strip_prefix(root).is_ok_and(|local| {
            !local.as_os_str().is_empty()
                && local
                    .components()
                    .all(|component| matches!(component, Component::Normal(_)))
        })
}

trait WorkspaceOperations {
    fn copy_file(&mut self, source: &Path, destination: &Path) -> Result<(), WorkspaceError>;
    fn rename(&mut self, source: &Path, destination: &Path) -> Result<(), WorkspaceError>;
}

struct SystemWorkspaceOperations;

impl WorkspaceOperations for SystemWorkspaceOperations {
    fn copy_file(&mut self, source: &Path, destination: &Path) -> Result<(), WorkspaceError> {
        // Per-file APFS clonefile calls can cost tens of milliseconds for tiny
        // files—far more than copying their bytes. Preserve CoW for genuinely
        // large artifacts, while ordinary source/config files take the fast
        // portable path.
        const REFLINK_MINIMUM_BYTES: u64 = 1024 * 1024;
        let bytes = fs::symlink_metadata(source)
            .map_err(|source_error| io_error(source, source_error))?
            .len();
        if bytes < REFLINK_MINIMUM_BYTES {
            fs::copy(source, destination)
                .map(|_| ())
                .map_err(|source_error| io_error(destination, source_error))
        } else {
            reflink_copy::reflink_or_copy(source, destination)
                .map(|_| ())
                .map_err(|source_error| io_error(destination, source_error))
        }
    }

    fn rename(&mut self, source: &Path, destination: &Path) -> Result<(), WorkspaceError> {
        atomic_rename(source, destination).map_err(WorkspaceError::from)
    }
}

#[cfg(unix)]
pub(crate) fn create_link(target: &Path, destination: &Path, _directory: bool) -> io::Result<()> {
    std::os::unix::fs::symlink(target, destination)
}

#[cfg(windows)]
pub(crate) fn create_link(target: &Path, destination: &Path, directory: bool) -> io::Result<()> {
    if directory {
        // Junctions work on ordinary NTFS installations without requiring
        // Developer Mode or SeCreateSymbolicLinkPrivilege. This is the common
        // path for dependency mounts and internal directory links. A project
        // may still live on a non-NTFS/network filesystem where junctions are
        // unavailable but unprivileged symlinks are enabled, so preserve that
        // valid fallback instead of assuming one Windows filesystem.
        match junction::create(target, destination) {
            Ok(()) => Ok(()),
            Err(junction_error) => {
                let _ = fs::remove_dir(destination);
                std::os::windows::fs::symlink_dir(target, destination).map_err(
                    |symlink_error| {
                        io::Error::new(
                            symlink_error.kind(),
                            format!(
                                "junction creation failed ({junction_error}); directory symlink fallback failed ({symlink_error})"
                            ),
                        )
                    },
                )
            }
        }
    } else {
        // A project that already contains a file symlink necessarily runs in
        // an environment capable of creating one. Do not silently copy it:
        // that can change Node realpath/module-identity semantics.
        std::os::windows::fs::symlink_file(target, destination)
    }
}

/// Clone a directory copy-on-write. `false` means the platform or filesystem
/// cannot (Linux has no directory reflink; APFS refuses across volumes and for
/// trees holding entries a clone cannot carry), and the caller falls back to
/// entry links. Any partial destination is removed first so the fallback
/// starts from a clean slate.
#[cfg(target_os = "macos")]
fn clone_directory(source: &Path, destination: &Path) -> bool {
    use std::{ffi::CString, os::unix::ffi::OsStrExt};

    let (Ok(source_c), Ok(destination_c)) = (
        CString::new(source.as_os_str().as_bytes()),
        CString::new(destination.as_os_str().as_bytes()),
    ) else {
        return false;
    };
    // SAFETY: both arguments are valid NUL-terminated paths that outlive the
    // call, and clonefile touches nothing else.
    let status = unsafe { libc::clonefile(source_c.as_ptr(), destination_c.as_ptr(), 0) };
    if status == 0 {
        return true;
    }
    let _ = fs::remove_dir_all(destination);
    false
}

#[cfg(all(unix, not(target_os = "macos")))]
fn clone_directory(_source: &Path, _destination: &Path) -> bool {
    false
}

/// Materialise `source` at `destination` without copying bytes: real
/// directories, one hard link per file, symlinks recreated verbatim. This is
/// the mount-safe form where a directory cannot be cloned (Linux has no
/// directory reflink): a container or VM that mounts the workspace sees real
/// dependency files where entry links would dangle. Hard links share inodes
/// with the originals, so an in-place write still reaches the user's tree --
/// the caveat entry links already carry -- but replacing an entry, which is
/// what npm does, only unlinks the workspace's name. `false` means the tree
/// could not be linked (another volume, protected files, an entry that is
/// neither file, directory nor symlink) and the caller falls back to entry
/// links after the partial destination is removed.
#[cfg(unix)]
fn hard_link_tree(source: &Path, destination: &Path) -> bool {
    fn link(source: &Path, destination: &Path) -> io::Result<()> {
        fs::create_dir(destination)?;
        for entry in fs::read_dir(source)? {
            let entry = entry?;
            let from = entry.path();
            let to = destination.join(entry.file_name());
            let file_type = entry.file_type()?;
            if file_type.is_dir() {
                link(&from, &to)?;
            } else if file_type.is_symlink() {
                std::os::unix::fs::symlink(fs::read_link(&from)?, &to)?;
            } else if file_type.is_file() {
                fs::hard_link(&from, &to)?;
            } else {
                return Err(io::Error::new(
                    io::ErrorKind::Unsupported,
                    "entry is neither a file, a directory nor a symlink",
                ));
            }
        }
        Ok(())
    }
    if link(source, destination).is_ok() {
        return true;
    }
    let _ = fs::remove_dir_all(destination);
    false
}

#[derive(Clone, Copy)]
struct CopyRoots<'a> {
    source: &'a Path,
    destination: &'a Path,
    final_destination: &'a Path,
    canonical_source: &'a Path,
}

fn copy_tree<Operations: WorkspaceOperations>(
    source: &Path,
    destination: &Path,
    roots: CopyRoots<'_>,
    root_level: bool,
    operations: &mut Operations,
) -> Result<(), WorkspaceError> {
    fs::create_dir_all(destination).map_err(|source| io_error(destination, source))?;
    let mut entries = fs::read_dir(source)
        .map_err(|error| io_error(source, error))?
        .collect::<Result<Vec<_>, _>>()
        .map_err(|error| io_error(source, error))?;
    entries.sort_by_key(fs::DirEntry::file_name);
    for entry in entries {
        let name = entry.file_name();
        let name_text = name
            .to_str()
            .ok_or_else(|| WorkspaceError::UnsafePath(entry.path()))?;
        if (root_level && ROOT_EXCLUSIONS.contains(&name_text))
            || NESTED_EXCLUSIONS.contains(&name_text)
        {
            continue;
        }
        let from = entry.path();
        let metadata = fs::symlink_metadata(&from).map_err(|error| io_error(&from, error))?;
        if metadata.file_type().is_dir() && owned_workspace_path(&from) {
            continue;
        }
        let to = destination.join(&name);
        // A Python virtual environment is never instrumented, and copied it
        // loses its interpreter: `bin/python` links outside the project, and
        // such links are left out, so a server started as `.venv/bin/python`
        // did not start at all. The copy uses the environment itself.
        if metadata.file_type().is_dir() && from.join("pyvenv.cfg").is_file() {
            create_link(&from, &to, true).map_err(|error| io_error(&to, error))?;
            continue;
        }
        if metadata.file_type().is_dir() {
            // Nested node_modules are never instrumented, so they are not
            // mirrored file by file: on a real monorepo (many packages and
            // examples, each with node_modules) the deep copy was 43-52
            // seconds of every run's startup.
            //
            // Preferred: a copy-on-write clone of the whole directory. It is
            // one call on APFS (85k files in ~1.2s, no bytes duplicated until
            // written) and leaves the workspace self-contained -- a suite that
            // mounts the workspace into a VM or container sees real dependency
            // trees, and writes stay in the workspace instead of passing
            // through to the user's tree.
            //
            // Next best, where directories cannot be cloned (Linux): the same
            // tree as hard links -- real directories inside a mount, no bytes
            // copied, one link call per file.
            //
            // Last resort: one symlink per package pointing at the original
            // tree, the same semantics the root has. The directory itself is
            // real, so tools creating NEW entries (vite's .cache) still write
            // into the workspace, but the links dangle wherever the original
            // path is not visible (a mounted VM), and npm then treats them as
            // broken installs and fails to re-link them across the mount.
            #[cfg(unix)]
            if name_text == "node_modules" && !root_level {
                if clone_directory(&from, &to) || hard_link_tree(&from, &to) {
                    // A workspace package's own tools are started from its
                    // own `.bin`, which comes before the root's on PATH.
                    crate::source_tools::place_launchers(&from.join(".bin"), &to.join(".bin"))
                        .map_err(|error| io_error(&to, error))?;
                    continue;
                }
                fs::create_dir_all(&to).map_err(|error| io_error(&to, error))?;
                let mut packages = fs::read_dir(&from)
                    .map_err(|error| io_error(&from, error))?
                    .collect::<Result<Vec<_>, _>>()
                    .map_err(|error| io_error(&from, error))?;
                packages.sort_by_key(fs::DirEntry::file_name);
                for package in packages {
                    let link_destination = to.join(package.file_name());
                    if package.file_name() == ".bin" && package.path().is_dir() {
                        link_tools(&package.path(), &link_destination, operations)?;
                        continue;
                    }
                    create_link(&package.path(), &link_destination, false)
                        .map_err(|error| io_error(&link_destination, error))?;
                }
                continue;
            }
            copy_tree(&from, &to, roots, false, operations)?;
            if name_text == "node_modules" && !root_level {
                crate::source_tools::place_launchers(&from.join(".bin"), &to.join(".bin"))
                    .map_err(|error| io_error(&to, error))?;
            }
        } else if metadata.file_type().is_symlink() {
            let link = fs::read_link(&from).map_err(|error| io_error(&from, error))?;
            let unresolved_target = if link.is_absolute() {
                link.clone()
            } else {
                from.parent().expect("entry parent").join(&link)
            };
            let Some(lexical_target) = lexical_normalize(&unresolved_target) else {
                omit_escaping_link(&from, &link);
                continue;
            };
            // A dangling symlink is a fact of the user's tree that their own
            // tooling tolerates: npm and pnpm workspaces routinely leave links
            // to packages that are not installed, and plain `npm test` never
            // resolves them. Refusing to mirror the project over one broke a
            // real monorepo on first touch. It resolves to nothing, so it can
            // leak nothing; the LEXICAL containment check still applies, and
            // the link is preserved as-is so the workspace matches the source.
            let canonical_target = match canonicalize_simplified(&from) {
                Ok(target) => Some(target),
                Err(error) if error.kind() == io::ErrorKind::NotFound => None,
                Err(error) => return Err(io_error(&from, error)),
            };
            let escapes = !inside(roots.source, &lexical_target)
                || canonical_target
                    .as_deref()
                    .is_some_and(|target| !inside(roots.canonical_source, target));
            if escapes {
                omit_escaping_link(&from, &link);
                continue;
            }
            let local_target = lexical_target
                .strip_prefix(roots.source)
                .map_err(|_| WorkspaceError::UnsafePath(lexical_target.clone()))?;
            let relocated = roots.destination.join(local_target);
            let Some(canonical_target) = canonical_target else {
                if cfg!(windows) {
                    // Windows link creation needs the target's type, which a
                    // dangling link cannot provide; the entry resolves to
                    // nothing either way.
                    continue;
                }
                let isolated_link = if link.is_absolute() {
                    pathdiff(&to, &relocated)?
                } else {
                    link
                };
                create_link(&isolated_link, &to, false).map_err(|error| io_error(&to, error))?;
                continue;
            };
            let target_metadata = fs::metadata(&canonical_target)
                .map_err(|error| io_error(&canonical_target, error))?;
            let isolated_link = if cfg!(windows) && target_metadata.is_dir() {
                roots.final_destination.join(local_target)
            } else if link.is_absolute() {
                pathdiff(&to, &relocated)?
            } else {
                link
            };
            create_link(&isolated_link, &to, target_metadata.is_dir())
                .map_err(|error| io_error(&to, error))?;
        } else if metadata.file_type().is_file() {
            operations.copy_file(&from, &to)?;
        }
        // A socket or FIFO -- a dev server's `tmp/puma.sock`, a database
        // socket kept under the project -- cannot be copied and holds no
        // source. Refusing it stopped the run over a file no test reads by
        // copy.
    }
    Ok(())
}

fn pathdiff(from: &Path, to: &Path) -> Result<PathBuf, WorkspaceError> {
    let from = from
        .parent()
        .ok_or_else(|| WorkspaceError::UnsafePath(from.into()))?;
    let from_components = from.components().collect::<Vec<_>>();
    let to_components = to.components().collect::<Vec<_>>();
    let common = from_components
        .iter()
        .zip(&to_components)
        .take_while(|(left, right)| left == right)
        .count();
    if common == 0 {
        return Err(WorkspaceError::UnsafePath(to.into()));
    }
    let mut relative = PathBuf::new();
    for _ in common..from_components.len() {
        relative.push("..");
    }
    for component in &to_components[common..] {
        relative.push(component.as_os_str());
    }
    Ok(relative)
}

/// One entry of the root `node_modules`. A package of the project's own
/// workspaces -- `node_modules/money` linked to `../packages/money` by npm,
/// pnpm or Yarn -- links to its copy in the workspace, which is the
/// instrumented one: linked to the project's entry it resolved to the
/// original, so another package's tests ran its code unmeasured and a change
/// to it never named them. Everything else links to the project's entry. A
/// scope directory (`@acme/`) is mirrored and its packages linked the same
/// way. Links to a copy are relative, so they hold when the staged workspace
/// is moved into place.
#[cfg(unix)]
fn link_package(
    root: &Path,
    workspace: &Path,
    target: &Path,
    to: &Path,
) -> Result<(), WorkspaceError> {
    let metadata = fs::symlink_metadata(target).map_err(|error| io_error(target, error))?;
    let scope = target
        .file_name()
        .and_then(|name| name.to_str())
        .is_some_and(|name| name.starts_with('@'));
    if scope && metadata.file_type().is_dir() {
        fs::create_dir_all(to).map_err(|error| io_error(to, error))?;
        let mut packages = fs::read_dir(target)
            .map_err(|error| io_error(target, error))?
            .collect::<Result<Vec<_>, _>>()
            .map_err(|error| io_error(target, error))?;
        packages.sort_by_key(fs::DirEntry::file_name);
        for package in packages {
            link_package(
                root,
                workspace,
                &package.path(),
                &to.join(package.file_name()),
            )?;
        }
        return Ok(());
    }
    if metadata.file_type().is_symlink()
        && let (Ok(resolved), Ok(canonical_root)) = (
            canonicalize_simplified(target),
            canonicalize_simplified(root),
        )
        && let Ok(local) = resolved.strip_prefix(&canonical_root)
        && !local.as_os_str().is_empty()
        && !local.components().any(|part| {
            matches!(
                part.as_os_str().to_str(),
                Some("node_modules" | ".supercov")
            )
        })
    {
        let copy = workspace.join(local);
        if fs::symlink_metadata(&copy).is_ok() {
            return create_link(&pathdiff(to, &copy)?, to, true)
                .map_err(|error| io_error(to, error));
        }
    }
    create_link(target, to, false).map_err(|error| io_error(to, error))
}

/// The workspace's own `node_modules/.bin`: every tool of the project's, and
/// in place of each that judges source as text a launcher that runs it on
/// the source as it was written (see `source_tools`). Package managers put
/// this directory first on a script's PATH, so nothing else decides what
/// `biome` in a test script is.
fn link_tools<Operations: WorkspaceOperations>(
    tools: &Path,
    to: &Path,
    operations: &mut Operations,
) -> Result<(), WorkspaceError> {
    #[cfg(unix)]
    let _ = &operations;
    fs::create_dir_all(to).map_err(|error| io_error(to, error))?;
    let judges = crate::source_tools::source_tools();
    let binary = crate::source_tools::launching_binary();
    let mut entries = fs::read_dir(tools)
        .map_err(|error| io_error(tools, error))?
        .collect::<Result<Vec<_>, _>>()
        .map_err(|error| io_error(tools, error))?;
    entries.sort_by_key(fs::DirEntry::file_name);
    for entry in entries {
        let (tool, link) = (entry.path(), to.join(entry.file_name()));
        let launcher = binary.as_deref().and_then(|binary| {
            crate::source_tools::launcher_for(&judges, binary, tools, &entry.file_name())
        });
        if let Some(launcher) = launcher {
            crate::source_tools::write_launcher(&link, &launcher)
                .map_err(|error| io_error(&link, error))?;
            continue;
        }
        // A link, where one needs no privilege. On Windows the shims are
        // small files that find their package from where they are, which
        // the workspace's `node_modules` has a link for.
        #[cfg(unix)]
        create_link(&tool, &link, false).map_err(|error| io_error(&link, error))?;
        #[cfg(windows)]
        if fs::metadata(&tool).is_ok_and(|metadata| metadata.is_file()) {
            operations.copy_file(&tool, &link)?;
        }
    }
    Ok(())
}

fn link_node_modules<Operations: WorkspaceOperations>(
    root: &Path,
    workspace: &Path,
    operations: &mut Operations,
) -> Result<(), WorkspaceError> {
    #[cfg(unix)]
    let _ = &operations;
    let source = root.join("node_modules");
    let source_metadata = match fs::symlink_metadata(&source) {
        Ok(metadata) => metadata,
        Err(error) if error.kind() == io::ErrorKind::NotFound => return Ok(()),
        Err(error) => return Err(io_error(&source, error)),
    };
    // pnpm setups routinely make the root node_modules itself a symlink into a
    // store elsewhere. The mirror links each entry to its absolute target
    // anyway, so following the root link loses no isolation -- refusing it
    // forced one user to materialise a 3.7 GB tree by hand.
    let source = if source_metadata.file_type().is_symlink() {
        let resolved =
            canonicalize_simplified(&source).map_err(|error| io_error(&source, error))?;
        if !fs::symlink_metadata(&resolved)
            .map_err(|error| io_error(&resolved, error))?
            .file_type()
            .is_dir()
        {
            return Err(WorkspaceError::UnsafePath(source));
        }
        resolved
    } else if source_metadata.file_type().is_dir() {
        source
    } else {
        return Err(WorkspaceError::UnsafePath(source));
    };
    let destination = workspace.join("node_modules");
    fs::create_dir_all(&destination).map_err(|error| io_error(&destination, error))?;
    let mut entries = fs::read_dir(&source)
        .map_err(|error| io_error(&source, error))?
        .collect::<Result<Vec<_>, _>>()
        .map_err(|error| io_error(&source, error))?;
    entries.sort_by_key(fs::DirEntry::file_name);
    for entry in entries {
        let target = entry.path();
        let to = destination.join(entry.file_name());
        if entry.file_name() == ".bin"
            && fs::metadata(&target).is_ok_and(|metadata| metadata.is_dir())
        {
            link_tools(&target, &to, operations)?;
            continue;
        }
        #[cfg(unix)]
        link_package(root, workspace, &target, &to)?;
        #[cfg(windows)]
        {
            let file_type = entry
                .file_type()
                .map_err(|error| io_error(&target, error))?;
            let resolved = if file_type.is_symlink() {
                canonicalize_simplified(&target).map_err(|error| io_error(&target, error))?
            } else {
                target.clone()
            };
            let metadata = fs::metadata(&resolved).map_err(|error| io_error(&resolved, error))?;
            if metadata.is_dir() {
                create_link(&resolved, &to, true).map_err(|error| io_error(&to, error))?;
            } else if metadata.is_file() {
                // Top-level node_modules metadata files are cheap to copy and a
                // link would unnecessarily require Windows symlink privileges.
                operations.copy_file(&resolved, &to)?;
            } else {
                return Err(WorkspaceError::UnsupportedEntry(target));
            }
        }
    }
    Ok(())
}

fn require_lock(root: &Path, lock: &ProjectLock) -> Result<(), WorkspaceError> {
    if lock.protects(root) {
        Ok(())
    } else {
        Err(WorkspaceError::MissingLock)
    }
}

pub fn prepare_isolated_workspace(
    root: &Path,
    run_id: &str,
    lock: &ProjectLock,
) -> Result<PathBuf, WorkspaceError> {
    require_lock(root, lock)?;
    ensure_container(root)?;
    let mut operations = SystemWorkspaceOperations;
    let workspace = isolated_workspace_path(root, run_id)?;
    remove_stored_tree_deferred(root, &workspace)?;
    let canonical_root = canonicalize_simplified(root).map_err(|error| io_error(root, error))?;
    copy_tree(
        root,
        &workspace,
        CopyRoots {
            source: root,
            destination: &workspace,
            final_destination: &workspace,
            canonical_source: &canonical_root,
        },
        true,
        &mut operations,
    )?;
    link_node_modules(root, &workspace, &mut operations)?;
    Ok(workspace)
}

fn transaction_prefix(root: &Path, kind: &str) -> Result<String, WorkspaceError> {
    Ok(format!(
        ".{}.{}-",
        project_name(root)?.to_string_lossy(),
        kind
    ))
}

fn transaction_path(
    root: &Path,
    lock: &ProjectLock,
    kind: &str,
) -> Result<PathBuf, WorkspaceError> {
    let workspace = cached_workspace_path(root, lock)?;
    Ok(workspace.parent().expect("workspace parent").join(format!(
        "{}{}",
        transaction_prefix(root, kind)?,
        unique()
    )))
}

#[derive(Debug, Clone, PartialEq, Eq, Serialize)]
#[serde(rename_all = "camelCase")]
pub struct CacheRecoveryResult {
    pub restored_previous: bool,
    pub removed_staging: usize,
    pub removed_previous: usize,
}

pub fn recover_cached_workspace(
    root: &Path,
    lock: &ProjectLock,
) -> Result<CacheRecoveryResult, WorkspaceError> {
    require_lock(root, lock)?;
    let workspace = cached_workspace_path(root, lock)?;
    let parent = workspace.parent().expect("workspace parent");
    let entries = match fs::read_dir(parent) {
        Ok(entries) => entries,
        Err(error) if error.kind() == io::ErrorKind::NotFound => {
            return Ok(CacheRecoveryResult {
                restored_previous: false,
                removed_staging: 0,
                removed_previous: 0,
            });
        }
        Err(error) => return Err(io_error(parent, error)),
    };
    let staging_prefix = transaction_prefix(root, "staging")?;
    let previous_prefix = transaction_prefix(root, "previous")?;
    let mut staging = Vec::new();
    let mut previous = Vec::new();
    let mut invalid_previous = Vec::new();
    for entry in entries {
        let entry = entry.map_err(|error| io_error(parent, error))?;
        let name = entry.file_name().to_string_lossy().into_owned();
        if name.starts_with(&staging_prefix) {
            staging.push(entry.path());
        } else if name.starts_with(&previous_prefix) {
            let metadata = entry
                .file_type()
                .map_err(|error| io_error(&entry.path(), error))?;
            if metadata.is_dir() {
                let modified = entry
                    .metadata()
                    .and_then(|metadata| metadata.modified())
                    .unwrap_or(UNIX_EPOCH);
                previous.push((modified, entry.path()));
            } else {
                invalid_previous.push(entry.path());
            }
        }
    }
    previous.sort_by_key(|entry| std::cmp::Reverse(entry.0));
    let mut restored = false;
    if fs::symlink_metadata(&workspace).is_err()
        && let Some((_, newest)) = previous.first()
    {
        atomic_rename(newest, &workspace)?;
        previous.remove(0);
        restored = true;
    }
    let removed_staging = staging.len();
    let removed_previous = previous.len() + invalid_previous.len();
    for path in staging
        .into_iter()
        .chain(previous.into_iter().map(|(_, path)| path))
        .chain(invalid_previous)
    {
        remove_stored_tree_deferred(root, &path)?;
    }
    Ok(CacheRecoveryResult {
        restored_previous: restored,
        removed_staging,
        removed_previous,
    })
}

fn checked_reuse_path(workspace: &Path, requested: &Path) -> Result<PathBuf, WorkspaceError> {
    if requested.is_absolute()
        || requested
            .components()
            .any(|component| !matches!(component, Component::Normal(_)))
    {
        return Err(WorkspaceError::UnsafePath(requested.into()));
    }
    let path = workspace.join(requested);
    if path == workspace || fs::symlink_metadata(&path).is_err() {
        return Err(WorkspaceError::UnsafePath(requested.into()));
    }
    Ok(path)
}

pub fn prepare_cached_workspace(
    root: &Path,
    lock: &ProjectLock,
    reuse_paths: &[PathBuf],
) -> Result<PathBuf, WorkspaceError> {
    let mut operations = SystemWorkspaceOperations;
    prepare_cached_workspace_with_operations(root, lock, reuse_paths, &mut operations)
}

fn prepare_cached_workspace_with_operations<Operations: WorkspaceOperations>(
    root: &Path,
    lock: &ProjectLock,
    reuse_paths: &[PathBuf],
    operations: &mut Operations,
) -> Result<PathBuf, WorkspaceError> {
    require_lock(root, lock)?;
    ensure_container(root)?;
    recover_cached_workspace(root, lock)?;
    let workspace = cached_workspace_path(root, lock)?;
    let staging = transaction_path(root, lock, "staging")?;
    let previous = transaction_path(root, lock, "previous")?;
    let result = (|| {
        let canonical_root =
            canonicalize_simplified(root).map_err(|error| io_error(root, error))?;
        copy_tree(
            root,
            &staging,
            CopyRoots {
                source: root,
                destination: &staging,
                final_destination: &workspace,
                canonical_source: &canonical_root,
            },
            true,
            operations,
        )?;
        link_node_modules(root, &staging, operations)?;
        for requested in reuse_paths {
            let from = checked_reuse_path(&workspace, requested)?;
            let to = staging.join(requested);
            // Output directories are excluded from the mirror only at the
            // root, so the staging tree may already hold a stale source copy
            // of a nested artifact. The cached artifact is the exact
            // post-build state and replaces it wholesale.
            match fs::symlink_metadata(&to) {
                Ok(existing) if existing.file_type().is_dir() => {
                    fs::remove_dir_all(&to).map_err(|error| io_error(&to, error))?;
                }
                Ok(_) => {
                    fs::remove_file(&to).map_err(|error| io_error(&to, error))?;
                }
                Err(_) => {}
            }
            let metadata = fs::symlink_metadata(&from).map_err(|error| io_error(&from, error))?;
            if metadata.file_type().is_dir() {
                let canonical_workspace = canonicalize_simplified(&workspace)
                    .map_err(|error| io_error(&workspace, error))?;
                copy_tree(
                    &from,
                    &to,
                    CopyRoots {
                        source: &workspace,
                        destination: &staging,
                        final_destination: &workspace,
                        canonical_source: &canonical_workspace,
                    },
                    false,
                    operations,
                )?;
            } else if metadata.file_type().is_file() {
                fs::create_dir_all(to.parent().expect("reuse parent"))
                    .map_err(|error| io_error(&to, error))?;
                operations.copy_file(&from, &to)?;
            } else {
                return Err(WorkspaceError::UnsupportedEntry(from));
            }
        }
        let mut moved_previous = false;
        if fs::symlink_metadata(&workspace).is_ok() {
            operations.rename(&workspace, &previous)?;
            moved_previous = true;
        }
        if let Err(error) = operations.rename(&staging, &workspace) {
            if moved_previous && fs::symlink_metadata(&workspace).is_err() {
                let _ = operations.rename(&previous, &workspace);
            }
            return Err(error);
        }
        if fs::symlink_metadata(&previous).is_ok() {
            remove_stored_tree_deferred(root, &previous)?;
        }
        Ok(workspace.clone())
    })();
    if fs::symlink_metadata(&staging).is_ok() {
        remove_stored_tree_deferred(root, &staging)?;
    }
    result
}

/// The state of every regular file in the mirror the moment before the
/// wrapped command starts: relative path to (length, modified time). Cheap to
/// take (stat only) and precise enough to attribute changes to the command.
pub struct WorkspaceOutputBaseline {
    entries: BTreeMap<PathBuf, (u64, SystemTime)>,
}

/// What flowed back to the real project after the wrapped command finished.
#[derive(Debug, Default, PartialEq, Eq)]
pub struct CommandOutputSync {
    pub synced: usize,
    /// Where the synced files went, for the line that reports them.
    pub synced_paths: Vec<PathBuf>,
    /// Source files the command modified inside the workspace. Their mirror
    /// copies are instrumented, so copying them back would inject probes into
    /// the user's repository; they are reported instead.
    pub skipped_instrumented: Vec<PathBuf>,
    /// Files the command deleted inside the workspace. Deletions are reported
    /// rather than propagated: a defect here would destroy user data.
    pub deleted_in_workspace: Vec<PathBuf>,
}

fn walk_output_files(
    workspace: &Path,
    directory: &Path,
    entries: &mut BTreeMap<PathBuf, (u64, SystemTime)>,
) -> Result<(), WorkspaceError> {
    for entry in fs::read_dir(directory)
        .map_err(|error| io_error(directory, error))?
        .collect::<Result<Vec<_>, _>>()
        .map_err(|error| io_error(directory, error))?
    {
        let path = entry.path();
        let name = entry.file_name();
        let relative = path
            .strip_prefix(workspace)
            .map_err(|_| WorkspaceError::UnsafePath(path.clone()))?
            .to_owned();
        // Supercov's generated state never flows back, `.git` is not a
        // command output, and symlinks (the node_modules link above all) point
        // at the real project already.
        if relative.components().count() == 1
            && matches!(name.to_str(), Some(".supercov") | Some(".git"))
        {
            continue;
        }
        let metadata = entry.metadata().map_err(|error| io_error(&path, error))?;
        // Dependencies are never command outputs. Nested node_modules may be
        // materialised clones (see copy_tree), and a tool's cache inside any
        // node_modules must not flow back into the project's dependency tree.
        if metadata.is_dir() && name.to_str() == Some("node_modules") {
            continue;
        }
        if fs::symlink_metadata(&path)
            .map_err(|error| io_error(&path, error))?
            .file_type()
            .is_symlink()
        {
            continue;
        }
        if metadata.is_dir() {
            walk_output_files(workspace, &path, entries)?;
        } else if metadata.is_file() {
            let modified = metadata
                .modified()
                .map_err(|error| io_error(&path, error))?;
            entries.insert(relative, (metadata.len(), modified));
        }
    }
    Ok(())
}

pub fn workspace_output_baseline(
    workspace: &Path,
) -> Result<WorkspaceOutputBaseline, WorkspaceError> {
    let mut entries = BTreeMap::new();
    walk_output_files(workspace, workspace, &mut entries)?;
    Ok(WorkspaceOutputBaseline { entries })
}

/// Refuse to copy through any pre-existing symlink component under the
/// project root, so a command output can never be redirected outside it.
fn validate_writeback_destination(root: &Path, relative: &Path) -> Result<(), WorkspaceError> {
    if relative.as_os_str().is_empty()
        || relative
            .components()
            .any(|component| !matches!(component, Component::Normal(_)))
    {
        return Err(WorkspaceError::UnsafePath(relative.into()));
    }
    let mut current = root.to_owned();
    for component in relative.components() {
        current.push(component.as_os_str());
        match fs::symlink_metadata(&current) {
            Ok(metadata) if metadata.file_type().is_symlink() => {
                return Err(WorkspaceError::UnsafePath(current));
            }
            Ok(_) => {}
            Err(error) if error.kind() == io::ErrorKind::NotFound => break,
            Err(error) => return Err(io_error(&current, error)),
        }
    }
    Ok(())
}

/// Paths by the place a reader would look for them: each top-level directory
/// with how many of the files it holds, the largest first, and files at the
/// root by name. "synced 2553 file(s)" named none of them, and 2,547 were a
/// build nobody expected in the checkout.
pub fn summarize_paths(paths: &[PathBuf]) -> String {
    const SHOWN: usize = 4;
    let mut places = BTreeMap::<String, usize>::new();
    for path in paths {
        let mut components = path.components();
        let first = components
            .next()
            .map(|component| component.as_os_str().to_string_lossy().into_owned())
            .unwrap_or_default();
        let place = if components.next().is_some() {
            format!("{first}/")
        } else {
            first
        };
        *places.entry(place).or_default() += 1;
    }
    let mut places = places.into_iter().collect::<Vec<_>>();
    places.sort_by(|left, right| right.1.cmp(&left.1).then_with(|| left.0.cmp(&right.0)));
    let mut shown = places
        .iter()
        .take(SHOWN)
        .map(|(place, count)| {
            if place.ends_with('/') {
                format!("{place} {count}")
            } else {
                place.clone()
            }
        })
        .collect::<Vec<_>>();
    if places.len() > SHOWN {
        let rest = places[SHOWN..]
            .iter()
            .map(|(_, count)| count)
            .sum::<usize>();
        shown.push(format!("{rest} elsewhere"));
    }
    shown.join(", ")
}

/// The build output directories of files built from instrumented source.
///
/// Such a directory is the outermost one holding the file that the project
/// either ignores in git or does not have: `.next`, `dist`, `build`. A
/// directory the project tracks is not one, so a generated file next to
/// updated snapshots withholds only itself.
struct BuiltDirectories<'a> {
    root: &'a Path,
    judged: BTreeMap<PathBuf, bool>,
    directories: BTreeSet<PathBuf>,
}

impl<'a> BuiltDirectories<'a> {
    fn new(root: &'a Path) -> Self {
        Self {
            root,
            judged: BTreeMap::new(),
            directories: BTreeSet::new(),
        }
    }

    fn add(&mut self, relative: &Path) {
        let mut directory = PathBuf::new();
        let Some(parent) = relative.parent() else {
            return;
        };
        for component in parent.components() {
            directory.push(component);
            let root = self.root;
            let output = *self
                .judged
                .entry(directory.clone())
                .or_insert_with(|| is_build_output(root, &directory));
            if output {
                self.directories.insert(directory);
                return;
            }
        }
    }

    fn holds(&self, relative: &Path) -> bool {
        relative
            .ancestors()
            .skip(1)
            .any(|directory| self.directories.contains(directory))
    }
}

/// A bundle, a module or the source map of one.
fn compiled_javascript(path: &Path) -> bool {
    path.extension()
        .and_then(|extension| extension.to_str())
        .is_some_and(|extension| matches!(extension, "js" | "mjs" | "cjs" | "map"))
}

fn is_build_output(root: &Path, directory: &Path) -> bool {
    if fs::symlink_metadata(root.join(directory)).is_err() {
        return true;
    }
    std::process::Command::new("git")
        .arg("-C")
        .arg(root)
        .args(["check-ignore", "-q", "--"])
        .arg(directory)
        .env("GIT_TERMINAL_PROMPT", "0")
        .stdin(std::process::Stdio::null())
        .stdout(std::process::Stdio::null())
        .stderr(std::process::Stdio::null())
        .status()
        .is_ok_and(|status| status.code() == Some(0))
}

/// A text file larger than this is copied as it is.
const NAMED_FILE_LIMIT: u64 = 16 << 20;

/// The spellings of the copy's directory a file can hold, each with the
/// project's directory spelled the same way: as the path is, as JSON writes
/// a Windows path (`C:\\project`), and as a URL writes one (`C:/project`).
fn copy_and_project(root: &Path, workspace: &Path) -> Vec<(String, String)> {
    let mut places = Vec::new();
    let mut add = |copy: &Path, project: &Path| {
        let (copy, project) = (copy.to_string_lossy(), project.to_string_lossy());
        for (from, to) in [("\\", "\\"), ("\\", "\\\\"), ("\\", "/")] {
            let pair = (copy.replace(from, to), project.replace(from, to));
            if !pair.0.is_empty() && pair.0 != pair.1 && !places.contains(&pair) {
                places.push(pair);
            }
        }
    };
    add(workspace, root);
    if let (Ok(copy), Ok(project)) = (
        canonicalize_simplified(workspace),
        canonicalize_simplified(root),
    ) {
        add(&copy, &project);
    }
    // The longest first, so one spelling inside another is not cut short.
    places.sort_by_key(|(copy, _)| std::cmp::Reverse(copy.len()));
    places
}

/// The text of a file the command wrote, with the copy's directory replaced
/// by the project's, when the file is text that names the copy.
///
/// The command ran in the copy, so anything that wrote where it was wrote
/// the copy's path: a coverage report's `coverage-final.json` keyed every
/// file by it, `clover.xml` and a Cobertura report's `<source>` named it, and
/// a snapshot of a working directory holds it. Run without Supercov the same
/// files name the project, and a file that flows back to the project is to
/// be the file the command alone would have left there.
fn naming_the_project(
    file: &Path,
    places: &[(String, String)],
) -> Result<Option<String>, WorkspaceError> {
    let metadata = fs::metadata(file).map_err(|error| io_error(file, error))?;
    if !metadata.is_file() || metadata.len() > NAMED_FILE_LIMIT {
        return Ok(None);
    }
    let bytes = fs::read(file).map_err(|error| io_error(file, error))?;
    if bytes.contains(&0) {
        return Ok(None);
    }
    let Ok(text) = String::from_utf8(bytes) else {
        return Ok(None);
    };
    Ok(project_named(&text, places))
}

/// What a Go or JVM command prints, passed on with the copy named as the
/// project. Maven ends a failed run with "Please refer to
/// <copy>/target/surefire-reports", Gradle with a `file://` address of its
/// report in the copy, and Go names a file in a compile error from where it
/// runs: `../../../../crypto.go:9:2` for `./crypto.go:9:2`.
pub fn console_names(root: &Path, workspace: &Path) -> crate::process_supervision::ConsoleNames {
    let up = workspace
        .strip_prefix(root)
        .ok()
        .map(|inside| inside.components().count())
        .filter(|levels| *levels > 0)
        .map(|levels| "../".repeat(levels));
    crate::process_supervision::ConsoleNames {
        places: copy_and_project(root, workspace),
        up_to_the_project: up,
    }
}

/// `text` with each spelling of the copy's directory replaced by the
/// project's, when it holds one.
pub(crate) fn project_named(text: &str, places: &[(String, String)]) -> Option<String> {
    if !places.iter().any(|(copy, _)| text.contains(copy.as_str())) {
        return None;
    }
    let mut out = String::with_capacity(text.len());
    let mut rest = text;
    // The next place the copy is named, the longest spelling where two start
    // together (`places` is sorted so).
    while let Some((at, copy, project)) = places
        .iter()
        .filter_map(|(copy, project)| Some((rest.find(copy.as_str())?, copy, project)))
        .min_by_key(|(at, _, _)| *at)
    {
        out.push_str(&rest[..at]);
        let after = &rest[at + copy.len()..];
        // The directory itself, not a neighbour whose name begins with it
        // (`app` beside `app-old`).
        let continues = after
            .chars()
            .next()
            .is_some_and(|next| next.is_alphanumeric() || matches!(next, '-' | '_'));
        if continues {
            let first = rest[at..].chars().next().expect("a match is not empty");
            out.push(first);
            rest = &rest[at + first.len_utf8()..];
        } else {
            out.push_str(project);
            rest = after;
        }
    }
    out.push_str(rest);
    (out != text).then_some(out)
}

/// Copy files the wrapped command created or changed in the mirror back to
/// the real project, so `supercov -- <command>` leaves the working tree in
/// the same state `<command>` alone would have: updated snapshots, generated
/// fixtures, and reports land in the repository, not in a cache directory.
/// `protected` names relative paths whose mirror copies are instrumented and
/// must never flow back.
pub fn sync_command_outputs(
    root: &Path,
    workspace: &Path,
    baseline: &WorkspaceOutputBaseline,
    protected: &BTreeSet<PathBuf>,
) -> Result<CommandOutputSync, WorkspaceError> {
    sync_outputs_of(
        CommandOutputs::JavaScript,
        root,
        workspace,
        baseline,
        protected,
    )
}

/// Whose command ran in the copy, for what of its output is the copy's own.
///
/// A JavaScript build carries the instrumentation in what it compiles, and
/// is recognised by the runtime it imports. Go builds nothing into the tree
/// unless asked, and what it is asked to build holds the runtime's package.
/// Maven and Gradle compile into the tree, `target/` and `build/`, and what
/// they compile there is the copy's: only the reports they write beside it
/// are the command's own. Cargo builds outside the copy.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum CommandOutputs {
    JavaScript,
    Go,
    Jvm,
    Rust,
}

/// A Go binary built in the copy: it links the runtime, whose package path
/// is in it.
fn links_the_go_runtime(path: &Path) -> Result<bool, WorkspaceError> {
    const MARKER: &[u8] = b"_supercov_runtime";
    let bytes = fs::read(path).map_err(|error| io_error(path, error))?;
    Ok(bytes.contains(&0) && bytes.windows(MARKER.len()).any(|window| window == MARKER))
}

/// The directories a JVM build writes its reports to, inside its output.
const JVM_REPORTS: &[&str] = &[
    "surefire-reports",
    "failsafe-reports",
    "test-results",
    "reports",
    "site",
];

/// Whether a file a JVM build wrote is the build's own state or what it
/// compiled, rather than a report: anything in Gradle's own directory, and
/// anything in a module's output directory outside the report directories.
/// An output directory is `target` beside a `pom.xml` and `build` beside a
/// Gradle build file, so a package named `build` is not one.
fn jvm_build_state(workspace: &Path, relative: &Path) -> bool {
    let parts = relative
        .components()
        .map(|part| part.as_os_str().to_string_lossy().into_owned())
        .collect::<Vec<_>>();
    if parts.first().is_some_and(|first| first == ".gradle") {
        return true;
    }
    // The copy's build files carry what Supercov added to them.
    if parts.last().is_some_and(|name| {
        matches!(
            name.as_str(),
            "pom.xml"
                | "build.gradle"
                | "build.gradle.kts"
                | "settings.gradle"
                | "settings.gradle.kts"
        )
    }) {
        return true;
    }
    if relative
        .extension()
        .is_some_and(|extension| extension == "class" || extension == "jar")
    {
        return true;
    }
    let mut module = workspace.to_path_buf();
    for (index, part) in parts.iter().enumerate() {
        let output = match part.as_str() {
            "target" => module.join("pom.xml").is_file(),
            "build" => [
                "build.gradle",
                "build.gradle.kts",
                "settings.gradle",
                "settings.gradle.kts",
            ]
            .iter()
            .any(|file| module.join(file).is_file()),
            _ => false,
        };
        if output {
            return !parts[index + 1..]
                .iter()
                .any(|inside| JVM_REPORTS.contains(&inside.as_str()));
        }
        module.push(part);
    }
    false
}

/// `sync_command_outputs` for a command of any of the languages that run in
/// a copy.
pub fn sync_outputs_of(
    kind: CommandOutputs,
    root: &Path,
    workspace: &Path,
    baseline: &WorkspaceOutputBaseline,
    protected: &BTreeSet<PathBuf>,
) -> Result<CommandOutputSync, WorkspaceError> {
    let mut current = BTreeMap::new();
    walk_output_files(workspace, workspace, &mut current)?;
    let mut sync = CommandOutputSync::default();
    // A file the command built FROM an instrumented source carries the
    // instrumentation with it. Copying that into the project would leave
    // probes and a workspace-only runtime import in the application's own
    // build output, which is the one thing isolation promises never
    // happens. The generated runtime lives only inside a workspace, so a
    // reference to it is proof the file is not the command's own output.
    //
    // Nor is the rest of the build it belongs to. `next build` wrote 2,547
    // files of which a handful imported the runtime; the others (chunks with
    // probes, manifests naming workspace paths) were copied, and a later
    // `next start` in the project would have served an instrumented build.
    let places = copy_and_project(root, workspace);
    let mut changed = Vec::new();
    let mut built = BTreeSet::new();
    let mut built_directories = BuiltDirectories::new(root);
    for (relative, state) in &current {
        if baseline.entries.get(relative) == Some(state) {
            continue;
        }
        if !protected.contains(relative) {
            match kind {
                CommandOutputs::JavaScript => {
                    // Compiled code marks a build. A report that quotes a
                    // stack frame of the runtime does not: the screenshots
                    // and traces beside a failed Playwright test's
                    // `error-context.md` are the command's own output.
                    let compiled = compiled_javascript(relative);
                    if built_from_instrumented_source(&workspace.join(relative), compiled)? {
                        built.insert(relative.clone());
                        if compiled {
                            built_directories.add(relative);
                        }
                    }
                }
                CommandOutputs::Go => {
                    if links_the_go_runtime(&workspace.join(relative))? {
                        built.insert(relative.clone());
                    }
                }
                CommandOutputs::Jvm => {
                    if jvm_build_state(workspace, relative) {
                        built.insert(relative.clone());
                    }
                }
                // Any Rust file in the copy may have been rewritten, the
                // tests among them for their assertions. And the lockfile
                // Cargo writes when a project has none is what the run's
                // dependencies are read from: returned, it made the run
                // that wrote it stale the moment it ended, and every test
                // affected by a change to one line.
                CommandOutputs::Rust => {
                    if relative
                        .extension()
                        .is_some_and(|extension| extension == "rs")
                        || relative
                            .file_name()
                            .is_some_and(|name| name == "Cargo.lock")
                    {
                        built.insert(relative.clone());
                    }
                }
            }
        }
        changed.push(relative);
    }
    for relative in changed {
        if protected.contains(relative)
            || built.contains(relative)
            || built_directories.holds(relative)
        {
            sync.skipped_instrumented.push(relative.clone());
            continue;
        }
        let from = workspace.join(relative);
        if unaccompanied_install_manifest(root, workspace, relative) {
            continue;
        }
        validate_writeback_destination(root, relative)?;
        let to = root.join(relative);
        if let Some(parent) = to.parent() {
            fs::create_dir_all(parent).map_err(|error| io_error(parent, error))?;
        }
        match naming_the_project(&from, &places)? {
            Some(text) => {
                fs::write(&to, text).map_err(|error| io_error(&to, error))?;
                if let Ok(metadata) = fs::metadata(&from) {
                    let _ = fs::set_permissions(&to, metadata.permissions());
                }
            }
            None => {
                fs::copy(&from, &to).map_err(|error| io_error(&to, error))?;
            }
        }
        sync.synced += 1;
        sync.synced_paths.push(relative.clone());
    }
    for relative in baseline.entries.keys() {
        // Only what the project still has is worth a line: a rebuilt `.next`
        // that never left the workspace "deleted" files nobody can find.
        if !current.contains_key(relative) && fs::symlink_metadata(root.join(relative)).is_ok() {
            sync.deleted_in_workspace.push(relative.clone());
        }
    }
    Ok(sync)
}

/// What flowed back and what stayed, as the lines a run prints about it.
pub fn describe_command_outputs(outputs: &CommandOutputSync) -> Vec<String> {
    let mut lines = Vec::new();
    if outputs.synced > 0 {
        lines.push(format!(
            "[supercov] synced {} file(s) the command created or changed back to the project: {}",
            outputs.synced,
            summarize_paths(&outputs.synced_paths)
        ));
    }
    if !outputs.skipped_instrumented.is_empty() {
        lines.push(format!(
            "[supercov] {} file(s) stayed in the isolated workspace: they are instrumented copies, or were built from them, and must not overwrite your project: {}",
            outputs.skipped_instrumented.len(),
            summarize_paths(&outputs.skipped_instrumented)
        ));
    }
    if !outputs.deleted_in_workspace.is_empty() {
        lines.push(format!(
            "[supercov] the command deleted {} file(s) in the isolated workspace; deletions are not propagated to the project: {}",
            outputs.deleted_in_workspace.len(),
            summarize_paths(&outputs.deleted_in_workspace)
        ));
    }
    lines
}

/// Give the workspace a git repository of its own, as the project has one.
///
/// Tools in a test command read git: npm's template-oss-check derives the
/// repository and release branches it expects from it, and semver's posttest
/// failed without one ("\"repository\" ... expected to be removed"). The
/// project's own .git is never shared -- a test's `git add` or `commit`
/// would then change the user's index and refs -- so this is a clone that
/// borrows the project's objects and has its own refs, index and config: the
/// project's HEAD, remotes and remote branches, and an index read from HEAD.
/// Best effort: without git, or on any failure, the workspace has no .git.
pub fn mirror_git_repository(root: &Path, workspace: &Path) -> Result<(), String> {
    if fs::symlink_metadata(root.join(".git")).is_err() {
        return Ok(());
    }
    let target = workspace.join(".git");
    let git = |directory: &Path, arguments: &[&str]| -> Result<String, String> {
        let output = std::process::Command::new("git")
            .arg("-C")
            .arg(directory)
            .args(arguments)
            .env("GIT_TERMINAL_PROMPT", "0")
            .stdin(std::process::Stdio::null())
            .output()
            .map_err(|error| format!("git {}: {error}", arguments.join(" ")))?;
        if output.status.success() {
            Ok(String::from_utf8_lossy(&output.stdout).trim().to_owned())
        } else {
            Err(format!(
                "git {}: {}",
                arguments.join(" "),
                String::from_utf8_lossy(&output.stderr).trim()
            ))
        }
    };
    let result = (|| {
        if fs::symlink_metadata(&target).is_ok() {
            fs::remove_dir_all(&target).map_err(|error| error.to_string())?;
        }
        let source = root.to_string_lossy();
        let destination = target.to_string_lossy();
        git(
            workspace,
            &[
                "clone",
                "--quiet",
                "--shared",
                "--bare",
                &source,
                &destination,
            ],
        )?;
        git(workspace, &["config", "core.bare", "false"])?;
        match git(root, &["symbolic-ref", "-q", "HEAD"]) {
            Ok(branch) => git(workspace, &["symbolic-ref", "HEAD", &branch])?,
            Err(_) => {
                let commit = git(root, &["rev-parse", "HEAD"])?;
                git(workspace, &["update-ref", "--no-deref", "HEAD", &commit])?
            }
        };
        git(workspace, &["remote", "remove", "origin"])?;
        let remotes = git(root, &["remote"])?;
        for name in remotes.lines().filter(|name| !name.is_empty()) {
            // As configured: get-url applies the user's insteadOf rewrites,
            // which the clone's own reads apply again.
            let url = git(root, &["config", "--get", &format!("remote.{name}.url")])?;
            git(workspace, &["remote", "add", name, &url])?;
        }
        git(
            workspace,
            &[
                "fetch",
                "--quiet",
                "--no-tags",
                &source,
                "+refs/remotes/*:refs/remotes/*",
            ],
        )?;
        git(workspace, &["read-tree", "HEAD"])?;
        // Supercov's own directories are not the command's untracked files.
        let exclude = target.join("info/exclude");
        fs::create_dir_all(target.join("info")).map_err(|error| error.to_string())?;
        let mut patterns = fs::read_to_string(&exclude).unwrap_or_default();
        patterns.push_str("\n.supercov/\n");
        fs::write(&exclude, patterns).map_err(|error| error.to_string())?;
        Ok(())
    })();
    if result.is_err() {
        let _ = fs::remove_dir_all(&target);
    }
    result
}

/// Keep the files Supercov rewrote out of the workspace repository's status:
/// natively they are unmodified, and a test that checks for a clean tree must
/// find one. Best effort, as the repository is.
pub fn hide_rewritten_files(workspace: &Path, files: &[String]) {
    if files.is_empty() || !workspace.join(".git").is_dir() {
        return;
    }
    let run = |arguments: &[&str], input: &str| {
        let mut child = std::process::Command::new("git")
            .arg("-C")
            .arg(workspace)
            .args(arguments)
            .stdin(std::process::Stdio::piped())
            .stdout(std::process::Stdio::piped())
            .stderr(std::process::Stdio::null())
            .spawn()
            .ok()?;
        child.stdin.take()?.write_all(input.as_bytes()).ok()?;
        let output = child.wait_with_output().ok()?;
        output
            .status
            .success()
            .then(|| String::from_utf8_lossy(&output.stdout).into_owned())
    };
    // Only tracked files carry the bit; git refuses the rest.
    let Some(listed) = run(&["ls-files", "-z"], "") else {
        return;
    };
    let tracked = listed.split('\0').collect::<BTreeSet<_>>();
    let hidden = files
        .iter()
        .filter(|file| tracked.contains(file.as_str()))
        .map(String::as_str)
        .collect::<Vec<_>>();
    if !hidden.is_empty() {
        let _ = run(
            &["update-index", "--skip-worktree", "--stdin"],
            &hidden.join("\n"),
        );
    }
}

/// Whether `relative` is the manifest or lockfile of an install the command
/// made in the workspace alone. node_modules never flows back, so copying
/// these would leave the project claiming an install it does not have: tap
/// installs its plugins into .tap/plugins, found package.json and a lockfile
/// there on the next run without Supercov, and failed to import them. Left
/// out, the tool installs them again as it would the first time.
fn unaccompanied_install_manifest(root: &Path, workspace: &Path, relative: &Path) -> bool {
    let manifest = relative
        .file_name()
        .and_then(|name| name.to_str())
        .is_some_and(|name| {
            matches!(
                name,
                "package.json"
                    | "package-lock.json"
                    | "npm-shrinkwrap.json"
                    | "yarn.lock"
                    | "pnpm-lock.yaml"
                    | "bun.lock"
                    | "bun.lockb"
            )
        });
    let Some(directory) = relative.parent() else {
        return false;
    };
    manifest
        && workspace.join(directory).join("node_modules").is_dir()
        && !root.join(directory).join("node_modules").exists()
}

/// Whether `path` mentions the generated runtime module, which exists only
/// inside an instrumented workspace. Read as bytes: build output can be
/// minified, source-mapped or not valid UTF-8, and the marker is ASCII.
/// Whether a file the command wrote came out of instrumented source: it
/// names the generated runtime, which lives only inside a workspace, or it is
/// compiled code that calls the probes. A bundler that inlines the runtime
/// leaves no path to it in a chunk (Turbopack's name it only in their source
/// maps), but the probe calls are still there. A minifier renames the
/// probes' local names and cannot rename the global they are read from: a
/// Vite production build of the instrumented copy held none of the first
/// two, and was synced over the project's `dist/`.
fn built_from_instrumented_source(path: &Path, compiled: bool) -> Result<bool, WorkspaceError> {
    const RUNTIME: &[u8] = b".supercov/node_modules/";
    const PROBE: &[u8] = b"__supercov";
    const GLOBAL: &[u8] = b"__SUPERCOV_DIRECT_RUNTIME__";
    let contents = match fs::read(path) {
        Ok(contents) => contents,
        Err(error) if error.kind() == std::io::ErrorKind::NotFound => return Ok(false),
        Err(error) => return Err(io_error(path, error)),
    };
    let holds = |marker: &[u8]| {
        contents
            .windows(marker.len())
            .any(|window| window == marker)
    };
    Ok(holds(RUNTIME) || (compiled && (holds(PROBE) || holds(GLOBAL))))
}

pub fn prune_cached_workspace_sources(
    root: &Path,
    lock: &ProjectLock,
) -> Result<Vec<String>, WorkspaceError> {
    require_lock(root, lock)?;
    let workspace = cached_workspace_path(root, lock)?;
    if fs::symlink_metadata(&workspace).is_err() {
        return Ok(Vec::new());
    }
    let keep = BTreeSet::from(["node_modules".to_owned(), ".supercov".to_owned()]);
    let mut removed = Vec::new();
    for entry in fs::read_dir(&workspace).map_err(|error| io_error(&workspace, error))? {
        let entry = entry.map_err(|error| io_error(&workspace, error))?;
        let name = entry
            .file_name()
            .into_string()
            .map_err(|_| WorkspaceError::UnsafePath(entry.path()))?;
        if keep.contains(&name) {
            continue;
        }
        remove_stored_tree_deferred(root, &entry.path())?;
        removed.push(name);
    }
    removed.sort();
    Ok(removed)
}

#[cfg(test)]
mod tests {
    #[test]
    fn file_urls_match_what_node_derives_for_every_path_shape() {
        use super::file_url_from_slash_path;
        assert_eq!(
            file_url_from_slash_path("C:/Users/runner/AppData/Local/Temp/x/register.mjs"),
            "file:///C:/Users/runner/AppData/Local/Temp/x/register.mjs"
        );
        assert_eq!(
            file_url_from_slash_path("C:/Users/a b/#1/register.mjs"),
            "file:///C:/Users/a%20b/%231/register.mjs"
        );
        assert_eq!(
            file_url_from_slash_path("//server/share/dir/register.mjs"),
            "file://server/share/dir/register.mjs"
        );
        assert_eq!(
            file_url_from_slash_path("/w/p/register.mjs"),
            "file:///w/p/register.mjs"
        );
        assert_eq!(
            file_url_from_slash_path("/w/p x?q/ü.mjs"),
            "file:///w/p%20x%3Fq/%C3%BC.mjs"
        );
    }

    #[test]
    fn verbatim_prefixes_are_stripped_and_plain_paths_left_alone() {
        use super::strip_verbatim;
        assert_eq!(strip_verbatim(r"\\?\C:\a\b").as_deref(), Some(r"C:\a\b"));
        assert_eq!(strip_verbatim(r"\\?\d:\x").as_deref(), Some(r"d:\x"));
        assert_eq!(
            strip_verbatim(r"\\?\UNC\server\share\dir").as_deref(),
            Some(r"\\server\share\dir")
        );
        assert_eq!(strip_verbatim(r"C:\a\b"), None);
        assert_eq!(strip_verbatim("/w/project"), None);
        assert_eq!(strip_verbatim(r"\\server\share"), None);
        assert_eq!(strip_verbatim(r"\\?\"), None);
    }

    use super::*;

    fn writeback_fixture(name: &str) -> (PathBuf, PathBuf) {
        let base =
            std::env::temp_dir().join(format!("supercov-writeback-{}-{name}", std::process::id()));
        if base.exists() {
            fs::remove_dir_all(&base).unwrap();
        }
        let root = base.join("project");
        let workspace = base.join("workspace");
        fs::create_dir_all(root.join("src")).unwrap();
        fs::create_dir_all(workspace.join("src")).unwrap();
        fs::create_dir_all(workspace.join(".supercov")).unwrap();
        fs::write(root.join("src/app.ts"), "original\n").unwrap();
        fs::write(workspace.join("src/app.ts"), "instrumented\n").unwrap();
        fs::write(workspace.join(".supercov/state.json"), "{}").unwrap();
        (root, workspace)
    }

    #[test]
    fn the_workspace_has_a_repository_of_its_own_that_reads_like_the_project() {
        let (root, workspace) = writeback_fixture("git-mirror");
        let git = |directory: &Path, arguments: &[&str]| {
            let output = std::process::Command::new("git")
                .arg("-C")
                .arg(directory)
                .args(arguments)
                .output()
                .unwrap();
            assert!(output.status.success(), "git {arguments:?}: {output:?}");
            String::from_utf8(output.stdout).unwrap().trim().to_owned()
        };
        git(&root, &["init", "--quiet", "--initial-branch=release/v2"]);
        git(&root, &["config", "user.email", "test@example.com"]);
        git(&root, &["config", "user.name", "Test"]);
        git(
            &root,
            &[
                "remote",
                "add",
                "origin",
                "https://github.com/example/app.git",
            ],
        );
        git(&root, &["add", "src/app.ts"]);
        git(&root, &["commit", "--quiet", "-m", "first"]);
        let head = git(&root, &["rev-parse", "HEAD"]);

        mirror_git_repository(&root, &workspace).unwrap();
        assert_eq!(git(&workspace, &["rev-parse", "HEAD"]), head);
        assert_eq!(
            git(&workspace, &["symbolic-ref", "HEAD"]),
            "refs/heads/release/v2"
        );
        assert_eq!(
            git(&workspace, &["config", "--get", "remote.origin.url"]),
            "https://github.com/example/app.git"
        );
        // The workspace copy is instrumented and .supercov is Supercov's.
        hide_rewritten_files(&workspace, &["src/app.ts".to_owned()]);
        assert_eq!(git(&workspace, &["status", "--porcelain"]), "");

        // A commit a test makes stays in the workspace's own repository.
        fs::write(workspace.join("new.txt"), "new\n").unwrap();
        git(&workspace, &["add", "new.txt"]);
        git(
            &workspace,
            &[
                "-c",
                "user.email=t@example.com",
                "-c",
                "user.name=T",
                "commit",
                "--quiet",
                "-m",
                "second",
            ],
        );
        assert_eq!(git(&root, &["rev-parse", "HEAD"]), head);
        assert_eq!(git(&root, &["status", "--porcelain"]), "");
        fs::remove_dir_all(root.parent().unwrap()).unwrap();
    }

    #[test]
    fn an_install_the_command_made_does_not_flow_back_without_its_dependencies() {
        let (root, workspace) = writeback_fixture("tool-install");
        fs::write(workspace.join("package.json"), "{}").unwrap();
        fs::write(root.join("package.json"), "{}").unwrap();
        fs::create_dir_all(workspace.join("node_modules")).unwrap();
        fs::create_dir_all(root.join("node_modules")).unwrap();
        let baseline = workspace_output_baseline(&workspace).unwrap();
        // tap installs the plugins its config names into .tap/plugins.
        fs::create_dir_all(workspace.join(".tap/plugins/node_modules/@tapjs/clock")).unwrap();
        fs::write(workspace.join(".tap/plugins/package.json"), "{}").unwrap();
        fs::write(workspace.join(".tap/plugins/package-lock.json"), "{}").unwrap();
        fs::create_dir_all(workspace.join(".tap/test-results")).unwrap();
        fs::write(workspace.join(".tap/test-results/basic.tap"), "ok").unwrap();
        // The project's own manifest still flows back beside its own install.
        fs::write(workspace.join("package.json"), "{\"version\":\"2\"}").unwrap();
        let sync = sync_command_outputs(&root, &workspace, &baseline, &BTreeSet::new()).unwrap();
        assert!(!root.join(".tap/plugins/package.json").exists());
        assert!(!root.join(".tap/plugins/package-lock.json").exists());
        assert!(root.join(".tap/test-results/basic.tap").is_file());
        assert_eq!(
            fs::read_to_string(root.join("package.json")).unwrap(),
            "{\"version\":\"2\"}"
        );
        assert_eq!(sync.synced, 2);
        fs::remove_dir_all(root.parent().unwrap()).unwrap();
    }

    #[test]
    fn a_file_that_flows_back_names_the_project_and_not_the_copy() {
        // Vitest's coverage report, written by a command that ran in the copy:
        // `coverage-final.json` keyed every file by the copy's path, where the
        // same command without Supercov keys it by the project's.
        let (root, workspace) = writeback_fixture("names");
        let baseline = workspace_output_baseline(&workspace).unwrap();
        let (copy, project) = (workspace.display().to_string(), root.display().to_string());
        fs::create_dir_all(workspace.join("coverage")).unwrap();
        fs::write(
            workspace.join("coverage/coverage-final.json"),
            format!("{{\"{copy}/src/fee.js\": {{\"path\": \"{copy}/src/fee.js\"}}}}"),
        )
        .unwrap();
        fs::write(
            workspace.join("coverage/cobertura.xml"),
            format!("<sources><source>{copy}</source></sources>"),
        )
        .unwrap();
        // A neighbour whose name only begins with the copy's is not the copy.
        fs::write(
            workspace.join("coverage/neighbour.txt"),
            format!("{copy}-old/file and {copy}"),
        )
        .unwrap();
        // Bytes that are not text are copied as they are.
        let mut binary = copy.clone().into_bytes();
        binary.push(0);
        fs::write(workspace.join("coverage/cache.bin"), &binary).unwrap();
        fs::write(workspace.join("coverage/plain.txt"), "nothing to name").unwrap();
        let sync = sync_command_outputs(&root, &workspace, &baseline, &BTreeSet::new()).unwrap();
        assert_eq!(sync.synced, 5);
        assert_eq!(
            fs::read_to_string(root.join("coverage/coverage-final.json")).unwrap(),
            format!("{{\"{project}/src/fee.js\": {{\"path\": \"{project}/src/fee.js\"}}}}")
        );
        assert_eq!(
            fs::read_to_string(root.join("coverage/cobertura.xml")).unwrap(),
            format!("<sources><source>{project}</source></sources>")
        );
        assert_eq!(
            fs::read_to_string(root.join("coverage/neighbour.txt")).unwrap(),
            format!("{copy}-old/file and {project}")
        );
        assert_eq!(fs::read(root.join("coverage/cache.bin")).unwrap(), binary);
        assert_eq!(
            fs::read_to_string(root.join("coverage/plain.txt")).unwrap(),
            "nothing to name"
        );
        // The copy keeps what the command wrote.
        assert!(
            fs::read_to_string(workspace.join("coverage/cobertura.xml"))
                .unwrap()
                .contains(&copy)
        );
        fs::remove_dir_all(root.parent().unwrap()).unwrap();
    }

    #[test]
    fn what_a_go_jvm_or_rust_command_writes_flows_back_and_what_it_builds_stays() {
        // Only JavaScript's command had its files returned. `go test
        // -coverprofile=cover.out` left no `cover.out` in the project, `mvn
        // test` no `target/surefire-reports`, and a snapshot a Rust test
        // accepted stayed in the copy.
        let write = |workspace: &Path, file: &str, contents: &[u8]| {
            let path = workspace.join(file);
            fs::create_dir_all(path.parent().unwrap()).unwrap();
            fs::write(path, contents).unwrap();
        };
        let flows = |kind: CommandOutputs, name: &str, files: &[(&str, &[u8])]| {
            let (root, workspace) = writeback_fixture(name);
            // The build files are the copy's before the command runs.
            write(&workspace, "pom.xml", b"<project/>");
            write(&workspace, "app/build.gradle.kts", b"plugins {}");
            let baseline = workspace_output_baseline(&workspace).unwrap();
            for (file, contents) in files {
                write(&workspace, file, contents);
            }
            let sync =
                sync_outputs_of(kind, &root, &workspace, &baseline, &BTreeSet::new()).unwrap();
            let synced = files
                .iter()
                .filter(|(file, _)| root.join(file).is_file())
                .map(|(file, _)| (*file).to_owned())
                .collect::<Vec<_>>();
            assert_eq!(sync.synced, synced.len(), "{sync:?}");
            fs::remove_dir_all(root.parent().unwrap()).unwrap();
            synced
        };
        let mut binary = b"\x7fELF\0\0example.com/shop/_supercov_runtime\0".to_vec();
        binary.extend([0u8; 16]);
        assert_eq!(
            flows(
                CommandOutputs::Go,
                "go",
                &[
                    ("cover.out", b"mode: set\n"),
                    ("testdata/golden.png", b"\x89PNG\0\0"),
                    ("shop.test", &binary),
                ]
            ),
            ["cover.out", "testdata/golden.png"]
        );
        assert_eq!(
            flows(
                CommandOutputs::Jvm,
                "jvm",
                &[
                    (
                        "target/surefire-reports/TEST-shop.CryptoTest.xml",
                        b"<testsuite/>"
                    ),
                    ("target/site/jacoco/index.html", b"<html/>"),
                    ("app/build/test-results/test/TEST-a.xml", b"<testsuite/>"),
                    ("app/build/reports/tests/test/index.html", b"<html/>"),
                    ("src/test/resources/approved/output.txt", b"approved"),
                    // What the build compiled, and its own state.
                    ("target/classes/shop/Crypto.class", b"\xca\xfe\xba\xbe"),
                    (
                        "target/maven-status/compile/createdFiles.lst",
                        b"shop/Crypto.class"
                    ),
                    ("app/build/classes/kotlin/main/A.class", b"\xca\xfe\xba\xbe"),
                    ("app/build/tmp/compileKotlin/cache.txt", b"state"),
                    (".gradle/8.10/fileHashes/fileHashes.lock", b"lock"),
                    ("pom.xml", b"<project><!-- changed --></project>"),
                    // A package named `build` is not a build's output.
                    ("src/build/notes.txt", b"kept"),
                ]
            ),
            [
                "target/surefire-reports/TEST-shop.CryptoTest.xml",
                "target/site/jacoco/index.html",
                "app/build/test-results/test/TEST-a.xml",
                "app/build/reports/tests/test/index.html",
                "src/test/resources/approved/output.txt",
                "src/build/notes.txt",
            ]
        );
        assert_eq!(
            flows(
                CommandOutputs::Rust,
                "rust",
                &[
                    ("src/snapshots/sealed.snap", b"SNAP"),
                    ("report.json", b"{}"),
                    ("Cargo.lock", b"# lock"),
                    ("src/generated.rs", b"pub fn made() {}"),
                ]
            ),
            ["src/snapshots/sealed.snap", "report.json"]
        );
    }

    #[test]
    fn dependency_trees_never_flow_back() {
        // Nested node_modules may be materialised clones, and any node_modules
        // can hold a tool's cache: neither is a command output.
        let (root, workspace) = writeback_fixture("dependencies");
        let baseline = workspace_output_baseline(&workspace).unwrap();
        fs::create_dir_all(workspace.join("node_modules/.vite")).unwrap();
        fs::write(workspace.join("node_modules/.vite/deps.json"), "{}").unwrap();
        fs::create_dir_all(workspace.join("packages/app/node_modules/dep")).unwrap();
        fs::write(
            workspace.join("packages/app/node_modules/dep/index.js"),
            "dep",
        )
        .unwrap();
        let sync = sync_command_outputs(&root, &workspace, &baseline, &BTreeSet::new()).unwrap();
        assert_eq!(sync.synced, 0);
        assert!(sync.deleted_in_workspace.is_empty());
        assert!(!root.join("node_modules").exists());
        assert!(!root.join("packages").exists());
        fs::remove_dir_all(root.parent().unwrap()).unwrap();
    }

    #[test]
    fn output_built_from_instrumented_sources_never_flows_back() {
        // A build the command runs inside the workspace compiles the
        // instrumented copies, so its output carries probes and an import of a
        // runtime that exists only in the workspace. Writing that into the
        // project would leave the application's own build output instrumented
        // after the run, which isolation promises never happens.
        let (root, workspace) = writeback_fixture("built-output");
        let baseline = workspace_output_baseline(&workspace).unwrap();
        fs::create_dir_all(workspace.join("dist")).unwrap();
        fs::write(
            workspace.join("dist/app.js"),
            "import { coverageHit } from \"./.supercov/node_modules/runtime.mjs\";\ncoverageHit(0);\n",
        )
        .unwrap();
        fs::write(
            workspace.join("dist/app.d.ts"),
            "export declare const a: number;\n",
        )
        .unwrap();

        let sync = sync_command_outputs(&root, &workspace, &baseline, &BTreeSet::new()).unwrap();

        // The declarations beside it were inferred from the instrumented
        // copies too, so the whole build stays behind, not a part of it.
        assert_eq!(
            sync.skipped_instrumented,
            vec![PathBuf::from("dist/app.d.ts"), PathBuf::from("dist/app.js")],
            "the instrumented build is reported, not copied"
        );
        assert_eq!(sync.synced, 0);
        assert!(!root.join("dist").exists());
        fs::remove_dir_all(root.parent().unwrap()).unwrap();
    }

    #[test]
    fn a_built_file_in_a_directory_the_project_keeps_withholds_only_itself() {
        // `src` is the project's own directory, not a build's: the snapshot a
        // test updated next to a generated bundle still belongs to the project.
        let (root, workspace) = writeback_fixture("built-beside-outputs");
        let baseline = workspace_output_baseline(&workspace).unwrap();
        fs::write(
            workspace.join("src/bundle.js"),
            "import \"../.supercov/node_modules/runtime.mjs\";\n",
        )
        .unwrap();
        fs::write(workspace.join("src/app.snap"), "updated\n").unwrap();

        let sync = sync_command_outputs(&root, &workspace, &baseline, &BTreeSet::new()).unwrap();

        assert_eq!(
            sync.skipped_instrumented,
            vec![PathBuf::from("src/bundle.js")]
        );
        assert_eq!(sync.synced, 1);
        assert_eq!(
            fs::read_to_string(root.join("src/app.snap")).unwrap(),
            "updated\n"
        );
        fs::remove_dir_all(root.parent().unwrap()).unwrap();
    }

    #[test]
    fn paths_are_summarized_by_where_they_are() {
        let paths = [
            ".next/BUILD_ID",
            ".next/server/app.js",
            ".next/server/chunks/a.js",
            "tests/e2e/runs/1/report.json",
            "coverage/report.json",
            "playwright-report/index.html",
            "notes.txt",
            "out/a.txt",
        ]
        .map(PathBuf::from);
        assert_eq!(
            summarize_paths(&paths),
            ".next/ 3, coverage/ 1, notes.txt, out/ 1, 2 elsewhere"
        );
        assert_eq!(summarize_paths(&paths[..1]), ".next/ 1");
    }

    #[test]
    fn a_report_that_quotes_the_runtime_does_not_make_its_directory_a_build() {
        // A failed Playwright test writes its stack, runtime frames included,
        // next to the screenshot and trace a developer opens to debug it.
        let (root, workspace) = writeback_fixture("report-quotes-runtime");
        let baseline = workspace_output_baseline(&workspace).unwrap();
        fs::create_dir_all(workspace.join("test-results/adds-an-item")).unwrap();
        fs::write(
            workspace.join("test-results/adds-an-item/error-context.md"),
            "at launch (/w/.supercov/node_modules/playwright.mjs:907:44)\n",
        )
        .unwrap();
        fs::write(
            workspace.join("test-results/adds-an-item/trace.zip"),
            "trace",
        )
        .unwrap();

        let sync = sync_command_outputs(&root, &workspace, &baseline, &BTreeSet::new()).unwrap();

        assert_eq!(
            sync.skipped_instrumented,
            vec![PathBuf::from("test-results/adds-an-item/error-context.md")]
        );
        assert_eq!(sync.synced, 1);
        assert!(root.join("test-results/adds-an-item/trace.zip").exists());
        fs::remove_dir_all(root.parent().unwrap()).unwrap();
    }

    #[test]
    fn a_build_directory_git_ignores_stays_behind_even_when_the_project_has_one() {
        let (root, workspace) = writeback_fixture("built-ignored");
        let git = |arguments: &[&str]| {
            std::process::Command::new("git")
                .arg("-C")
                .arg(&root)
                .args(arguments)
                .output()
                .unwrap()
        };
        if !git(&["init", "-q"]).status.success() {
            fs::remove_dir_all(root.parent().unwrap()).unwrap();
            return;
        }
        fs::write(root.join(".gitignore"), ".next\n").unwrap();
        fs::create_dir_all(root.join(".next")).unwrap();
        fs::write(root.join(".next/BUILD_ID"), "mine\n").unwrap();
        let baseline = workspace_output_baseline(&workspace).unwrap();
        fs::create_dir_all(workspace.join(".next/server")).unwrap();
        fs::write(
            workspace.join(".next/server/chunk.js"),
            // Turbopack inlines the runtime: a chunk holds probe calls and
            // no path to it.
            "__supercovCoverageHitV3(__supercovProbeFileV2, 0);\n",
        )
        .unwrap();
        fs::write(workspace.join(".next/BUILD_ID"), "instrumented\n").unwrap();

        let sync = sync_command_outputs(&root, &workspace, &baseline, &BTreeSet::new()).unwrap();

        assert_eq!(sync.synced, 0);
        assert_eq!(sync.skipped_instrumented.len(), 2);
        assert_eq!(
            fs::read_to_string(root.join(".next/BUILD_ID")).unwrap(),
            "mine\n"
        );
        fs::remove_dir_all(root.parent().unwrap()).unwrap();
    }

    #[test]
    fn a_minified_build_of_the_instrumented_copy_stays_in_the_workspace() {
        let (root, workspace) = writeback_fixture("built-minified");
        let initialized = std::process::Command::new("git")
            .arg("-C")
            .arg(&root)
            .args(["init", "-q"])
            .output()
            .unwrap();
        if !initialized.status.success() {
            fs::remove_dir_all(root.parent().unwrap()).unwrap();
            return;
        }
        fs::write(root.join(".gitignore"), "dist\n").unwrap();
        fs::create_dir_all(root.join("dist")).unwrap();
        fs::write(root.join("dist/index.html"), "mine\n").unwrap();
        let baseline = workspace_output_baseline(&workspace).unwrap();
        fs::create_dir_all(workspace.join("dist/assets")).unwrap();
        fs::write(
            workspace.join("dist/assets/index-DzZxFCVp.js"),
            // A minifier renames each probe's local name. The global they
            // are read from is a property name, which it leaves.
            "const e=globalThis.__SUPERCOV_DIRECT_RUNTIME__.coverageHitV2;e(t,0);\n",
        )
        .unwrap();
        fs::write(workspace.join("dist/index.html"), "instrumented\n").unwrap();

        let sync = sync_command_outputs(&root, &workspace, &baseline, &BTreeSet::new()).unwrap();

        assert_eq!(sync.synced, 0);
        assert_eq!(sync.skipped_instrumented.len(), 2);
        assert_eq!(
            fs::read_to_string(root.join("dist/index.html")).unwrap(),
            "mine\n"
        );
        fs::remove_dir_all(root.parent().unwrap()).unwrap();
    }

    #[test]
    fn command_outputs_flow_back_to_the_project() {
        let (root, workspace) = writeback_fixture("outputs");
        let baseline = workspace_output_baseline(&workspace).unwrap();
        fs::create_dir_all(workspace.join("src/__snapshots__")).unwrap();
        fs::write(
            workspace.join("src/__snapshots__/app.snap"),
            "updated snapshot\n",
        )
        .unwrap();
        fs::write(workspace.join(".supercov/state.json"), "{\"changed\":1}").unwrap();
        let sync = sync_command_outputs(&root, &workspace, &baseline, &BTreeSet::new()).unwrap();
        assert_eq!(sync.synced, 1);
        assert_eq!(
            fs::read_to_string(root.join("src/__snapshots__/app.snap")).unwrap(),
            "updated snapshot\n"
        );
        assert!(!root.join(".supercov/state.json").exists());
        fs::remove_dir_all(root.parent().unwrap()).unwrap();
    }

    #[test]
    fn instrumented_sources_never_flow_back() {
        let (root, workspace) = writeback_fixture("protected");
        let baseline = workspace_output_baseline(&workspace).unwrap();
        // A formatter run by the command rewrites the instrumented copy; the
        // real source must keep its original bytes.
        fs::write(workspace.join("src/app.ts"), "instrumented, reformatted\n").unwrap();
        let protected = BTreeSet::from([PathBuf::from("src/app.ts")]);
        let sync = sync_command_outputs(&root, &workspace, &baseline, &protected).unwrap();
        assert_eq!(sync.synced, 0);
        assert_eq!(sync.skipped_instrumented, [PathBuf::from("src/app.ts")]);
        assert_eq!(
            fs::read_to_string(root.join("src/app.ts")).unwrap(),
            "original\n"
        );
        fs::remove_dir_all(root.parent().unwrap()).unwrap();
    }

    #[test]
    fn workspace_deletions_are_reported_but_never_propagated() {
        let (root, workspace) = writeback_fixture("deletions");
        fs::write(workspace.join("stale.txt"), "old\n").unwrap();
        fs::write(root.join("stale.txt"), "old\n").unwrap();
        // Gone from the workspace only: the project never had it.
        fs::write(workspace.join("built.txt"), "old\n").unwrap();
        let baseline = workspace_output_baseline(&workspace).unwrap();
        fs::remove_file(workspace.join("stale.txt")).unwrap();
        fs::remove_file(workspace.join("built.txt")).unwrap();
        let sync = sync_command_outputs(&root, &workspace, &baseline, &BTreeSet::new()).unwrap();
        assert_eq!(sync.deleted_in_workspace, [PathBuf::from("stale.txt")]);
        assert!(root.join("stale.txt").exists());
        fs::remove_dir_all(root.parent().unwrap()).unwrap();
    }

    #[cfg(unix)]
    #[test]
    fn writeback_refuses_a_symlinked_destination() {
        let (root, workspace) = writeback_fixture("symlink");
        let outside = root.parent().unwrap().join("outside");
        fs::create_dir_all(&outside).unwrap();
        std::os::unix::fs::symlink(&outside, root.join("reports")).unwrap();
        let baseline = workspace_output_baseline(&workspace).unwrap();
        fs::create_dir_all(workspace.join("reports")).unwrap();
        fs::write(workspace.join("reports/result.txt"), "output\n").unwrap();
        let error =
            sync_command_outputs(&root, &workspace, &baseline, &BTreeSet::new()).unwrap_err();
        assert!(matches!(error, WorkspaceError::UnsafePath(_)));
        assert!(!outside.join("result.txt").exists());
        fs::remove_dir_all(root.parent().unwrap()).unwrap();
    }

    struct OrdinaryCopyOperations;

    impl WorkspaceOperations for OrdinaryCopyOperations {
        fn copy_file(&mut self, source: &Path, destination: &Path) -> Result<(), WorkspaceError> {
            fs::copy(source, destination)
                .map(|_| ())
                .map_err(|error| io_error(destination, error))
        }

        fn rename(&mut self, source: &Path, destination: &Path) -> Result<(), WorkspaceError> {
            atomic_rename(source, destination).map_err(WorkspaceError::from)
        }
    }

    struct FaultOperations {
        copy_count: usize,
        fail_copy_at: Option<usize>,
        rename_count: usize,
        fail_rename_at: Option<usize>,
    }

    impl WorkspaceOperations for FaultOperations {
        fn copy_file(&mut self, source: &Path, destination: &Path) -> Result<(), WorkspaceError> {
            self.copy_count += 1;
            if self.fail_copy_at == Some(self.copy_count) {
                return Err(io_error(
                    destination,
                    io::Error::new(io::ErrorKind::StorageFull, "injected disk full"),
                ));
            }
            SystemWorkspaceOperations.copy_file(source, destination)
        }

        fn rename(&mut self, source: &Path, destination: &Path) -> Result<(), WorkspaceError> {
            self.rename_count += 1;
            if self.fail_rename_at == Some(self.rename_count) {
                return Err(io_error(
                    destination,
                    io::Error::other("injected publication rename failure"),
                ));
            }
            SystemWorkspaceOperations.rename(source, destination)
        }
    }

    fn transaction_debris(root: &Path) -> Vec<String> {
        let workspace = copy_path(root, 1).unwrap();
        let prefix = format!(".{}.", project_name(root).unwrap().to_string_lossy());
        fs::read_dir(workspace.parent().unwrap())
            .unwrap()
            .filter_map(Result::ok)
            .map(|entry| entry.file_name().to_string_lossy().into_owned())
            .filter(|name| name.starts_with(&prefix))
            .collect()
    }

    fn project() -> PathBuf {
        let root = std::env::temp_dir().join(format!("supercov-workspace-rust-{}", unique()));
        fs::create_dir_all(root.join("src")).unwrap();
        fs::write(root.join("src/index.js"), "one").unwrap();
        fs::write(root.join("package.json"), "{}").unwrap();
        root
    }

    #[test]
    #[cfg(unix)]
    fn root_node_modules_symlink_is_followed() {
        // pnpm layouts often make the project's node_modules a symlink into an
        // external store. Refusing it forced a 3.7 GB manual materialisation;
        // entries are linked to absolute targets regardless, so following the
        // root link is isolation-neutral.
        let root = project();
        let store = std::env::temp_dir().join(format!("supercov-store-{}", unique()));
        fs::create_dir_all(store.join("left-pad")).unwrap();
        fs::write(store.join("left-pad/package.json"), "{}").unwrap();
        std::os::unix::fs::symlink(&store, root.join("node_modules")).unwrap();
        let mut lock = ProjectLock::acquire(&root, "run", "now").unwrap();
        let workspace = prepare_isolated_workspace(&root, "run", &lock).unwrap();
        assert!(
            fs::symlink_metadata(workspace.join("node_modules/left-pad"))
                .unwrap()
                .file_type()
                .is_symlink()
        );
        lock.release().unwrap();
        fs::remove_dir_all(root).unwrap();
        fs::remove_dir_all(store).unwrap();
    }

    #[test]
    #[cfg(unix)]
    fn dangling_symlinks_are_preserved_and_escaping_ones_still_refused() {
        // npm and pnpm workspaces routinely leave symlinks to packages that
        // are not installed. superinterface's examples/*/node_modules carried
        // one, plain `npm test` never resolves it, and the mirror died on
        // `canonicalize` with ENOENT before any test ran. A dangling link
        // resolves to nothing, so it can leak nothing: it is preserved as-is,
        // while the lexical containment check still applies.
        let root = project();
        let nested = root.join("examples/app/node_modules/@scope");
        fs::create_dir_all(&nested).unwrap();
        std::os::unix::fs::symlink(
            "../../../../packages/react/node_modules/@scope/pkg",
            nested.join("pkg"),
        )
        .unwrap();
        let mut lock = ProjectLock::acquire(&root, "run", "now").unwrap();
        let workspace = prepare_isolated_workspace(&root, "run", &lock).unwrap();
        let mirrored = workspace.join("examples/app/node_modules/@scope/pkg");
        let metadata = fs::symlink_metadata(&mirrored).unwrap();
        assert!(metadata.file_type().is_symlink());
        assert_eq!(
            fs::read_link(&mirrored).unwrap(),
            PathBuf::from("../../../../packages/react/node_modules/@scope/pkg")
        );
        lock.release().unwrap();
        fs::remove_dir_all(root).unwrap();
    }

    #[test]
    #[cfg(target_os = "macos")]
    fn nested_node_modules_are_cloned_not_linked() {
        // A suite that mounts the workspace into a VM sees no host paths, so
        // entry links into the original tree dangle there and npm fails to
        // re-link them across the mount. APFS clones the directory instead:
        // real files, relative links inside kept verbatim, and writes staying
        // in the workspace.
        let root = project();
        let nested = root.join("packages/app/node_modules");
        fs::create_dir_all(nested.join("dep")).unwrap();
        fs::create_dir_all(nested.join(".bin")).unwrap();
        fs::write(nested.join("dep/index.js"), "dep").unwrap();
        std::os::unix::fs::symlink("../dep/index.js", nested.join(".bin/dep")).unwrap();
        let mut lock = ProjectLock::acquire(&root, "run", "now").unwrap();
        let workspace = prepare_isolated_workspace(&root, "run", &lock).unwrap();
        let mirrored = workspace.join("packages/app/node_modules");
        assert!(
            fs::symlink_metadata(mirrored.join("dep"))
                .unwrap()
                .file_type()
                .is_dir()
        );
        assert_eq!(
            fs::read_to_string(mirrored.join("dep/index.js")).unwrap(),
            "dep"
        );
        assert_eq!(
            fs::read_link(mirrored.join(".bin/dep")).unwrap(),
            PathBuf::from("../dep/index.js")
        );
        fs::write(mirrored.join("dep/index.js"), "changed in the workspace").unwrap();
        assert_eq!(
            fs::read_to_string(nested.join("dep/index.js")).unwrap(),
            "dep"
        );
        lock.release().unwrap();
        fs::remove_dir_all(root).unwrap();
    }

    #[test]
    #[cfg(unix)]
    fn hard_linked_dependency_tree_is_real_and_replaceable() {
        // The Linux materialisation: files share inodes with the originals,
        // symlinks are carried verbatim, and replacing an entry in the
        // workspace (npm's reify) leaves the user's tree untouched.
        use std::os::unix::fs::MetadataExt;
        let base = std::env::temp_dir().join(format!("supercov-hardlink-{}", unique()));
        let source = base.join("node_modules");
        fs::create_dir_all(source.join("dep")).unwrap();
        fs::create_dir_all(source.join(".bin")).unwrap();
        fs::write(source.join("dep/index.js"), "dep").unwrap();
        std::os::unix::fs::symlink("../dep/index.js", source.join(".bin/dep")).unwrap();
        let destination = base.join("workspace/node_modules");
        fs::create_dir_all(destination.parent().unwrap()).unwrap();
        assert!(hard_link_tree(&source, &destination));
        assert_eq!(
            fs::metadata(destination.join("dep/index.js"))
                .unwrap()
                .ino(),
            fs::metadata(source.join("dep/index.js")).unwrap().ino()
        );
        assert_eq!(
            fs::read_link(destination.join(".bin/dep")).unwrap(),
            PathBuf::from("../dep/index.js")
        );
        fs::remove_dir_all(destination.join("dep")).unwrap();
        fs::create_dir_all(destination.join("dep")).unwrap();
        fs::write(destination.join("dep/index.js"), "replaced").unwrap();
        assert_eq!(
            fs::read_to_string(source.join("dep/index.js")).unwrap(),
            "dep"
        );
        fs::remove_dir_all(base).unwrap();
    }

    #[test]
    #[cfg(unix)]
    fn symlink_escaping_the_project_is_omitted_from_the_mirror() {
        // The escape check guards the MIRRORED source tree. node_modules (root
        // and nested) are linked or cloned from the user's originals instead,
        // so the escaping fixture lives outside node_modules here.
        // Leftover tool state (a Chrome test profile linking into /tmp) must
        // not abort measurement: the entry is omitted and the run proceeds.
        let root = project();
        let nested = root.join("examples/app/lib");
        fs::create_dir_all(&nested).unwrap();
        std::os::unix::fs::symlink("../../../../../outside-the-project/pkg", nested.join("pkg"))
            .unwrap();
        let mut lock = ProjectLock::acquire(&root, "run", "now").unwrap();
        let workspace = prepare_isolated_workspace(&root, "run", &lock).unwrap();
        assert!(fs::symlink_metadata(workspace.join("examples/app/lib/pkg")).is_err());
        assert!(workspace.join("src/index.js").exists());
        lock.release().unwrap();
        fs::remove_dir_all(root).unwrap();
    }

    #[test]
    fn isolated_copy_never_changes_source_and_requires_the_lock() {
        let root = project();
        let mut lock = ProjectLock::acquire(&root, "run", "now").unwrap();
        let workspace = prepare_isolated_workspace(&root, "run", &lock).unwrap();
        fs::write(workspace.join("src/index.js"), "instrumented").unwrap();
        assert_eq!(
            fs::read_to_string(root.join("src/index.js")).unwrap(),
            "one"
        );
        lock.release().unwrap();
        assert!(matches!(
            prepare_isolated_workspace(&root, "other", &lock),
            Err(WorkspaceError::MissingLock)
        ));
        fs::remove_dir_all(root).unwrap();
    }

    #[test]
    fn everything_supercov_writes_lives_under_the_store() {
        let root = project();
        assert_eq!(
            workspace_container(&root),
            root.join(".supercov/workspaces")
        );
        let mut lock = ProjectLock::acquire(&root, "run", "now").unwrap();
        let workspace = prepare_cached_workspace(&root, &lock, &[]).unwrap();
        assert!(workspace.starts_with(root.join(".supercov")));
        assert_eq!(
            fs::read_to_string(root.join(".supercov/.gitignore")).unwrap(),
            "*\n"
        );
        lock.release().unwrap();
        fs::remove_dir_all(root).unwrap();
    }

    #[test]
    fn an_unowned_container_gets_a_deterministic_fallback() {
        let root = project();
        fs::create_dir_all(root.join(".supercov/workspaces")).unwrap();
        fs::write(root.join(".supercov/workspaces/user-file"), "mine\n").unwrap();
        let container = workspace_container(&root);
        assert_ne!(container, root.join(".supercov/workspaces"));
        let name = container
            .file_name()
            .unwrap()
            .to_string_lossy()
            .into_owned();
        assert!(name.starts_with("workspaces-"));
        fs::remove_dir_all(root).unwrap();
    }

    #[test]
    fn user_supercov_directory_is_copied_and_never_adopted() {
        let root = project();
        fs::create_dir(root.join("supercov")).unwrap();
        fs::write(root.join("supercov/user-module.js"), "export default 1;\n").unwrap();
        let mut lock = ProjectLock::acquire(&root, "run", "now").unwrap();
        let workspace = prepare_cached_workspace(&root, &lock, &[]).unwrap();
        assert_ne!(workspace_container(&root), root.join("supercov"));
        assert_eq!(
            fs::read_to_string(workspace.join("supercov/user-module.js")).unwrap(),
            "export default 1;\n"
        );
        assert_eq!(
            fs::read_to_string(root.join("supercov/user-module.js")).unwrap(),
            "export default 1;\n"
        );
        assert!(!root.join("supercov").join(WORKSPACE_MARKER).exists());
        lock.release().unwrap();
        fs::remove_dir_all(root).unwrap();
    }

    #[cfg(unix)]
    #[test]
    fn stable_cache_refreshes_atomically_and_reuses_only_explicit_artifacts() {
        let root = project();
        let mut lock = ProjectLock::acquire(&root, "run", "now").unwrap();
        let first = prepare_cached_workspace(&root, &lock, &[]).unwrap();
        fs::create_dir_all(first.join("build")).unwrap();
        fs::write(first.join("build/output.js"), "instrumented").unwrap();
        fs::write(first.join("stale.txt"), "stale").unwrap();
        fs::write(root.join("src/index.js"), "two").unwrap();
        let second = prepare_cached_workspace(&root, &lock, &[PathBuf::from("build")]).unwrap();
        assert_eq!(first, second);
        assert_eq!(
            fs::read_to_string(second.join("src/index.js")).unwrap(),
            "two"
        );
        assert_eq!(
            fs::read_to_string(second.join("build/output.js")).unwrap(),
            "instrumented"
        );
        assert!(!second.join("stale.txt").exists());
        lock.release().unwrap();
        fs::remove_dir_all(root).unwrap();
    }

    #[test]
    fn reused_artifact_replaces_the_mirrored_stale_copy_wholesale() {
        let root = project();
        fs::create_dir_all(root.join("packages/app/dist")).unwrap();
        fs::write(root.join("packages/app/dist/stale.js"), "uninstrumented").unwrap();
        let mut lock = ProjectLock::acquire(&root, "run", "now").unwrap();
        let first = prepare_cached_workspace(&root, &lock, &[]).unwrap();
        fs::remove_file(first.join("packages/app/dist/stale.js")).unwrap();
        fs::write(first.join("packages/app/dist/built.js"), "instrumented").unwrap();
        let second =
            prepare_cached_workspace(&root, &lock, &[PathBuf::from("packages/app/dist")]).unwrap();
        assert_eq!(
            fs::read_to_string(second.join("packages/app/dist/built.js")).unwrap(),
            "instrumented"
        );
        assert!(!second.join("packages/app/dist/stale.js").exists());
        lock.release().unwrap();
        fs::remove_dir_all(root).unwrap();
    }

    #[test]
    fn terminal_workspace_keeps_flat_frontend_cache_without_discoverable_test_sources() {
        let root = project();
        fs::create_dir_all(root.join("tests/e2e")).unwrap();
        fs::write(root.join("tests/e2e/app.spec.js"), "test source").unwrap();
        let mut lock = ProjectLock::acquire(&root, "run", "now").unwrap();
        let workspace = prepare_cached_workspace(&root, &lock, &[]).unwrap();
        let artifacts = workspace.join(".supercov/frontend-cache-artifacts");
        fs::create_dir_all(&artifacts).unwrap();
        fs::write(artifacts.join("digest"), "instrumented test").unwrap();
        fs::write(
            workspace.join(".supercov/frontend-cache.json"),
            "{\"schemaVersion\":2}",
        )
        .unwrap();
        prune_cached_workspace_sources(&root, &lock).unwrap();
        assert!(!workspace.join("tests/e2e/app.spec.js").exists());
        assert_eq!(
            fs::read_to_string(artifacts.join("digest")).unwrap(),
            "instrumented test"
        );
        lock.release().unwrap();
        fs::remove_dir_all(root).unwrap();
    }

    #[test]
    fn ordinary_copy_fallback_preserves_workspace_semantics() {
        let root = project();
        let mut lock = ProjectLock::acquire(&root, "run", "now").unwrap();
        let mut operations = OrdinaryCopyOperations;
        let workspace =
            prepare_cached_workspace_with_operations(&root, &lock, &[], &mut operations).unwrap();
        assert_eq!(
            fs::read_to_string(workspace.join("src/index.js")).unwrap(),
            "one"
        );
        assert_eq!(
            fs::read_to_string(root.join("src/index.js")).unwrap(),
            "one"
        );
        assert!(transaction_debris(&root).is_empty());
        lock.release().unwrap();
        fs::remove_dir_all(root).unwrap();
    }

    #[test]
    fn enospc_during_copy_preserves_the_complete_generation() {
        let root = project();
        let mut lock = ProjectLock::acquire(&root, "run", "now").unwrap();
        let workspace = prepare_cached_workspace(&root, &lock, &[]).unwrap();
        fs::write(workspace.join("generation"), "complete").unwrap();
        fs::write(root.join("src/index.js"), "new source").unwrap();
        let mut operations = FaultOperations {
            copy_count: 0,
            fail_copy_at: Some(1),
            rename_count: 0,
            fail_rename_at: None,
        };
        let error = prepare_cached_workspace_with_operations(&root, &lock, &[], &mut operations)
            .unwrap_err();
        assert!(matches!(
            error,
            WorkspaceError::Io { ref source, .. }
                if source.kind() == io::ErrorKind::StorageFull
        ));
        assert_eq!(
            fs::read_to_string(workspace.join("generation")).unwrap(),
            "complete"
        );
        assert_eq!(
            fs::read_to_string(workspace.join("src/index.js")).unwrap(),
            "one"
        );
        assert!(transaction_debris(&root).is_empty());
        lock.release().unwrap();
        fs::remove_dir_all(root).unwrap();
    }

    #[test]
    fn failed_publication_rename_restores_the_complete_generation() {
        let root = project();
        let mut lock = ProjectLock::acquire(&root, "run", "now").unwrap();
        let workspace = prepare_cached_workspace(&root, &lock, &[]).unwrap();
        fs::write(workspace.join("generation"), "complete").unwrap();
        fs::write(root.join("src/index.js"), "new source").unwrap();
        let mut operations = FaultOperations {
            copy_count: 0,
            fail_copy_at: None,
            rename_count: 0,
            fail_rename_at: Some(2),
        };
        let error = prepare_cached_workspace_with_operations(&root, &lock, &[], &mut operations)
            .unwrap_err();
        assert!(matches!(
            error,
            WorkspaceError::Io { ref source, .. }
                if source.kind() == io::ErrorKind::Other
        ));
        assert_eq!(
            operations.rename_count, 3,
            "prior generation was not restored"
        );
        assert_eq!(
            fs::read_to_string(workspace.join("generation")).unwrap(),
            "complete"
        );
        assert_eq!(
            fs::read_to_string(workspace.join("src/index.js")).unwrap(),
            "one"
        );
        assert!(transaction_debris(&root).is_empty());
        lock.release().unwrap();
        fs::remove_dir_all(root).unwrap();
    }

    #[cfg(unix)]
    #[test]
    fn relocates_internal_links_and_rejects_external_links() {
        use std::os::unix::fs::symlink;

        let root = project();
        fs::create_dir_all(root.join("shared")).unwrap();
        fs::write(root.join("shared/value"), "inside").unwrap();
        symlink("../shared/value", root.join("src/value-link")).unwrap();
        let external = project();
        fs::write(external.join("outside"), "outside").unwrap();
        let mut lock = ProjectLock::acquire(&root, "run", "now").unwrap();
        let workspace = prepare_cached_workspace(&root, &lock, &[]).unwrap();
        assert_eq!(
            fs::read_to_string(workspace.join("src/value-link")).unwrap(),
            "inside"
        );
        symlink(external.join("outside"), root.join("src/external-link")).unwrap();
        let refreshed = prepare_cached_workspace(&root, &lock, &[]).unwrap();
        assert!(fs::symlink_metadata(refreshed.join("src/external-link")).is_err());
        assert_eq!(
            fs::read_to_string(workspace.join("src/index.js")).unwrap(),
            "one"
        );
        lock.release().unwrap();
        fs::remove_dir_all(root).unwrap();
        fs::remove_dir_all(external).unwrap();
    }

    #[cfg(windows)]
    #[test]
    fn relocates_internal_junctions_without_symlink_privileges() {
        let root = project();
        fs::create_dir_all(root.join("shared")).unwrap();
        fs::write(root.join("shared/value"), "inside").unwrap();
        junction::create(root.join("shared"), root.join("linked-shared")).unwrap();
        let mut lock = ProjectLock::acquire(&root, "run", "now").unwrap();
        let workspace = prepare_cached_workspace(&root, &lock, &[]).unwrap();
        let isolated_link = workspace.join("linked-shared");
        assert!(junction::exists(&isolated_link).unwrap());
        assert_eq!(
            canonicalize_simplified(junction::get_target(&isolated_link).unwrap()).unwrap(),
            canonicalize_simplified(workspace.join("shared")).unwrap()
        );
        assert_eq!(
            fs::read_to_string(isolated_link.join("value")).unwrap(),
            "inside"
        );
        lock.release().unwrap();
        fs::remove_dir_all(root).unwrap();
    }

    #[cfg(windows)]
    #[test]
    fn mounts_node_modules_with_junctions_and_copies_metadata_files() {
        let root = project();
        fs::create_dir_all(root.join("node_modules/example")).unwrap();
        fs::write(root.join("node_modules/example/index.js"), "module").unwrap();
        fs::write(root.join("node_modules/.package-lock.json"), "lock").unwrap();
        let mut lock = ProjectLock::acquire(&root, "run", "now").unwrap();
        let workspace = prepare_cached_workspace(&root, &lock, &[]).unwrap();
        let package = workspace.join("node_modules/example");
        assert!(junction::exists(&package).unwrap());
        assert_eq!(
            canonicalize_simplified(junction::get_target(&package).unwrap()).unwrap(),
            canonicalize_simplified(root.join("node_modules/example")).unwrap()
        );
        let metadata = workspace.join("node_modules/.package-lock.json");
        assert!(fs::symlink_metadata(&metadata).unwrap().is_file());
        assert_eq!(fs::read_to_string(metadata).unwrap(), "lock");
        lock.release().unwrap();
        fs::remove_dir_all(root).unwrap();
    }

    #[test]
    fn recovery_restores_the_newest_previous_generation_and_discards_staging() {
        let root = project();
        let mut lock = ProjectLock::acquire(&root, "run", "now").unwrap();
        ensure_container(&root).unwrap();
        let workspace = cached_workspace_path(&root, &lock).unwrap();
        let parent = workspace.parent().unwrap();
        fs::create_dir_all(parent).unwrap();
        let previous = parent.join(format!(
            "{}old",
            transaction_prefix(&root, "previous").unwrap()
        ));
        let staging = parent.join(format!(
            "{}new",
            transaction_prefix(&root, "staging").unwrap()
        ));
        fs::create_dir_all(&previous).unwrap();
        fs::write(previous.join("complete"), "yes").unwrap();
        fs::create_dir_all(&staging).unwrap();
        let result = recover_cached_workspace(&root, &lock).unwrap();
        assert!(result.restored_previous);
        assert_eq!(result.removed_staging, 1);
        assert_eq!(
            fs::read_to_string(workspace.join("complete")).unwrap(),
            "yes"
        );
        lock.release().unwrap();
        fs::remove_dir_all(root).unwrap();
    }
}