supercov_engine/

rust_worker.rs

//! A Rust Worker a browser suite drives through `wrangler dev`.
//!
//! The crate is instrumented in the suite's copy and `worker-build` compiles
//! it to wasm, where the region runtime keeps its region in linear memory
//! behind two exports per crate: its address and a clear. Supercov points the
//! copy's wrangler configuration at an entry module that remembers every wasm
//! instance the isolate creates, and at an inspector port of its own, so the
//! suite's own `wrangler dev` command is left as written. While the suite
//! runs, a poller takes each instance's region through the inspector and
//! clears it every few milliseconds. Each take is a window, credited by time
//! to the one browser test whose span it overlaps, as for a Go or JVM server;
//! what no single test's window holds is the run's background.

use std::{
    collections::BTreeMap,
    fs,
    io::Write,
    net::{SocketAddr, TcpListener, TcpStream},
    path::{Path, PathBuf},
    sync::{
        Arc,
        atomic::{AtomicBool, Ordering},
    },
    thread::JoinHandle,
    time::{Duration, Instant, SystemTime, UNIX_EPOCH},
};

use base64::Engine as _;
use serde_json::{Value, json};

use crate::{
    integrity::{FrontendIntegrityInputs, create_explicit_run_integrity},
    lifecycle::{note_kept_evidence, remove_stored_tree_deferred},
    run_store::{RunIntegrity, RunMetadata},
    rust_project::PreparedRustProject,
    served_tests::ServedTest,
};

/// The module wrangler bundles in place of the project's own entry: it
/// installs the registry, then is the project's entry.
const ENTRY_FILE: &str = "supercov-worker-entry.mjs";
const REGISTRY_FILE: &str = "supercov-worker-registry.mjs";

/// Remember every wasm instance the isolate creates, for the inspector:
/// workerd answers no heap queries, so instances cannot be found later.
const REGISTRY: &str = r#"// Generated by Supercov: remembers this isolate's wasm instances.
const instances = (globalThis.__supercovWasm ??= []);
const Instance = WebAssembly.Instance;
function Recorded(module, imports) {
  const instance = new Instance(module, imports);
  instances.push(instance);
  return instance;
}
Recorded.prototype = Instance.prototype;
WebAssembly.Instance = Recorded;
const instantiate = WebAssembly.instantiate;
WebAssembly.instantiate = async function (source, imports) {
  const result = await instantiate.call(WebAssembly, source, imports);
  instances.push(result instanceof Instance ? result : result.instance);
  return result;
};
"#;

/// Evaluated in the Worker's context on every poll: each instrumented
/// crate's region as its header and the pages a probe touched since the
/// last take, then cleared. A region with nothing new is left out, except
/// the first time, which carries what `cfg!` said of the crate's gates.
const TAKE: &str = r#"(() => {
  const seen = (globalThis.__supercovTaken ??= new WeakSet());
  const out = [];
  for (const instance of globalThis.__supercovWasm ?? []) {
    const exports = instance.exports;
    for (const name of Object.keys(exports)) {
      if (!name.startsWith('__supercov_region_')) continue;
      const crate = name.slice('__supercov_region_'.length);
      const packed = BigInt.asUintN(64, BigInt(exports[name]()));
      const at = Number(packed >> 32n), length = Number(packed & 0xffffffffn);
      if (!length || !exports.memory) continue;
      const view = new DataView(exports.memory.buffer, at, length);
      const first = !seen.has(exports);
      if (!first && view.getUint32(40, true) === 0 && view.getUint32(44, true) === 0) continue;
      seen.add(exports);
      const bytes = new Uint8Array(exports.memory.buffer, at, length);
      const encode = (from, to) => {
        let text = '';
        for (let index = from; index < to; index += 0x8000)
          text += String.fromCharCode.apply(null, bytes.subarray(index, Math.min(to, index + 0x8000)));
        return btoa(text);
      };
      const bodyAt = view.getUint32(32, true);
      const segments = [[0, encode(0, bodyAt)]];
      for (let page = 0; bodyAt + page * 4096 < length; page++) {
        if (bytes[128 + (page >> 3)] & (1 << (page & 7))) {
          const from = bodyAt + page * 4096;
          segments.push([from, encode(from, Math.min(length, from + 4096))]);
        }
      }
      out.push([crate, length, segments]);
      exports['__supercov_clear_' + crate]();
    }
  }
  return out;
})()"#;

/// How often the poller takes the regions. A window shorter than the gap
/// between two tests is what lets each be credited its own.
const POLL_INTERVAL: Duration = Duration::from_millis(5);

/// A wrangler configuration Supercov can point at its entry.
#[derive(Debug, Clone, PartialEq, Eq)]
struct WorkerConfig {
    path: PathBuf,
    main: String,
    inspector_port: Option<u16>,
}

/// The wrangler configurations under `root` that build a Rust Worker: a
/// `[build] command` running `worker-build`, beside the crate's Cargo.toml.
///
/// A plain walk: a `.gitignore` above the project (a monorepo's, or the
/// checkout a fixture sits in) must not hide the project itself.
fn worker_configs(root: &Path) -> Vec<PathBuf> {
    const SKIPPED: &[&str] = &[
        "node_modules",
        "target",
        "build",
        "dist",
        ".git",
        ".supercov",
        ".wrangler",
    ];
    let mut found = Vec::new();
    let mut directories = vec![(root.to_path_buf(), 0)];
    while let Some((directory, depth)) = directories.pop() {
        let Ok(entries) = fs::read_dir(&directory) else {
            continue;
        };
        for entry in entries.flatten() {
            let path = entry.path();
            let name = entry.file_name();
            let Some(name) = name.to_str() else {
                continue;
            };
            let Ok(kind) = entry.file_type() else {
                continue;
            };
            if kind.is_dir() {
                if depth < 6 && !SKIPPED.contains(&name) {
                    directories.push((path, depth + 1));
                }
                continue;
            }
            if !kind.is_file()
                || !matches!(name, "wrangler.toml" | "wrangler.json" | "wrangler.jsonc")
            {
                continue;
            }
            let builds_rust = read_config(&path).is_some_and(|(_, document)| {
                build_command(&document).is_some_and(|command| command.contains("worker-build"))
            });
            if builds_rust && path.with_file_name("Cargo.toml").is_file() {
                found.push(path);
            }
        }
    }
    found.sort();
    found
}

/// A configuration as JSON whatever it was written in.
fn read_config(path: &Path) -> Option<(String, Value)> {
    let text = fs::read_to_string(path).ok()?;
    let document = if path
        .extension()
        .is_some_and(|extension| extension == "toml")
    {
        let table = text.parse::<toml::Table>().ok()?;
        serde_json::to_value(table).ok()?
    } else {
        serde_json::from_str(&strip_jsonc(&text)).ok()?
    };
    Some((text, document))
}

fn build_command(document: &Value) -> Option<String> {
    match document.get("build")?.get("command")? {
        Value::String(command) => Some(command.clone()),
        Value::Array(parts) => Some(
            parts
                .iter()
                .filter_map(Value::as_str)
                .collect::<Vec<_>>()
                .join(" "),
        ),
        _ => None,
    }
}

fn parse_config(path: &Path) -> Option<WorkerConfig> {
    let (_, document) = read_config(path)?;
    Some(WorkerConfig {
        path: path.to_owned(),
        main: document.get("main")?.as_str()?.to_owned(),
        inspector_port: document
            .get("dev")
            .and_then(|dev| dev.get("inspector_port"))
            .and_then(Value::as_u64)
            .and_then(|port| u16::try_from(port).ok()),
    })
}

/// JSON with comments and trailing commas, as wrangler.jsonc allows, made
/// plain JSON. Strings are copied as they are.
fn strip_jsonc(text: &str) -> String {
    let mut out = String::with_capacity(text.len());
    let mut characters = text.chars().peekable();
    while let Some(character) = characters.next() {
        match character {
            '"' => {
                out.push('"');
                while let Some(inner) = characters.next() {
                    out.push(inner);
                    match inner {
                        '\\' => {
                            if let Some(escaped) = characters.next() {
                                out.push(escaped);
                            }
                        }
                        '"' => break,
                        _ => {}
                    }
                }
            }
            '/' if characters.peek() == Some(&'/') => {
                for inner in characters.by_ref() {
                    if inner == '\n' {
                        out.push('\n');
                        break;
                    }
                }
            }
            '/' if characters.peek() == Some(&'*') => {
                characters.next();
                let mut previous = '\0';
                for inner in characters.by_ref() {
                    if previous == '*' && inner == '/' {
                        break;
                    }
                    previous = inner;
                }
            }
            _ => out.push(character),
        }
    }
    // A comma before a closing bracket, past any whitespace.
    let mut cleaned = String::with_capacity(out.len());
    let mut in_string = false;
    let mut escaped = false;
    let bytes = out.as_bytes();
    for (index, character) in out.char_indices() {
        if in_string {
            cleaned.push(character);
            if escaped {
                escaped = false;
            } else if character == '\\' {
                escaped = true;
            } else if character == '"' {
                in_string = false;
            }
            continue;
        }
        if character == '"' {
            in_string = true;
        } else if character == ',' {
            let next = bytes[index + 1..]
                .iter()
                .find(|byte| !byte.is_ascii_whitespace());
            if matches!(next, Some(b'}' | b']')) {
                continue;
            }
        }
        cleaned.push(character);
    }
    cleaned
}

/// Point the copy's configuration at the entry module and, given one, the
/// inspector port. The rest of the configuration is the project's.
fn rewrite_config(config: &WorkerConfig, entry: &str, port: Option<u16>) -> Result<(), String> {
    let path = &config.path;
    let text = fs::read_to_string(path).map_err(|error| format!("{}: {error}", path.display()))?;
    let rewritten = if path
        .extension()
        .is_some_and(|extension| extension == "toml")
    {
        let mut document = text
            .parse::<toml_edit::DocumentMut>()
            .map_err(|error| format!("{}: {error}", path.display()))?;
        document["main"] = toml_edit::value(entry);
        if let (None, Some(port)) = (config.inspector_port, port) {
            if !document.contains_key("dev") {
                document["dev"] = toml_edit::table();
            }
            document["dev"]["inspector_port"] = toml_edit::value(i64::from(port));
        }
        document.to_string()
    } else {
        let mut document: Value = serde_json::from_str(&strip_jsonc(&text))
            .map_err(|error| format!("{}: {error}", path.display()))?;
        document["main"] = json!(entry);
        if let (None, Some(port)) = (config.inspector_port, port) {
            if !document.get("dev").is_some_and(Value::is_object) {
                document["dev"] = json!({});
            }
            document["dev"]["inspector_port"] = json!(port);
        }
        serde_json::to_string_pretty(&document).map_err(|error| error.to_string())?
    };
    fs::write(path, rewritten).map_err(|error| format!("{}: {error}", path.display()))
}

/// Write the registry and Supercov's entry beside a configuration and point
/// the configuration at it. A configuration that already names the entry --
/// a copy kept from an earlier run -- is left as it is: the entry would
/// otherwise import itself.
fn point_at_entry(config: &WorkerConfig, port: Option<u16>) -> Result<(), String> {
    if config.main.trim_start_matches("./") == ENTRY_FILE {
        return Ok(());
    }
    let directory = config.path.parent().unwrap_or(Path::new("."));
    fs::write(directory.join(REGISTRY_FILE), REGISTRY).map_err(|error| error.to_string())?;
    let main = config.main.trim_start_matches("./");
    let entry = format!(
        "// Generated by Supercov: the project's entry, with its wasm instances remembered.\nimport './{REGISTRY_FILE}';\nexport * from './{main}';\nexport {{ default }} from './{main}';\n"
    );
    fs::write(directory.join(ENTRY_FILE), entry).map_err(|error| error.to_string())?;
    rewrite_config(config, ENTRY_FILE, port)
}

/// A port nothing listens on now.
fn free_port() -> Result<u16, String> {
    let listener = TcpListener::bind("127.0.0.1:0").map_err(|error| error.to_string())?;
    listener
        .local_addr()
        .map(|address| address.port())
        .map_err(|error| error.to_string())
}

/// The cfg values of wasm32-unknown-unknown, as the crate's own toolchain
/// reports them: what decides which items the target leaves out.
pub(crate) fn wasm_target_cfg(
    crate_directory: &Path,
) -> Option<crate::rust_instrumenter::TargetCfg> {
    let output = std::process::Command::new("rustc")
        .args(["--print", "cfg", "--target", "wasm32-unknown-unknown"])
        .current_dir(crate_directory)
        .output()
        .ok()?;
    if !output.status.success() {
        return None;
    }
    let values = String::from_utf8_lossy(&output.stdout)
        .lines()
        .filter_map(|line| line.parse::<cargo_platform::Cfg>().ok())
        .collect::<Vec<_>>();
    Some(crate::rust_instrumenter::TargetCfg::new(&values))
}

fn now_ms() -> i64 {
    SystemTime::now()
        .duration_since(UNIX_EPOCH)
        .map(|elapsed| elapsed.as_millis() as i64)
        .unwrap_or(0)
}

/// Takes the regions of one `wrangler dev` through its inspector until told
/// to stop, appending a window per take with anything in it.
struct Poller {
    stop: Arc<AtomicBool>,
    thread: Option<JoinHandle<()>>,
}

impl Poller {
    fn start(port: u16, windows: PathBuf) -> Self {
        let stop = Arc::new(AtomicBool::new(false));
        let flag = Arc::clone(&stop);
        let thread = std::thread::Builder::new()
            .name("supercov-worker-poller".into())
            .spawn(move || poll(port, &windows, &flag))
            .ok();
        Self { stop, thread }
    }

    /// Take once more if wrangler is still there, then stop.
    fn finish(mut self) {
        self.stop.store(true, Ordering::Release);
        if let Some(thread) = self.thread.take() {
            let _ = thread.join();
        }
    }
}

impl Drop for Poller {
    fn drop(&mut self) {
        self.stop.store(true, Ordering::Release);
    }
}

fn poll(port: u16, windows: &Path, stop: &AtomicBool) {
    let Ok(mut file) = fs::OpenOptions::new()
        .create(true)
        .append(true)
        .open(windows)
    else {
        return;
    };
    let address = SocketAddr::from(([127, 0, 0, 1], port));
    // wrangler starts when the suite asks for it, and restarts the Worker on
    // a rebuild: connect, and connect again, until the run is over.
    while !stop.load(Ordering::Acquire) {
        let Ok(stream) = TcpStream::connect_timeout(&address, Duration::from_millis(200)) else {
            std::thread::sleep(Duration::from_millis(100));
            continue;
        };
        let _ = stream.set_nodelay(true);
        let _ = stream.set_read_timeout(Some(Duration::from_millis(20)));
        let Ok((mut socket, _)) =
            tungstenite::client::client(format!("ws://127.0.0.1:{port}/ws"), stream)
        else {
            std::thread::sleep(Duration::from_millis(100));
            continue;
        };
        session(&mut socket, &mut file, stop);
        let _ = socket.close(None);
    }
}

/// One inspector connection: enable the runtime, learn the Worker's
/// execution context, and take its regions on the clock. Returns when the
/// connection ends, or after the last take once the run is over.
fn session(socket: &mut tungstenite::WebSocket<TcpStream>, file: &mut fs::File, stop: &AtomicBool) {
    use tungstenite::{Error, Message};
    let send = |socket: &mut tungstenite::WebSocket<TcpStream>, message: Value| {
        socket
            .send(Message::text(message.to_string()))
            .map_err(|_| ())
    };
    if send(socket, json!({"id": 1, "method": "Runtime.enable"})).is_err() {
        return;
    }
    let mut contexts = Vec::<i64>::new();
    let mut next_id = 2_u64;
    // The take in flight: its id, when it was sent, and where its window
    // starts.
    let mut awaiting: Option<(u64, Instant)> = None;
    let mut window_start = now_ms();
    let mut last_take = Instant::now() - POLL_INTERVAL;
    let mut final_sent = false;
    loop {
        let stopping = stop.load(Ordering::Acquire);
        if stopping && awaiting.is_none() && (final_sent || contexts.is_empty()) {
            return;
        }
        if awaiting.is_none()
            && let Some(context) = contexts.last()
            && (stopping || last_take.elapsed() >= POLL_INTERVAL)
        {
            let id = next_id;
            next_id += 1;
            let request = json!({
                "id": id,
                "method": "Runtime.evaluate",
                "params": {"expression": TAKE, "contextId": context, "returnByValue": true},
            });
            if send(socket, request).is_err() {
                return;
            }
            awaiting = Some((id, Instant::now()));
            last_take = Instant::now();
            final_sent = stopping;
        }
        match socket.read() {
            Ok(Message::Text(text)) => {
                let Ok(message) = serde_json::from_str::<Value>(text.as_str()) else {
                    continue;
                };
                match message.get("method").and_then(Value::as_str) {
                    Some("Runtime.executionContextCreated") => {
                        if let Some(id) = message["params"]["context"]["id"].as_i64() {
                            contexts.push(id);
                        }
                    }
                    Some("Runtime.executionContextDestroyed") => {
                        if let Some(id) = message["params"]["executionContextId"].as_i64() {
                            contexts.retain(|context| *context != id);
                        }
                    }
                    Some("Runtime.executionContextsCleared") => contexts.clear(),
                    _ => {}
                }
                if let Some((id, _)) = awaiting
                    && message.get("id").and_then(Value::as_u64) == Some(id)
                {
                    awaiting = None;
                    let end = now_ms();
                    let regions = &message["result"]["result"]["value"];
                    if regions
                        .as_array()
                        .is_some_and(|regions| !regions.is_empty())
                    {
                        let line = json!({"start": window_start, "end": end, "regions": regions});
                        let _ = writeln!(file, "{line}");
                    }
                    window_start = end;
                }
            }
            Ok(Message::Close(_)) => return,
            Ok(_) => {}
            Err(Error::Io(error))
                if matches!(
                    error.kind(),
                    std::io::ErrorKind::WouldBlock | std::io::ErrorKind::TimedOut
                ) => {}
            Err(_) => return,
        }
        // A take the Worker never answered (it was replaced mid-evaluation)
        // is given up; the next one covers the same span.
        if awaiting.is_some_and(|(_, sent)| sent.elapsed() > Duration::from_secs(5)) {
            awaiting = None;
        }
    }
}

/// One take as the poller appended it.
struct Window {
    start: i64,
    end: i64,
    /// Each region, rebuilt: crate key and bytes.
    regions: Vec<(String, Vec<u8>)>,
}

fn read_windows(directory: &Path) -> Result<Vec<Window>, String> {
    let mut files = fs::read_dir(directory)
        .map(|entries| {
            entries
                .filter_map(Result::ok)
                .map(|entry| entry.path())
                .filter(|path| {
                    path.extension()
                        .is_some_and(|extension| extension == "windows")
                })
                .collect::<Vec<_>>()
        })
        .unwrap_or_default();
    files.sort();
    let mut windows = Vec::new();
    for file in &files {
        let text =
            fs::read_to_string(file).map_err(|error| format!("{}: {error}", file.display()))?;
        // A line cut short by Supercov being stopped mid-write is skipped.
        for line in text.lines() {
            let Ok(value) = serde_json::from_str::<Value>(line) else {
                continue;
            };
            if let Some(window) = window_from(&value) {
                windows.push(window);
            }
        }
    }
    Ok(windows)
}

fn window_from(value: &Value) -> Option<Window> {
    let mut regions = Vec::new();
    for region in value.get("regions")?.as_array()? {
        let crate_key = region.get(0)?.as_str()?.to_owned();
        let length = usize::try_from(region.get(1)?.as_u64()?).ok()?;
        let mut bytes = vec![0_u8; length];
        for segment in region.get(2)?.as_array()? {
            let offset = usize::try_from(segment.get(0)?.as_u64()?).ok()?;
            let data = base64::engine::general_purpose::STANDARD
                .decode(segment.get(1)?.as_str()?)
                .ok()?;
            bytes
                .get_mut(offset..offset + data.len())?
                .copy_from_slice(&data);
        }
        regions.push((crate_key, bytes));
    }
    Some(Window {
        start: value.get("start")?.as_i64()?,
        end: value.get("end")?.as_i64()?,
        regions,
    })
}

/// The Rust Workers of one Cargo workspace a browser suite may start.
pub struct RustWorkerServer {
    project: PreparedRustProject,
    pollers: Vec<Poller>,
    integrity: RunIntegrity,
    source_inputs: crate::source_manifest::Inputs,
    source_roots: Option<Vec<String>>,
    /// `.supercov/work/<run>/rust-worker`, holding `evidence/`.
    directory: PathBuf,
}

impl RustWorkerServer {
    /// Whether `root` holds a Rust Worker a suite could start.
    pub fn present(root: &Path) -> bool {
        !worker_configs(root).is_empty()
    }

    /// Instrument the copy's Worker crates and point their configurations at
    /// Supercov's entry, one server per Cargo workspace. Empty when the
    /// project has no Rust Worker.
    pub fn prepare(
        root: &Path,
        workspace: &Path,
        run_id: &str,
        command: &[String],
        diagnostics: &mut dyn Write,
    ) -> Result<Vec<Self>, String> {
        let configs = worker_configs(workspace);
        // Configurations by the Cargo workspace their crate belongs to: a
        // workspace is instrumented once, whatever number of Workers it holds.
        let mut by_workspace = BTreeMap::<PathBuf, Vec<WorkerConfig>>::new();
        for path in configs {
            let Some(config) = parse_config(&path) else {
                writeln!(
                    diagnostics,
                    "[supercov] {} names no `main`; its Worker is not measured",
                    path.display()
                )
                .map_err(|error| error.to_string())?;
                continue;
            };
            let crate_directory = path.parent().unwrap_or(workspace).to_owned();
            let cargo_root = cargo_workspace_root(&crate_directory).unwrap_or(crate_directory);
            by_workspace.entry(cargo_root).or_default().push(config);
        }
        let mut servers = Vec::new();
        for (index, (cargo_root, configs)) in by_workspace.into_iter().enumerate() {
            let Ok(relative) = cargo_root.strip_prefix(workspace) else {
                continue;
            };
            let source_roots = crate::source_discovery::configured_source_roots(
                &std::env::vars().collect::<BTreeMap<_, _>>(),
            );
            // The project's inputs, as a later query recomputes them to judge
            // whether the run is current.
            let mut integrity_inputs = crate::rust_run::collect_integrity_inputs(root, command)?;
            crate::source_discovery::fold_roots_into_configuration(
                &mut integrity_inputs.execution_configuration,
                source_roots.as_deref(),
            );
            let source_inputs =
                crate::source_capture::capture(root, "rust", integrity_inputs.assertion_paths())?;
            let integrity = create_explicit_run_integrity(
                root,
                &integrity_inputs,
                &FrontendIntegrityInputs::embedded_rust(),
            )
            .map_err(|error| error.to_string())?;
            let cfg = wasm_target_cfg(&cargo_root);
            let mut project =
                crate::rust_project::prepare_rust_project_for(&cargo_root, None, cfg.as_ref())
                    .map_err(|error| error.to_string())?;
            crate::rust_wasm_evidence::root_paths(&mut project, relative);
            let directory = root
                .join(".supercov/work")
                .join(run_id)
                .join(format!("rust-worker-{index}"));
            let evidence = directory.join("evidence");
            fs::create_dir_all(&evidence).map_err(|error| error.to_string())?;
            let mut pollers = Vec::new();
            for (number, config) in configs.iter().enumerate() {
                let port = match config.inspector_port {
                    Some(port) => port,
                    None => free_port()?,
                };
                point_at_entry(config, Some(port))?;
                pollers.push(Poller::start(
                    port,
                    evidence.join(format!("{number:04}.windows")),
                ));
            }
            servers.push(Self {
                project,
                pollers,
                integrity,
                source_inputs,
                source_roots,
                directory,
            });
        }
        Ok(servers)
    }

    pub fn source_files(&self) -> usize {
        self.project.source_files.len()
    }

    /// The source this Worker was instrumented from: an image built from an
    /// earlier copy holds probes placed in other source.
    pub fn source_fingerprint(&self) -> String {
        self.integrity.fingerprint.source.clone()
    }

    /// What the suite's environment needs: a lint on the rewritten source
    /// must not fail the Worker's build under the crate's own lint policy.
    ///
    /// Capped at `warn`, not `allow` as for `cargo test`: Cargo learns which
    /// crate types a target supports from rustc's "dropping unsupported crate
    /// type" warnings, and wasm32 drops `dylib` and `proc-macro`, so with
    /// every lint allowed it cannot read the answer and stops the build. The
    /// probes' parentheses, the one lint they raise, are allowed by name.
    pub fn environment(&self) -> BTreeMap<String, String> {
        let mut rustflags = std::env::var("RUSTFLAGS").unwrap_or_default();
        if !rustflags.is_empty() {
            rustflags.push(' ');
        }
        rustflags.push_str("--cap-lints=warn -A unused_parens");
        BTreeMap::from([("RUSTFLAGS".into(), rustflags)])
    }

    /// Publish what the Workers ran as a Rust run of its own: each window
    /// credited, by time, to the one browser test whose span it overlaps;
    /// what no single test's does is the run's background. None when no
    /// Worker ran or no browser test reported.
    pub fn publish(
        mut self,
        root: &Path,
        _evidence_run_id: &str,
        metadata: RunMetadata,
        served: &[ServedTest],
        diagnostics: &mut dyn Write,
    ) -> Result<Option<(PathBuf, RunMetadata)>, String> {
        for poller in std::mem::take(&mut self.pollers) {
            poller.finish();
        }
        let evidence_directory = self.directory.join("evidence");
        let result = read_windows(&evidence_directory).and_then(|windows| {
            crate::rust_wasm_evidence::publish(
                root,
                crate::rust_wasm_evidence::WasmRun {
                    language: "Rust Worker",
                    archive: "rust-worker-evidence.raw.gz",
                    project: &self.project,
                    integrity: &self.integrity,
                    source_inputs: &self.source_inputs,
                    source_roots: self.source_roots.clone(),
                },
                windows
                    .into_iter()
                    .map(|window| crate::rust_wasm_evidence::WasmRecord {
                        owner: crate::rust_wasm_evidence::Owner::Window(window.start, window.end),
                        regions: window.regions,
                    })
                    .collect(),
                metadata,
                served,
                diagnostics,
            )
        });
        let result = result.map_err(|error| {
            let run = self
                .directory
                .parent()
                .and_then(Path::file_name)
                .map(|name| name.to_string_lossy().into_owned())
                .unwrap_or_default();
            note_kept_evidence(
                root,
                &evidence_directory,
                &format!("{run}-rust-worker"),
                error,
            )
        });
        let _ = remove_stored_tree_deferred(root, &self.directory);
        result
    }
}

/// The program Supercov's `wrangler` stands in for, named in the test
/// processes' environment.
pub const REAL_WRANGLER: &str = "SUPERCOV_WRANGLER";
/// The hidden commands the stand-in and its poller run as.
pub const WRANGLER_COMMAND: &str = "__wrangler";
pub const WRANGLER_POLL_COMMAND: &str = "__wrangler-poll";

/// The Rust Workers a `cargo test` suite starts itself, with `wrangler dev`.
///
/// The crate is instrumented with the rest of the workspace, and `wrangler
/// dev` builds it in the copy as it builds it anywhere. What it ran is read
/// by a `wrangler` of Supercov's, first on the test processes' `PATH`: it
/// starts a poller on an inspector port of its own and then becomes the real
/// `wrangler`, so the test keeps the process and the signals it expects. The
/// poller writes what it takes into the evidence directory of the test that
/// started the Worker, and the run reads it there as that test's own: exact,
/// not credited by time.
pub struct CargoTestWorkers {
    environment: Vec<(String, std::ffi::OsString)>,
}

impl CargoTestWorkers {
    /// Point the copy's Worker configurations at Supercov's entry and put the
    /// stand-in in `tools`. None where the workspace has no Rust Worker, or
    /// where no `wrangler` is installed: a test that looks for one must find
    /// none, as it would without Supercov.
    pub fn prepare(workspace: &Path, tools: &Path) -> Result<Option<Self>, String> {
        let configs = worker_configs(workspace)
            .iter()
            .filter_map(|path| parse_config(path))
            .collect::<Vec<_>>();
        if configs.is_empty() {
            return Ok(None);
        }
        let path = std::env::var_os("PATH").unwrap_or_default();
        let Some(real) = on_path(&path, "wrangler") else {
            return Ok(None);
        };
        for config in &configs {
            point_at_entry(config, None)?;
        }
        let supercov = std::env::current_exe().map_err(|error| error.to_string())?;
        fs::create_dir_all(tools).map_err(|error| format!("{}: {error}", tools.display()))?;
        write_stand_in(tools, &supercov)?;
        let mut directories = vec![tools.to_owned()];
        directories.extend(std::env::split_paths(&path));
        let joined = std::env::join_paths(directories).map_err(|error| error.to_string())?;
        Ok(Some(Self {
            environment: vec![
                ("PATH".into(), joined),
                (REAL_WRANGLER.into(), real.into_os_string()),
            ],
        }))
    }

    /// What each test process is started with.
    pub fn environment(&self) -> impl Iterator<Item = (&str, &std::ffi::OsStr)> {
        self.environment
            .iter()
            .map(|(name, value)| (name.as_str(), value.as_os_str()))
    }
}

/// A program on `path`: a file of that name, or on Windows one with a
/// PATHEXT extension.
fn on_path(path: &std::ffi::OsStr, name: &str) -> Option<PathBuf> {
    let names = if cfg!(windows) {
        vec![
            format!("{name}.cmd"),
            format!("{name}.exe"),
            name.to_owned(),
        ]
    } else {
        vec![name.to_owned()]
    };
    std::env::split_paths(path).find_map(|directory| {
        names
            .iter()
            .map(|name| directory.join(name))
            .find(|candidate| fs::metadata(candidate).is_ok_and(|metadata| metadata.is_file()))
    })
}

fn write_stand_in(tools: &Path, supercov: &Path) -> Result<(), String> {
    #[cfg(unix)]
    {
        use std::os::unix::fs::PermissionsExt;
        let quoted = supercov.to_string_lossy().replace('\'', r"'\''");
        let script = tools.join("wrangler");
        fs::write(
            &script,
            format!("#!/bin/sh\nexec '{quoted}' {WRANGLER_COMMAND} \"$@\"\n"),
        )
        .map_err(|error| format!("{}: {error}", script.display()))?;
        fs::set_permissions(&script, fs::Permissions::from_mode(0o755))
            .map_err(|error| format!("{}: {error}", script.display()))?;
    }
    #[cfg(windows)]
    {
        let script = tools.join("wrangler.cmd");
        fs::write(
            &script,
            format!("@\"{}\" {WRANGLER_COMMAND} %*\r\n", supercov.display()),
        )
        .map_err(|error| format!("{}: {error}", script.display()))?;
    }
    Ok(())
}

/// `wrangler dev`, with options before or after the command: wrangler's
/// global options that take a value (`--cwd worker dev`) are read past.
fn starts_dev(arguments: &[std::ffi::OsString]) -> bool {
    const VALUED: &[&str] = &["--cwd", "-c", "--config", "-e", "--env", "--env-file"];
    let mut arguments = arguments.iter().map(|argument| argument.to_string_lossy());
    while let Some(argument) = arguments.next() {
        if VALUED.contains(&argument.as_ref()) {
            arguments.next();
        } else if !argument.starts_with('-') {
            return argument == "dev";
        }
    }
    false
}

/// The inspector port the arguments name, as `--inspector-port N` or
/// `--inspector-port=N`.
fn named_inspector_port(arguments: &[std::ffi::OsString]) -> Option<u16> {
    let mut arguments = arguments.iter().map(|argument| argument.to_string_lossy());
    while let Some(argument) = arguments.next() {
        if argument == "--inspector-port" {
            return arguments.next()?.parse().ok();
        }
        if let Some(port) = argument.strip_prefix("--inspector-port=") {
            return port.parse().ok();
        }
    }
    None
}

/// Supercov's `wrangler`: under a test, `wrangler dev` gets an inspector
/// port and a poller writing into the test's evidence; then this process
/// becomes the real `wrangler`. The build it runs caps lints at `warn`, as
/// for a Worker under a browser suite.
pub fn launch_wrangler(arguments: Vec<std::ffi::OsString>) -> i32 {
    let Some(real) = std::env::var_os(REAL_WRANGLER) else {
        eprintln!("[supercov] {WRANGLER_COMMAND} runs only as the wrangler of a measured test");
        return 127;
    };
    let mut arguments = arguments;
    let evidence = std::env::var_os("SUPERCOV_RUST_EVIDENCE_DIR").map(PathBuf::from);
    if starts_dev(&arguments)
        && let Some(evidence) = evidence
    {
        let port = match named_inspector_port(&arguments) {
            Some(port) => Some(port),
            None => free_port().ok().inspect(|port| {
                arguments.push("--inspector-port".into());
                arguments.push(port.to_string().into());
            }),
        };
        if let (Some(port), Ok(supercov)) = (port, std::env::current_exe()) {
            let windows = evidence.join(format!("worker-{}.windows", std::process::id()));
            let _ = std::process::Command::new(supercov)
                .arg(WRANGLER_POLL_COMMAND)
                .arg(std::process::id().to_string())
                .arg(port.to_string())
                .arg(&windows)
                .stdin(std::process::Stdio::null())
                .stdout(std::process::Stdio::null())
                .stderr(std::process::Stdio::null())
                .spawn();
        }
    }
    let mut rustflags = std::env::var("RUSTFLAGS").unwrap_or_default();
    if !rustflags.is_empty() {
        rustflags.push(' ');
    }
    rustflags.push_str("--cap-lints=warn -A unused_parens");
    let mut command = std::process::Command::new(&real);
    command.args(&arguments).env("RUSTFLAGS", rustflags);
    #[cfg(unix)]
    {
        use std::os::unix::process::CommandExt;
        let error = command.exec();
        eprintln!(
            "[supercov] could not start {}: {error}",
            Path::new(&real).display()
        );
        126
    }
    #[cfg(not(unix))]
    {
        match command.status() {
            Ok(status) => status.code().unwrap_or(1),
            Err(error) => {
                eprintln!(
                    "[supercov] could not start {}: {error}",
                    Path::new(&real).display()
                );
                126
            }
        }
    }
}

/// The poller of one `wrangler dev` a test started: takes through `port`
/// while process `pid` -- that wrangler -- lives, appending to a `.part`
/// file it renames to `windows` when done, so the run knows it is whole.
pub fn poll_wrangler(arguments: Vec<std::ffi::OsString>) -> i32 {
    let [pid, port, windows] = arguments.as_slice() else {
        return 2;
    };
    let (Ok(pid), Ok(port)) = (
        pid.to_string_lossy().parse::<u32>(),
        port.to_string_lossy().parse::<u16>(),
    ) else {
        return 2;
    };
    let windows = PathBuf::from(windows);
    let part = partial(&windows);
    let poller = Poller::start(port, part.clone());
    while process_alive(pid) {
        std::thread::sleep(Duration::from_millis(20));
    }
    poller.finish();
    if fs::symlink_metadata(&part).is_ok() {
        let _ = fs::rename(&part, &windows);
    }
    0
}

fn partial(windows: &Path) -> PathBuf {
    let mut name = windows.as_os_str().to_owned();
    name.push(".part");
    PathBuf::from(name)
}

#[cfg(unix)]
fn process_alive(pid: u32) -> bool {
    let Ok(pid) = libc::pid_t::try_from(pid) else {
        return false;
    };
    // SAFETY: signal 0 only asks whether the process exists.
    let answered = unsafe { libc::kill(pid, 0) };
    answered == 0 || std::io::Error::last_os_error().raw_os_error() == Some(libc::EPERM)
}

#[cfg(not(unix))]
fn process_alive(_pid: u32) -> bool {
    false
}

/// How long the run waits for a poller to finish after its test: the poller
/// notices wrangler is gone within a few polls.
const POLLER_SETTLE: Duration = Duration::from_secs(3);

/// The Worker takes in a test's evidence directory, written out as region
/// files the run reads as it reads a native process's; and the crates they
/// came from, which `wrangler dev` built though the run's own build did not.
/// A take whose region names another instrumentation is not this run's.
pub fn materialize_worker_windows(
    directory: &Path,
    token: &str,
) -> Result<std::collections::BTreeSet<String>, String> {
    let listed = |extension: &str| -> Vec<PathBuf> {
        let mut files = fs::read_dir(directory)
            .map(|entries| {
                entries
                    .filter_map(Result::ok)
                    .map(|entry| entry.path())
                    .filter(|path| {
                        path.file_name()
                            .and_then(|name| name.to_str())
                            .is_some_and(|name| {
                                name.starts_with("worker-") && name.ends_with(extension)
                            })
                    })
                    .collect::<Vec<_>>()
            })
            .unwrap_or_default();
        files.sort();
        files
    };
    let started = Instant::now();
    while !listed(".windows.part").is_empty() && started.elapsed() < POLLER_SETTLE {
        std::thread::sleep(Duration::from_millis(20));
    }
    // A poller still writing (its wrangler outlived the test) has given what
    // it has so far: every line it wrote is whole.
    let files = listed(".windows")
        .into_iter()
        .chain(listed(".windows.part"))
        .collect::<Vec<_>>();
    let mut crates = std::collections::BTreeSet::new();
    for (number, file) in files.iter().enumerate() {
        let text =
            fs::read_to_string(file).map_err(|error| format!("{}: {error}", file.display()))?;
        for (line, value) in text
            .lines()
            .filter_map(|line| serde_json::from_str::<Value>(line).ok())
            .enumerate()
        {
            let Some(window) = window_from(&value) else {
                continue;
            };
            for (index, (crate_key, bytes)) in window.regions.into_iter().enumerate() {
                if region_token(&bytes) != Some(token) {
                    continue;
                }
                let region = directory.join(format!(
                    "{token}-worker-{number:04}-{line:06}-{index}.region"
                ));
                fs::write(&region, &bytes)
                    .map_err(|error| format!("{}: {error}", region.display()))?;
                crates.insert(crate_key);
            }
        }
        fs::remove_file(file).map_err(|error| format!("{}: {error}", file.display()))?;
    }
    Ok(crates)
}

/// The instrumentation token a region's header carries.
fn region_token(bytes: &[u8]) -> Option<&str> {
    let field = bytes.get(56..72)?;
    let end = field
        .iter()
        .position(|byte| *byte == 0)
        .unwrap_or(field.len());
    std::str::from_utf8(&field[..end]).ok()
}

/// The root of the Cargo workspace `directory`'s crate belongs to.
pub(crate) fn cargo_workspace_root(directory: &Path) -> Option<PathBuf> {
    let output = std::process::Command::new("cargo")
        .args(["locate-project", "--workspace", "--message-format", "plain"])
        .current_dir(directory)
        .output()
        .ok()?;
    if !output.status.success() {
        return None;
    }
    let manifest = PathBuf::from(String::from_utf8_lossy(&output.stdout).trim());
    manifest.parent().map(Path::to_path_buf)
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn jsonc_comments_and_trailing_commas_are_dropped_and_strings_kept() {
        let text = r#"{
  // the Worker
  "name": "a // not a comment",
  /* block */ "main": "build/worker/shim.mjs",
  "build": { "command": "worker-build --release", },
  "list": [1, 2, ],
  "quote": "say \"hi\", ]"
}"#;
        let value: Value = serde_json::from_str(&strip_jsonc(text)).unwrap();
        assert_eq!(value["name"], "a // not a comment");
        assert_eq!(value["main"], "build/worker/shim.mjs");
        assert_eq!(value["build"]["command"], "worker-build --release");
        assert_eq!(value["list"], json!([1, 2]));
        assert_eq!(value["quote"], "say \"hi\", ]");
    }

    #[test]
    fn the_stand_in_reads_dev_and_an_inspector_port_wherever_they_stand() {
        let os = |arguments: &[&str]| {
            arguments
                .iter()
                .map(std::ffi::OsString::from)
                .collect::<Vec<_>>()
        };
        assert!(starts_dev(&os(&["dev", "--local"])));
        assert!(starts_dev(&os(&["--cwd", "worker", "dev"])));
        assert!(starts_dev(&os(&[
            "-c",
            "wrangler.toml",
            "--env=staging",
            "dev"
        ])));
        assert!(starts_dev(&os(&["--local", "dev"])));
        assert!(!starts_dev(&os(&["deploy"])));
        assert!(!starts_dev(&os(&[])));
        assert_eq!(
            named_inspector_port(&os(&["dev", "--inspector-port", "9230"])),
            Some(9230)
        );
        assert_eq!(
            named_inspector_port(&os(&["dev", "--inspector-port=9231"])),
            Some(9231)
        );
        assert_eq!(named_inspector_port(&os(&["dev", "--port", "8787"])), None);
        assert_eq!(
            named_inspector_port(&os(&["dev", "--inspector-port"])),
            None
        );
    }

    /// A take becomes a region file of the test's own evidence, named by the
    /// run's token, and says which crate it came from; a take of another
    /// instrumentation, or a line cut short, is left out.
    #[test]
    fn a_take_becomes_a_region_file_of_the_test_that_started_the_worker() {
        let directory = temporary("materialize");
        let region = |token: &str| {
            let mut bytes = vec![0_u8; 4096 + 256];
            bytes[56..56 + token.len()].copy_from_slice(token.as_bytes());
            bytes[200] = 7;
            bytes
        };
        let segment =
            |bytes: &[u8]| json!([0, base64::engine::general_purpose::STANDARD.encode(bytes)]);
        let ours = region("abc123");
        let theirs = region("fff999");
        let line = json!({
            "start": 1,
            "end": 2,
            "regions": [
                ["abc123k1", ours.len(), [segment(&ours)]],
                ["fff999k2", theirs.len(), [segment(&theirs)]],
            ],
        });
        fs::write(
            directory.join("worker-41.windows"),
            format!("{line}\n{{\"start\": 3, \"end\""),
        )
        .unwrap();
        let crates = materialize_worker_windows(&directory, "abc123").unwrap();
        assert_eq!(
            crates,
            std::collections::BTreeSet::from(["abc123k1".to_owned()])
        );
        let mut names = fs::read_dir(&directory)
            .unwrap()
            .map(|entry| entry.unwrap().file_name().to_string_lossy().into_owned())
            .collect::<Vec<_>>();
        names.sort();
        assert_eq!(names, ["abc123-worker-0000-000000-0.region"]);
        assert_eq!(fs::read(directory.join(&names[0])).unwrap(), ours);
        // Read once: a second pass finds nothing new.
        assert!(
            materialize_worker_windows(&directory, "abc123")
                .unwrap()
                .is_empty()
        );
        assert_eq!(region_token(&ours), Some("abc123"));
        assert_eq!(region_token(&[0_u8; 10]), None);
        fs::remove_dir_all(&directory).unwrap();
    }

    /// A workspace with a Worker and a `wrangler` on PATH gets the stand-in,
    /// and its configuration points at Supercov's entry once: a copy kept
    /// from an earlier run is not pointed at itself. Without a `wrangler`,
    /// nothing is put first on PATH.
    #[cfg(unix)]
    #[test]
    fn the_stand_in_is_put_first_only_where_a_wrangler_is_installed() {
        let workspace = temporary("stand-in");
        fs::create_dir_all(workspace.join("worker/src")).unwrap();
        fs::write(
            workspace.join("worker/Cargo.toml"),
            "[package]\nname = \"w\"\n",
        )
        .unwrap();
        fs::write(workspace.join("worker/src/lib.rs"), "").unwrap();
        fs::write(
            workspace.join("worker/wrangler.toml"),
            "name = \"w\"\nmain = \"build/worker/shim.mjs\"\n\n[build]\ncommand = \"worker-build --release\"\n",
        )
        .unwrap();
        let bin = workspace.join("bin");
        fs::create_dir_all(&bin).unwrap();
        let real = bin.join("wrangler");
        fs::write(&real, "#!/bin/sh\n").unwrap();
        let found = on_path(bin.as_os_str(), "wrangler");
        assert_eq!(found.as_deref(), Some(real.as_path()));
        assert_eq!(on_path(std::ffi::OsStr::new(""), "wrangler"), None);
        let tools = workspace.join("tools");
        let config = parse_config(&workspace.join("worker/wrangler.toml")).unwrap();
        point_at_entry(&config, None).unwrap();
        let pointed = fs::read_to_string(workspace.join("worker/wrangler.toml")).unwrap();
        assert!(
            pointed.contains(&format!("main = \"{ENTRY_FILE}\"")),
            "{pointed}"
        );
        assert!(!pointed.contains("inspector_port"), "{pointed}");
        let entry = fs::read_to_string(workspace.join("worker").join(ENTRY_FILE)).unwrap();
        // Again, as on a kept copy: the entry still names the project's.
        point_at_entry(
            &parse_config(&workspace.join("worker/wrangler.toml")).unwrap(),
            None,
        )
        .unwrap();
        assert_eq!(
            fs::read_to_string(workspace.join("worker").join(ENTRY_FILE)).unwrap(),
            entry
        );
        assert!(entry.contains("./build/worker/shim.mjs"), "{entry}");
        fs::create_dir_all(&tools).unwrap();
        write_stand_in(&tools, Path::new("/opt/it's/supercov")).unwrap();
        let script = fs::read_to_string(tools.join("wrangler")).unwrap();
        assert_eq!(
            script,
            format!("#!/bin/sh\nexec '/opt/it'\\''s/supercov' {WRANGLER_COMMAND} \"$@\"\n")
        );
        fs::remove_dir_all(&workspace).unwrap();
    }

    fn temporary(name: &str) -> PathBuf {
        let path = std::env::temp_dir().join(format!(
            "supercov-rust-worker-{}-{}-{name}",
            std::process::id(),
            SystemTime::now()
                .duration_since(UNIX_EPOCH)
                .unwrap()
                .as_nanos()
        ));
        fs::create_dir_all(&path).unwrap();
        path
    }

    #[test]
    fn only_a_configuration_that_builds_with_worker_build_beside_a_crate_is_a_rust_worker() {
        let root = temporary("configs");
        let write = |relative: &str, text: &str| {
            let path = root.join(relative);
            fs::create_dir_all(path.parent().unwrap()).unwrap();
            fs::write(path, text).unwrap();
        };
        write(
            "worker/wrangler.toml",
            "main = \"build/worker/shim.mjs\"\n[build]\ncommand = \"cargo install -q worker-build && worker-build --release\"\n",
        );
        write("worker/Cargo.toml", "[package]\nname = \"w\"\n");
        // JavaScript Workers and a Rust one without its crate are not.
        write("js/wrangler.toml", "main = \"src/index.js\"\n");
        write(
            "orphan/wrangler.jsonc",
            "{ \"main\": \"x.mjs\", // c\n \"build\": { \"command\": \"worker-build\" } }",
        );
        write(
            "json/wrangler.jsonc",
            "{ \"main\": \"build/worker/shim.mjs\", /* c */ \"build\": { \"command\": \"worker-build --dev\", }, }",
        );
        write("json/Cargo.toml", "[package]\nname = \"j\"\n");
        write(
            "node_modules/dep/wrangler.toml",
            "main = \"m\"\n[build]\ncommand = \"worker-build\"\n",
        );
        write("node_modules/dep/Cargo.toml", "");
        let found = worker_configs(&root)
            .into_iter()
            .map(|path| path.strip_prefix(&root).unwrap().to_owned())
            .collect::<Vec<_>>();
        assert_eq!(
            found,
            [
                PathBuf::from("json/wrangler.jsonc"),
                PathBuf::from("worker/wrangler.toml")
            ]
        );
        fs::remove_dir_all(root).unwrap();
    }

    #[test]
    fn the_copy_points_at_the_entry_and_an_inspector_port_and_keeps_the_rest() {
        let root = temporary("rewrite");
        let toml_path = root.join("wrangler.toml");
        fs::write(
            &toml_path,
            "# the Worker\nname = \"grades\"\nmain = \"build/worker/shim.mjs\"\n\n[build]\ncommand = \"worker-build --release\"\n",
        )
        .unwrap();
        let config = parse_config(&toml_path).unwrap();
        rewrite_config(&config, ENTRY_FILE, Some(9444)).unwrap();
        let rewritten = fs::read_to_string(&toml_path).unwrap();
        assert!(rewritten.contains("# the Worker"), "{rewritten}");
        let (_, document) = read_config(&toml_path).unwrap();
        assert_eq!(document["main"], ENTRY_FILE);
        assert_eq!(document["dev"]["inspector_port"], 9444);
        assert_eq!(document["build"]["command"], "worker-build --release");

        // A port the author chose is theirs, and the poller uses it.
        let json_path = root.join("wrangler.jsonc");
        fs::write(
            &json_path,
            "{\n  // mine\n  \"main\": \"./build/worker/shim.mjs\",\n  \"dev\": { \"inspector_port\": 9555 },\n}\n",
        )
        .unwrap();
        let config = parse_config(&json_path).unwrap();
        assert_eq!(config.inspector_port, Some(9555));
        assert_eq!(config.main, "./build/worker/shim.mjs");
        rewrite_config(&config, ENTRY_FILE, Some(1)).unwrap();
        let (_, document) = read_config(&json_path).unwrap();
        assert_eq!(document["main"], ENTRY_FILE);
        assert_eq!(document["dev"]["inspector_port"], 9555);
        fs::remove_dir_all(root).unwrap();
    }

    #[test]
    fn a_window_is_rebuilt_from_its_header_and_touched_pages() {
        let encode = |bytes: &[u8]| base64::engine::general_purpose::STANDARD.encode(bytes);
        let value = json!({
            "start": 10,
            "end": 15,
            "regions": [["abc", 8192, [[0, encode(&[1, 2, 3])], [4096, encode(&[9; 4])]]]],
        });
        let window = window_from(&value).unwrap();
        assert_eq!((window.start, window.end), (10, 15));
        let (crate_key, bytes) = &window.regions[0];
        assert_eq!(crate_key, "abc");
        assert_eq!(bytes.len(), 8192);
        assert_eq!(&bytes[..4], &[1, 2, 3, 0]);
        assert_eq!(&bytes[4096..4101], &[9, 9, 9, 9, 0]);
        // A segment past the region is a malformed window, not a panic.
        let bad = json!({"start": 0, "end": 1, "regions": [["a", 4, [[2, encode(&[1, 2, 3])]]]]});
        assert!(window_from(&bad).is_none());
    }
}