monochromatic-git-policy-cli 0.1.3

//! What:
//!  Private,
//!  crash-safe file primitives shared by every durable record of the wrapper:
//!       exclusive no-follow creation with private modes,
//!  file and directory sync,
//!  and
//!       no-follow reads that refuse links and non-regular files.
//! Why:
//!  Lock records,
//!  transaction journals and capture records must survive a crash in a
//!      readable state,
//!  must never be written through a planted link,
//!  and must be private to
//!      the account.
//!  The incumbent's `src/trust/registry-io.ts` defines these semantics;
//!  both
//!      wrappers write and read the same files.
//!
//! In TS you'd write (pseudocode):
//! ```ts
//! await writePrivateFile({ path, bytes }); await syncDirectory(dirname(path));
//! ```

/// What:
///  `File` is an open file handle;
///  `OpenOptions` configures how one is opened.
/// Why:
///   Exclusive creation and no-follow opening need the configurable form.
///
/// In TS you'd write (pseudocode):
/// ```ts
/// import { open, constants } from 'node:fs/promises';
/// ```
use std::fs::{File, OpenOptions};
/// The trait that gives files `.read_to_end` and `.take`.
use std::io::Read;
/// The trait that gives files `.write_all`.
use std::io::Write;
/// `Path` is a borrowed filesystem path of raw bytes.
use std::path::Path;

/// Mode of every private directory the wrapper creates:
///  owner only.
pub const PRIVATE_DIRECTORY_MODE: u32 = 0o700;

/// Mode of every private file the wrapper creates:
///  owner read and write only.
pub const PRIVATE_FILE_MODE: u32 = 0o600;

/// What:
///  Why a no-follow read refused a path.
///  An `enum` is a closed set of alternatives.
/// Why:
///   A missing file is an expected answer for optional records;
///  a link,
///  a directory or an
///       over-long file is unsafe or corrupt state that recovery must name,
///  never follow.
///
/// In TS you'd write (pseudocode):
/// ```ts
/// type ReadRefusal = { kind: 'missing' } | { kind: 'not-regular' } | { kind: 'too-large' } | { kind: 'io'; error: Error };
/// ```
#[derive(Debug)]
pub enum ReadRefusal {
    /// Nothing exists at the path.
    Missing,
    /// The path names a link,
    ///  directory or other non-regular file.
    NotRegular,
    /// The file holds more bytes than the caller allows.
    TooLarge,
    /// The operating system refused the read for another reason.
    Io(
        /// The operating-system error that refused the read.
        std::io::Error,
    ),
}

/// What:
///  Open options for an exclusive,
///  no-follow,
///  private-mode creation.
///       `#[cfg(unix)]` compiles the block only on Unix-like systems.
/// Why:
///   `O_EXCL` makes a second writer fail instead of replacing the record;
///  `O_NOFOLLOW`
///       refuses a planted final link;
///  mode `0600` keeps the record private from the start.
///
/// In TS you'd write (pseudocode):
/// ```ts
/// open(path, O_CREAT | O_EXCL | O_WRONLY | O_NOFOLLOW, 0o600)
/// ```
fn exclusive_options() -> OpenOptions {
    // `mut` allows the builder to be configured step by step.
    let mut options: OpenOptions = OpenOptions::new();
    options.write(true).create_new(true);
    #[cfg(unix)]
    {
        // The trait adds `.mode` and `.custom_flags` to open options on Unix.
        /// The Unix open-option extensions,
        ///  for the mode and no-follow flags.
        use std::os::unix::fs::OpenOptionsExt;
        options
            .mode(PRIVATE_FILE_MODE)
            .custom_flags(libc::O_NOFOLLOW);
    }
    return options;
}

/// What:
///  Set a path's permission bits to `mode`,
///  on Unix only.
///       `std::io::Result<()>` is "nothing,
///  or an operating-system error".
/// Why:
///   The creation mode is filtered by the process `umask`;
///  an explicit change makes the
///       mode exactly private whatever the caller's `umask` is,
///  as the incumbent's
///       `protectPath` does.
///  Windows keeps its inherited access rules (recorded under
///       "Choices open to veto" in `doc/handover/cli-git-native-transactions.md`).
///
/// In TS you'd write (pseudocode):
/// ```ts
/// if (process.platform !== 'win32') await chmod(path, mode);
/// ```
pub fn protect_path(path: &Path, mode: u32) -> std::io::Result<()> {
    #[cfg(unix)]
    {
        // The trait adds `.from_mode` to permissions on Unix.
        /// The Unix permission extensions,
        ///  for the mode.
        use std::os::unix::fs::PermissionsExt;
        std::fs::set_permissions(path, std::fs::Permissions::from_mode(mode))?;
    }
    // `let _ = mode;` marks the argument as used where no mode exists.
    #[cfg(not(unix))]
    let _ = (path, mode);
    return Ok(());
}

/// What:
///  Write one private file exclusively,
///  refusing links,
///  then make its bytes durable.
///       `&[u8]` borrows the exact bytes.
/// Why:
///   A journal state file is created once and never rewritten,
///  so a crash leaves either
///       no file or a complete one after the caller syncs the directory.
///
/// In TS you'd write (pseudocode):
/// ```ts
/// async function writePrivateFile({ path, bytes }): Promise<void>;
/// ```
pub fn write_private_file(path: &Path, bytes: &[u8]) -> std::io::Result<()> {
    let mut file: File = exclusive_options().open(path)?;
    file.write_all(bytes)?;
    // `sync_all` flushes data and metadata, the incumbent's `handle.sync()`.
    file.sync_all()?;
    return protect_path(path, PRIVATE_FILE_MODE);
}

/// What:
///  Write one file exclusively with mode `0600`,
///  refusing links,
///  and sync its bytes,
///       without the explicit mode change.
/// Why:
///   Owner-lock candidates are written this way by the incumbent's `writeCandidate`:
///  the
///       creation mode alone,
///  then a sync before publication.
///
/// In TS you'd write (pseudocode):
/// ```ts
/// const handle = await open(path, O_CREAT | O_EXCL | O_WRONLY | O_NOFOLLOW, 0o600); await handle.writeFile(bytes); await handle.sync();
/// ```
pub fn write_exclusive_synced(path: &Path, bytes: &[u8]) -> std::io::Result<()> {
    let mut file: File = exclusive_options().open(path)?;
    file.write_all(bytes)?;
    return file.sync_all();
}

/// What:
///  Create one directory with mode `0700`;
///  its parent must exist.
/// Why:
///   Lock candidates,
///  transaction directories and stores are private to the account.
///       Only the creation mode is applied,
///  as the incumbent's `mkdir(path, { mode })`.
///
/// In TS you'd write (pseudocode):
/// ```ts
/// await mkdir(path, { mode: 0o700 });
/// ```
pub fn create_private_directory(path: &Path) -> std::io::Result<()> {
    return private_directory_builder().create(path);
}

/// What:
///  The directory builder that creates private directories on this platform.
/// Why:
///   Only Unix configures a mode;
///  a separate function per platform keeps each body free of
///       a `mut` the other platform would leave unused.
///
/// In TS you'd write (pseudocode):
/// ```ts
/// const builder = { mode: 0o700 };
/// ```
#[cfg(unix)]
fn private_directory_builder() -> std::fs::DirBuilder {
    // The trait adds `.mode` to directory builders on Unix.
    /// The Unix directory-builder extensions,
    ///  for the mode.
    use std::os::unix::fs::DirBuilderExt;
    // `mut` allows the builder to be configured before it is returned.
    let mut builder: std::fs::DirBuilder = std::fs::DirBuilder::new();
    builder.mode(PRIVATE_DIRECTORY_MODE);
    return builder;
}

/// What:
///  The directory builder that creates private directories on this platform.
/// Why:
///   Windows has no mode to configure,
///  as `private_directory_builder` on Unix explains.
///
/// In TS you'd write (pseudocode):
/// ```ts
/// const builder = {};
/// ```
#[cfg(not(unix))]
fn private_directory_builder() -> std::fs::DirBuilder {
    return std::fs::DirBuilder::new();
}

/// What:
///  Make a directory's entries durable,
///  where the platform supports directory handles.
/// Why:
///   A new or renamed entry survives a crash only after its directory is synced;
///  Windows
///       has no directory handle for this,
///  as the incumbent's `syncDirectory` notes.
///
/// In TS you'd write (pseudocode):
/// ```ts
/// async function syncDirectory(path: string): Promise<void>;
/// ```
pub fn sync_directory(path: &Path) -> std::io::Result<()> {
    #[cfg(unix)]
    {
        // Opening a directory read-only and syncing it is the POSIX way to flush its entries.
        let directory: File = File::open(path)?;
        directory.sync_all()?;
    }
    #[cfg(not(unix))]
    let _ = path;
    return Ok(());
}

/// What:
///  Remove a directory tree,
///  treating an already absent tree as removed.
/// Why:
///   Retired locks and transactions are removed by name after a rename;
///  another process
///       may have removed them first,
///  which is the same end state (`rm -rf` with `force`).
///
/// In TS you'd write (pseudocode):
/// ```ts
/// await rm(path, { recursive: true, force: true });
/// ```
pub fn remove_tree(path: &Path) -> std::io::Result<()> {
    match std::fs::remove_dir_all(path) {
        Ok(()) => return Ok(()),
        Err(error) if error.kind() == std::io::ErrorKind::NotFound => return Ok(()),
        Err(error) => return Err(error),
    }
}

/// What:
///  Open a file for reading without following a final link.
/// Why:
///   Records are always files the wrappers created;
///  a link planted at a record path must
///       be refused,
///  never read through.
///
/// In TS you'd write (pseudocode):
/// ```ts
/// await open(path, O_RDONLY | O_NOFOLLOW);
/// ```
fn open_no_follow(path: &Path) -> std::io::Result<File> {
    // `mut` allows the builder to be configured step by step.
    let mut options: OpenOptions = OpenOptions::new();
    options.read(true);
    #[cfg(unix)]
    {
        // The trait adds `.custom_flags` to open options on Unix.
        /// The Unix open-option extensions,
        ///  for the no-follow flag.
        use std::os::unix::fs::OpenOptionsExt;
        options.custom_flags(libc::O_NOFOLLOW);
    }
    return options.open(path);
}

/// What:
///  Read a whole regular file without following links,
///  up to `limit` bytes.
///       `u64` is an unsigned 64-bit size;
///  `Result<Vec<u8>, ReadRefusal>` is "the bytes,
///  or why
///       not".
/// Why:
///   Recovery reads untrusted state:
///  a link or a directory must be named as unsafe,
///  and a
///       runaway file must not be read into memory.
///
/// In TS you'd write (pseudocode):
/// ```ts
/// async function readPrivateFile(path: string, limit: number): Promise<Uint8Array>; // throws ReadRefusal
/// ```
pub fn read_regular_file(path: &Path, limit: u64) -> Result<Vec<u8>, ReadRefusal> {
    let file: File = match open_no_follow(path) {
        Ok(opened) => opened,
        Err(error) => return Err(open_refusal(error)),
    };
    // The handle's own metadata, so a swap between the check and the read is impossible.
    let metadata: std::fs::Metadata = file.metadata().map_err(ReadRefusal::Io)?;
    if !metadata.is_file() {
        return Err(ReadRefusal::NotRegular);
    }
    if metadata.len() > limit {
        return Err(ReadRefusal::TooLarge);
    }
    // `.take(limit + 1)` caps the read even if the file grows after the size check.
    let mut bytes: Vec<u8> = Vec::new();
    file.take(limit + 1)
        .read_to_end(&mut bytes)
        .map_err(ReadRefusal::Io)?;
    if bytes.len() as u64 > limit {
        return Err(ReadRefusal::TooLarge);
    }
    return Ok(bytes);
}

/// What:
///  Classify an open failure as missing,
///  a refused link,
///  or another error.
/// Why:
///   `O_NOFOLLOW` on a link fails with `ELOOP` on Linux and macOS;
///  that is the unsafe-link
///       answer,
///  not an input-output failure.
///  A path through a non-directory (`ENOTDIR`) names
///       no file,
///  which the incumbent treats like a missing one.
///
/// In TS you'd write (pseudocode):
/// ```ts
/// function openRefusal(error: NodeJS.ErrnoException): ReadRefusal;
/// ```
fn open_refusal(error: std::io::Error) -> ReadRefusal {
    if error.kind() == std::io::ErrorKind::NotFound {
        return ReadRefusal::Missing;
    }
    #[cfg(unix)]
    {
        if error.raw_os_error() == Some(libc::ENOTDIR) {
            return ReadRefusal::Missing;
        }
        if error.raw_os_error() == Some(libc::ELOOP) {
            return ReadRefusal::NotRegular;
        }
    }
    return ReadRefusal::Io(error);
}

/// Disposable-directory controls stay out of the release executable.
#[cfg(test)]
#[path = "private_storage_tests.rs"]
mod tests;