struct Manifest [src]

Fields

cache: *Cache
hash: HashHelperCurrent state for incremental hashing.
manifest_file: ?fs.File
manifest_dirty: bool
want_shared_lock: bool = trueSet this flag to true before calling hit() in order to indicate that upon a cache hit, the code using the cache will not modify the files within the cache directory. This allows multiple processes to utilize the same cache directory at the same time.
have_exclusive_lock: bool = false
want_refresh_timestamp: bool = true
files: Files = .{}
hex_digest: HexDigest
diagnostic: Diagnostic = .none
recent_problematic_timestamp: i128 = 0Keeps track of the last time we performed a file system write to observe what time the file system thinks it is, according to its own granularity.
Members

addDepFile (Function)
addDepFilePost (Function)
addFile (Function)
addFilePath (Function)
addFilePost (Function)
addFilePostContents (Function)
addFilePostFetch (Function)
addListOfFiles (Function)
addOpenedFile (Function)
addOptionalFile (Function)
addOptionalFilePath (Function)
deinit (Function)
Diagnostic (union)
Files (Type)
FilesContext (struct)
final (Function)
finalBin (Function)
hit (Function)
HitError (Error Set)
populateFileSystemInputs (Function)
populateOtherManifest (Function)
toOwnedLock (Function)
unhit (Function)
writeManifest (Function)
Source

  pub const Manifest = struct {
    cache: *Cache,
    /// Current state for incremental hashing.
    hash: HashHelper,
    manifest_file: ?fs.File,
    manifest_dirty: bool,
    /// Set this flag to true before calling hit() in order to indicate that
    /// upon a cache hit, the code using the cache will not modify the files
    /// within the cache directory. This allows multiple processes to utilize
    /// the same cache directory at the same time.
    want_shared_lock: bool = true,
    have_exclusive_lock: bool = false,
    // Indicate that we want isProblematicTimestamp to perform a filesystem write in
    // order to obtain a problematic timestamp for the next call. Calls after that
    // will then use the same timestamp, to avoid unnecessary filesystem writes.
    want_refresh_timestamp: bool = true,
    files: Files = .{},
    hex_digest: HexDigest,
    diagnostic: Diagnostic = .none,
    /// Keeps track of the last time we performed a file system write to observe
    /// what time the file system thinks it is, according to its own granularity.
    recent_problematic_timestamp: i128 = 0,

    pub const Diagnostic = union(enum) {
        none,
        manifest_create: fs.File.OpenError,
        manifest_read: fs.File.ReadError,
        manifest_lock: fs.File.LockError,
        file_open: FileOp,
        file_stat: FileOp,
        file_read: FileOp,
        file_hash: FileOp,

        pub const FileOp = struct {
            file_index: usize,
            err: anyerror,
        };
    };

    pub const Files = std.ArrayHashMapUnmanaged(File, void, FilesContext, false);

    pub const FilesContext = struct {
        pub fn hash(fc: FilesContext, file: File) u32 {
            _ = fc;
            return file.prefixed_path.hash();
        }

        pub fn eql(fc: FilesContext, a: File, b: File, b_index: usize) bool {
            _ = fc;
            _ = b_index;
            return a.prefixed_path.eql(b.prefixed_path);
        }
    };

    const FilesAdapter = struct {
        pub fn eql(context: @This(), a: PrefixedPath, b: File, b_index: usize) bool {
            _ = context;
            _ = b_index;
            return a.eql(b.prefixed_path);
        }

        pub fn hash(context: @This(), key: PrefixedPath) u32 {
            _ = context;
            return key.hash();
        }
    };

    /// Add a file as a dependency of process being cached. When `hit` is
    /// called, the file's contents will be checked to ensure that it matches
    /// the contents from previous times.
    ///
    /// Max file size will be used to determine the amount of space the file contents
    /// are allowed to take up in memory. If max_file_size is null, then the contents
    /// will not be loaded into memory.
    ///
    /// Returns the index of the entry in the `files` array list. You can use it
    /// to access the contents of the file after calling `hit()` like so:
    ///
    /// ```
    /// var file_contents = cache_hash.files.keys()[file_index].contents.?;
    /// ```
    pub fn addFilePath(m: *Manifest, file_path: Path, max_file_size: ?usize) !usize {
        return addOpenedFile(m, file_path, null, max_file_size);
    }

    /// Same as `addFilePath` except the file has already been opened.
    pub fn addOpenedFile(m: *Manifest, path: Path, handle: ?fs.File, max_file_size: ?usize) !usize {
        const gpa = m.cache.gpa;
        try m.files.ensureUnusedCapacity(gpa, 1);
        const resolved_path = try fs.path.resolve(gpa, &.{
            path.root_dir.path orelse ".",
            path.subPathOrDot(),
        });
        errdefer gpa.free(resolved_path);
        const prefixed_path = try m.cache.findPrefixResolved(resolved_path);
        return addFileInner(m, prefixed_path, handle, max_file_size);
    }

    /// Deprecated; use `addFilePath`.
    pub fn addFile(self: *Manifest, file_path: []const u8, max_file_size: ?usize) !usize {
        assert(self.manifest_file == null);

        const gpa = self.cache.gpa;
        try self.files.ensureUnusedCapacity(gpa, 1);
        const prefixed_path = try self.cache.findPrefix(file_path);
        errdefer gpa.free(prefixed_path.sub_path);

        return addFileInner(self, prefixed_path, null, max_file_size);
    }

    fn addFileInner(self: *Manifest, prefixed_path: PrefixedPath, handle: ?fs.File, max_file_size: ?usize) usize {
        const gop = self.files.getOrPutAssumeCapacityAdapted(prefixed_path, FilesAdapter{});
        if (gop.found_existing) {
            self.cache.gpa.free(prefixed_path.sub_path);
            gop.key_ptr.updateMaxSize(max_file_size);
            gop.key_ptr.updateHandle(handle);
            return gop.index;
        }
        gop.key_ptr.* = .{
            .prefixed_path = prefixed_path,
            .contents = null,
            .max_file_size = max_file_size,
            .stat = undefined,
            .bin_digest = undefined,
            .handle = handle,
        };

        self.hash.add(prefixed_path.prefix);
        self.hash.addBytes(prefixed_path.sub_path);

        return gop.index;
    }

    /// Deprecated, use `addOptionalFilePath`.
    pub fn addOptionalFile(self: *Manifest, optional_file_path: ?[]const u8) !void {
        self.hash.add(optional_file_path != null);
        const file_path = optional_file_path orelse return;
        _ = try self.addFile(file_path, null);
    }

    pub fn addOptionalFilePath(self: *Manifest, optional_file_path: ?Path) !void {
        self.hash.add(optional_file_path != null);
        const file_path = optional_file_path orelse return;
        _ = try self.addFilePath(file_path, null);
    }

    pub fn addListOfFiles(self: *Manifest, list_of_files: []const []const u8) !void {
        self.hash.add(list_of_files.len);
        for (list_of_files) |file_path| {
            _ = try self.addFile(file_path, null);
        }
    }

    pub fn addDepFile(self: *Manifest, dir: fs.Dir, dep_file_basename: []const u8) !void {
        assert(self.manifest_file == null);
        return self.addDepFileMaybePost(dir, dep_file_basename);
    }

    pub const HitError = error{
        /// Unable to check the cache for a reason that has been recorded into
        /// the `diagnostic` field.
        CacheCheckFailed,
        /// A cache manifest file exists however it could not be parsed.
        InvalidFormat,
        OutOfMemory,
    };

    /// Check the cache to see if the input exists in it. If it exists, returns `true`.
    /// A hex encoding of its hash is available by calling `final`.
    ///
    /// This function will also acquire an exclusive lock to the manifest file. This means
    /// that a process holding a Manifest will block any other process attempting to
    /// acquire the lock. If `want_shared_lock` is `true`, a cache hit guarantees the
    /// manifest file to be locked in shared mode, and a cache miss guarantees the manifest
    /// file to be locked in exclusive mode.
    ///
    /// The lock on the manifest file is released when `deinit` is called. As another
    /// option, one may call `toOwnedLock` to obtain a smaller object which can represent
    /// the lock. `deinit` is safe to call whether or not `toOwnedLock` has been called.
    pub fn hit(self: *Manifest) HitError!bool {
        assert(self.manifest_file == null);

        self.diagnostic = .none;

        const ext = ".txt";
        var manifest_file_path: [hex_digest_len + ext.len]u8 = undefined;

        var bin_digest: BinDigest = undefined;
        self.hash.hasher.final(&bin_digest);

        self.hex_digest = binToHex(bin_digest);

        @memcpy(manifest_file_path[0..self.hex_digest.len], &self.hex_digest);
        manifest_file_path[hex_digest_len..][0..ext.len].* = ext.*;

        // We'll try to open the cache with an exclusive lock, but if that would block
        // and `want_shared_lock` is set, a shared lock might be sufficient, so we'll
        // open with a shared lock instead.
        while (true) {
            if (self.cache.manifest_dir.createFile(&manifest_file_path, .{
                .read = true,
                .truncate = false,
                .lock = .exclusive,
                .lock_nonblocking = self.want_shared_lock,
            })) |manifest_file| {
                self.manifest_file = manifest_file;
                self.have_exclusive_lock = true;
                break;
            } else |err| switch (err) {
                error.WouldBlock => {
                    self.manifest_file = self.cache.manifest_dir.openFile(&manifest_file_path, .{
                        .mode = .read_write,
                        .lock = .shared,
                    }) catch |e| {
                        self.diagnostic = .{ .manifest_create = e };
                        return error.CacheCheckFailed;
                    };
                    break;
                },
                error.FileNotFound => {
                    // There are no dir components, so the only possibility
                    // should be that the directory behind the handle has been
                    // deleted, however we have observed on macOS two processes
                    // racing to do openat() with O_CREAT manifest in ENOENT.
                    //
                    // As a workaround, we retry with exclusive=true which
                    // disambiguates by returning EEXIST, indicating original
                    // failure was a race, or ENOENT, indicating deletion of
                    // the directory of our open handle.
                    if (builtin.os.tag != .macos) {
                        self.diagnostic = .{ .manifest_create = error.FileNotFound };
                        return error.CacheCheckFailed;
                    }

                    if (self.cache.manifest_dir.createFile(&manifest_file_path, .{
                        .read = true,
                        .truncate = false,
                        .lock = .exclusive,
                        .lock_nonblocking = self.want_shared_lock,
                        .exclusive = true,
                    })) |manifest_file| {
                        self.manifest_file = manifest_file;
                        self.have_exclusive_lock = true;
                        break;
                    } else |excl_err| switch (excl_err) {
                        error.WouldBlock, error.PathAlreadyExists => continue,
                        error.FileNotFound => {
                            self.diagnostic = .{ .manifest_create = error.FileNotFound };
                            return error.CacheCheckFailed;
                        },
                        else => |e| {
                            self.diagnostic = .{ .manifest_create = e };
                            return error.CacheCheckFailed;
                        },
                    }
                },
                else => |e| {
                    self.diagnostic = .{ .manifest_create = e };
                    return error.CacheCheckFailed;
                },
            }
        }

        self.want_refresh_timestamp = true;

        const input_file_count = self.files.entries.len;

        // We're going to construct a second hash. Its input will begin with the digest we've
        // already computed (`bin_digest`), and then it'll have the digests of each input file,
        // including "post" files (see `addFilePost`). If this is a hit, we learn the set of "post"
        // files from the manifest on disk. If this is a miss, we'll learn those from future calls
        // to `addFilePost` etc. As such, the state of `self.hash.hasher` after this function
        // depends on whether this is a hit or a miss.
        //
        // If we return `true` indicating a cache hit, then `self.hash.hasher` must already include
        // the digests of the "post" files, so the caller can call `final`. Otherwise, on a cache
        // miss, `self.hash.hasher` will include the digests of all non-"post" files -- that is,
        // the ones we've already been told about. The rest will be discovered through calls to
        // `addFilePost` etc, which will update the hasher. After all files are added, the user can
        // use `final`, and will at some point `writeManifest` the file list to disk.

        self.hash.hasher = hasher_init;
        self.hash.hasher.update(&bin_digest);

        hit: {
            const file_digests_populated: usize = digests: {
                switch (try self.hitWithCurrentLock()) {
                    .hit => break :hit,
                    .miss => |m| if (!try self.upgradeToExclusiveLock()) {
                        break :digests m.file_digests_populated;
                    },
                }
                // We've just had a miss with the shared lock, and upgraded to an exclusive lock. Someone
                // else might have modified the digest, so we need to check again before deciding to miss.
                // Before trying again, we must reset `self.hash.hasher` and `self.files`.
                // This is basically just the first half of `unhit`.
                self.hash.hasher = hasher_init;
                self.hash.hasher.update(&bin_digest);
                while (self.files.count() != input_file_count) {
                    var file = self.files.pop().?;
                    file.key.deinit(self.cache.gpa);
                }
                switch (try self.hitWithCurrentLock()) {
                    .hit => break :hit,
                    .miss => |m| break :digests m.file_digests_populated,
                }
            };

            // This is a guaranteed cache miss. We're almost ready to return `false`, but there's a
            // little bookkeeping to do first. The first `file_digests_populated` entries in `files`
            // have their `bin_digest` populated; there may be some left in `input_file_count` which
            // we'll need to populate ourselves. Other than that, this is basically `unhit`.
            self.manifest_dirty = true;
            self.hash.hasher = hasher_init;
            self.hash.hasher.update(&bin_digest);
            while (self.files.count() != input_file_count) {
                var file = self.files.pop().?;
                file.key.deinit(self.cache.gpa);
            }
            for (self.files.keys(), 0..) |*file, idx| {
                if (idx < file_digests_populated) {
                    // `bin_digest` is already populated by `hitWithCurrentLock`, so we can use it directly.
                    self.hash.hasher.update(&file.bin_digest);
                } else {
                    self.populateFileHash(file) catch |err| {
                        self.diagnostic = .{ .file_hash = .{
                            .file_index = idx,
                            .err = err,
                        } };
                        return error.CacheCheckFailed;
                    };
                }
            }
            return false;
        }

        if (self.want_shared_lock) {
            self.downgradeToSharedLock() catch |err| {
                self.diagnostic = .{ .manifest_lock = err };
                return error.CacheCheckFailed;
            };
        }

        return true;
    }

    /// Assumes that `self.hash.hasher` has been updated only with the original digest and that
    /// `self.files` contains only the original input files.
    fn hitWithCurrentLock(self: *Manifest) HitError!union(enum) {
        hit,
        miss: struct {
            file_digests_populated: usize,
        },
    } {
        const gpa = self.cache.gpa;
        const input_file_count = self.files.entries.len;
        var tiny_buffer: [1]u8 = undefined; // allows allocRemaining to detect limit exceeded
        var manifest_reader = self.manifest_file.?.reader(&tiny_buffer); // Reads positionally from zero.
        const limit: std.Io.Limit = .limited(manifest_file_size_max);
        const file_contents = manifest_reader.interface.allocRemaining(gpa, limit) catch |err| switch (err) {
            error.OutOfMemory => return error.OutOfMemory,
            error.StreamTooLong => return error.OutOfMemory,
            error.ReadFailed => {
                self.diagnostic = .{ .manifest_read = manifest_reader.err.? };
                return error.CacheCheckFailed;
            },
        };
        defer gpa.free(file_contents);

        var any_file_changed = false;
        var line_iter = mem.tokenizeScalar(u8, file_contents, '\n');
        var idx: usize = 0;
        const header_valid = valid: {
            const line = line_iter.next() orelse break :valid false;
            break :valid std.mem.eql(u8, line, manifest_header);
        };
        if (!header_valid) {
            return .{ .miss = .{ .file_digests_populated = 0 } };
        }
        while (line_iter.next()) |line| {
            defer idx += 1;

            var iter = mem.tokenizeScalar(u8, line, ' ');
            const size = iter.next() orelse return error.InvalidFormat;
            const inode = iter.next() orelse return error.InvalidFormat;
            const mtime_nsec_str = iter.next() orelse return error.InvalidFormat;
            const digest_str = iter.next() orelse return error.InvalidFormat;
            const prefix_str = iter.next() orelse return error.InvalidFormat;
            const file_path = iter.rest();

            const stat_size = fmt.parseInt(u64, size, 10) catch return error.InvalidFormat;
            const stat_inode = fmt.parseInt(fs.File.INode, inode, 10) catch return error.InvalidFormat;
            const stat_mtime = fmt.parseInt(i64, mtime_nsec_str, 10) catch return error.InvalidFormat;
            const file_bin_digest = b: {
                if (digest_str.len != hex_digest_len) return error.InvalidFormat;
                var bd: BinDigest = undefined;
                _ = fmt.hexToBytes(&bd, digest_str) catch return error.InvalidFormat;
                break :b bd;
            };

            const prefix = fmt.parseInt(u8, prefix_str, 10) catch return error.InvalidFormat;
            if (prefix >= self.cache.prefixes_len) return error.InvalidFormat;

            if (file_path.len == 0) return error.InvalidFormat;

            const cache_hash_file = f: {
                const prefixed_path: PrefixedPath = .{
                    .prefix = prefix,
                    .sub_path = file_path, // expires with file_contents
                };
                if (idx < input_file_count) {
                    const file = &self.files.keys()[idx];
                    if (!file.prefixed_path.eql(prefixed_path))
                        return error.InvalidFormat;

                    file.stat = .{
                        .size = stat_size,
                        .inode = stat_inode,
                        .mtime = stat_mtime,
                    };
                    file.bin_digest = file_bin_digest;
                    break :f file;
                }
                const gop = try self.files.getOrPutAdapted(gpa, prefixed_path, FilesAdapter{});
                errdefer _ = self.files.pop();
                if (!gop.found_existing) {
                    gop.key_ptr.* = .{
                        .prefixed_path = .{
                            .prefix = prefix,
                            .sub_path = try gpa.dupe(u8, file_path),
                        },
                        .contents = null,
                        .max_file_size = null,
                        .handle = null,
                        .stat = .{
                            .size = stat_size,
                            .inode = stat_inode,
                            .mtime = stat_mtime,
                        },
                        .bin_digest = file_bin_digest,
                    };
                }
                break :f gop.key_ptr;
            };

            const pp = cache_hash_file.prefixed_path;
            const dir = self.cache.prefixes()[pp.prefix].handle;
            const this_file = dir.openFile(pp.sub_path, .{ .mode = .read_only }) catch |err| switch (err) {
                error.FileNotFound => {
                    // Every digest before this one has been populated successfully.
                    return .{ .miss = .{ .file_digests_populated = idx } };
                },
                else => |e| {
                    self.diagnostic = .{ .file_open = .{
                        .file_index = idx,
                        .err = e,
                    } };
                    return error.CacheCheckFailed;
                },
            };
            defer this_file.close();

            const actual_stat = this_file.stat() catch |err| {
                self.diagnostic = .{ .file_stat = .{
                    .file_index = idx,
                    .err = err,
                } };
                return error.CacheCheckFailed;
            };
            const size_match = actual_stat.size == cache_hash_file.stat.size;
            const mtime_match = actual_stat.mtime == cache_hash_file.stat.mtime;
            const inode_match = actual_stat.inode == cache_hash_file.stat.inode;

            if (!size_match or !mtime_match or !inode_match) {
                cache_hash_file.stat = .{
                    .size = actual_stat.size,
                    .mtime = actual_stat.mtime,
                    .inode = actual_stat.inode,
                };

                if (self.isProblematicTimestamp(cache_hash_file.stat.mtime)) {
                    // The actual file has an unreliable timestamp, force it to be hashed
                    cache_hash_file.stat.mtime = 0;
                    cache_hash_file.stat.inode = 0;
                }

                var actual_digest: BinDigest = undefined;
                hashFile(this_file, &actual_digest) catch |err| {
                    self.diagnostic = .{ .file_read = .{
                        .file_index = idx,
                        .err = err,
                    } };
                    return error.CacheCheckFailed;
                };

                if (!mem.eql(u8, &cache_hash_file.bin_digest, &actual_digest)) {
                    cache_hash_file.bin_digest = actual_digest;
                    // keep going until we have the input file digests
                    any_file_changed = true;
                }
            }

            if (!any_file_changed) {
                self.hash.hasher.update(&cache_hash_file.bin_digest);
            }
        }

        // If the manifest was somehow missing one of our input files, or if any file hash has changed,
        // then this is a cache miss. However, we have successfully populated some or all of the file
        // digests.
        if (any_file_changed or idx < input_file_count) {
            return .{ .miss = .{ .file_digests_populated = idx } };
        }

        return .hit;
    }

    /// Reset `self.hash.hasher` to the state it should be in after `hit` returns `false`.
    /// The hasher contains the original input digest, and all original input file digests (i.e.
    /// not including post files).
    /// Assumes that `bin_digest` is populated for all files up to `input_file_count`. As such,
    /// this is not necessarily safe to call within `hit`.
    pub fn unhit(self: *Manifest, bin_digest: BinDigest, input_file_count: usize) void {
        // Reset the hash.
        self.hash.hasher = hasher_init;
        self.hash.hasher.update(&bin_digest);

        // Remove files not in the initial hash.
        while (self.files.count() != input_file_count) {
            var file = self.files.pop().?;
            file.key.deinit(self.cache.gpa);
        }

        for (self.files.keys()) |file| {
            self.hash.hasher.update(&file.bin_digest);
        }
    }

    fn isProblematicTimestamp(man: *Manifest, file_time: i128) bool {
        // If the file_time is prior to the most recent problematic timestamp
        // then we don't need to access the filesystem.
        if (file_time < man.recent_problematic_timestamp)
            return false;

        // Next we will check the globally shared Cache timestamp, which is accessed
        // from multiple threads.
        man.cache.mutex.lock();
        defer man.cache.mutex.unlock();

        // Save the global one to our local one to avoid locking next time.
        man.recent_problematic_timestamp = man.cache.recent_problematic_timestamp;
        if (file_time < man.recent_problematic_timestamp)
            return false;

        // This flag prevents multiple filesystem writes for the same hit() call.
        if (man.want_refresh_timestamp) {
            man.want_refresh_timestamp = false;

            var file = man.cache.manifest_dir.createFile("timestamp", .{
                .read = true,
                .truncate = true,
            }) catch return true;
            defer file.close();

            // Save locally and also save globally (we still hold the global lock).
            man.recent_problematic_timestamp = (file.stat() catch return true).mtime;
            man.cache.recent_problematic_timestamp = man.recent_problematic_timestamp;
        }

        return file_time >= man.recent_problematic_timestamp;
    }

    fn populateFileHash(self: *Manifest, ch_file: *File) !void {
        if (ch_file.handle) |handle| {
            return populateFileHashHandle(self, ch_file, handle);
        } else {
            const pp = ch_file.prefixed_path;
            const dir = self.cache.prefixes()[pp.prefix].handle;
            const handle = try dir.openFile(pp.sub_path, .{});
            defer handle.close();
            return populateFileHashHandle(self, ch_file, handle);
        }
    }

    fn populateFileHashHandle(self: *Manifest, ch_file: *File, handle: fs.File) !void {
        const actual_stat = try handle.stat();
        ch_file.stat = .{
            .size = actual_stat.size,
            .mtime = actual_stat.mtime,
            .inode = actual_stat.inode,
        };

        if (self.isProblematicTimestamp(ch_file.stat.mtime)) {
            // The actual file has an unreliable timestamp, force it to be hashed
            ch_file.stat.mtime = 0;
            ch_file.stat.inode = 0;
        }

        if (ch_file.max_file_size) |max_file_size| {
            if (ch_file.stat.size > max_file_size) {
                return error.FileTooBig;
            }

            const contents = try self.cache.gpa.alloc(u8, @as(usize, @intCast(ch_file.stat.size)));
            errdefer self.cache.gpa.free(contents);

            // Hash while reading from disk, to keep the contents in the cpu cache while
            // doing hashing.
            var hasher = hasher_init;
            var off: usize = 0;
            while (true) {
                const bytes_read = try handle.pread(contents[off..], off);
                if (bytes_read == 0) break;
                hasher.update(contents[off..][0..bytes_read]);
                off += bytes_read;
            }
            hasher.final(&ch_file.bin_digest);

            ch_file.contents = contents;
        } else {
            try hashFile(handle, &ch_file.bin_digest);
        }

        self.hash.hasher.update(&ch_file.bin_digest);
    }

    /// Add a file as a dependency of process being cached, after the initial hash has been
    /// calculated. This is useful for processes that don't know all the files that
    /// are depended on ahead of time. For example, a source file that can import other files
    /// will need to be recompiled if the imported file is changed.
    pub fn addFilePostFetch(self: *Manifest, file_path: []const u8, max_file_size: usize) ![]const u8 {
        assert(self.manifest_file != null);

        const gpa = self.cache.gpa;
        const prefixed_path = try self.cache.findPrefix(file_path);
        errdefer gpa.free(prefixed_path.sub_path);

        const gop = try self.files.getOrPutAdapted(gpa, prefixed_path, FilesAdapter{});
        errdefer _ = self.files.pop();

        if (gop.found_existing) {
            gpa.free(prefixed_path.sub_path);
            return gop.key_ptr.contents.?;
        }

        gop.key_ptr.* = .{
            .prefixed_path = prefixed_path,
            .max_file_size = max_file_size,
            .stat = undefined,
            .bin_digest = undefined,
            .contents = null,
        };

        self.files.lockPointers();
        defer self.files.unlockPointers();

        try self.populateFileHash(gop.key_ptr);
        return gop.key_ptr.contents.?;
    }

    /// Add a file as a dependency of process being cached, after the initial hash has been
    /// calculated.
    ///
    /// This is useful for processes that don't know the all the files that are
    /// depended on ahead of time. For example, a source file that can import
    /// other files will need to be recompiled if the imported file is changed.
    pub fn addFilePost(self: *Manifest, file_path: []const u8) !void {
        assert(self.manifest_file != null);

        const gpa = self.cache.gpa;
        const prefixed_path = try self.cache.findPrefix(file_path);
        errdefer gpa.free(prefixed_path.sub_path);

        const gop = try self.files.getOrPutAdapted(gpa, prefixed_path, FilesAdapter{});
        errdefer _ = self.files.pop();

        if (gop.found_existing) {
            gpa.free(prefixed_path.sub_path);
            return;
        }

        gop.key_ptr.* = .{
            .prefixed_path = prefixed_path,
            .max_file_size = null,
            .handle = null,
            .stat = undefined,
            .bin_digest = undefined,
            .contents = null,
        };

        self.files.lockPointers();
        defer self.files.unlockPointers();

        try self.populateFileHash(gop.key_ptr);
    }

    /// Like `addFilePost` but when the file contents have already been loaded from disk.
    pub fn addFilePostContents(
        self: *Manifest,
        file_path: []const u8,
        bytes: []const u8,
        stat: File.Stat,
    ) !void {
        assert(self.manifest_file != null);
        const gpa = self.cache.gpa;

        const prefixed_path = try self.cache.findPrefix(file_path);
        errdefer gpa.free(prefixed_path.sub_path);

        const gop = try self.files.getOrPutAdapted(gpa, prefixed_path, FilesAdapter{});
        errdefer _ = self.files.pop();

        if (gop.found_existing) {
            gpa.free(prefixed_path.sub_path);
            return;
        }

        const new_file = gop.key_ptr;

        new_file.* = .{
            .prefixed_path = prefixed_path,
            .max_file_size = null,
            .handle = null,
            .stat = stat,
            .bin_digest = undefined,
            .contents = null,
        };

        if (self.isProblematicTimestamp(new_file.stat.mtime)) {
            // The actual file has an unreliable timestamp, force it to be hashed
            new_file.stat.mtime = 0;
            new_file.stat.inode = 0;
        }

        {
            var hasher = hasher_init;
            hasher.update(bytes);
            hasher.final(&new_file.bin_digest);
        }

        self.hash.hasher.update(&new_file.bin_digest);
    }

    pub fn addDepFilePost(self: *Manifest, dir: fs.Dir, dep_file_basename: []const u8) !void {
        assert(self.manifest_file != null);
        return self.addDepFileMaybePost(dir, dep_file_basename);
    }

    fn addDepFileMaybePost(self: *Manifest, dir: fs.Dir, dep_file_basename: []const u8) !void {
        const gpa = self.cache.gpa;
        const dep_file_contents = try dir.readFileAlloc(dep_file_basename, gpa, .limited(manifest_file_size_max));
        defer gpa.free(dep_file_contents);

        var error_buf: std.ArrayListUnmanaged(u8) = .empty;
        defer error_buf.deinit(gpa);

        var resolve_buf: std.ArrayListUnmanaged(u8) = .empty;
        defer resolve_buf.deinit(gpa);

        var it: DepTokenizer = .{ .bytes = dep_file_contents };
        while (it.next()) |token| {
            switch (token) {
                // We don't care about targets, we only want the prereqs
                // Clang is invoked in single-source mode but other programs may not
                .target, .target_must_resolve => {},
                .prereq => |file_path| if (self.manifest_file == null) {
                    _ = try self.addFile(file_path, null);
                } else try self.addFilePost(file_path),
                .prereq_must_resolve => {
                    resolve_buf.clearRetainingCapacity();
                    try token.resolve(gpa, &resolve_buf);
                    if (self.manifest_file == null) {
                        _ = try self.addFile(resolve_buf.items, null);
                    } else try self.addFilePost(resolve_buf.items);
                },
                else => |err| {
                    try err.printError(gpa, &error_buf);
                    log.err("failed parsing {s}: {s}", .{ dep_file_basename, error_buf.items });
                    return error.InvalidDepFile;
                },
            }
        }
    }

    /// Returns a binary hash of the inputs.
    pub fn finalBin(self: *Manifest) BinDigest {
        assert(self.manifest_file != null);

        // We don't close the manifest file yet, because we want to
        // keep it locked until the API user is done using it.
        // We also don't write out the manifest yet, because until
        // cache_release is called we still might be working on creating
        // the artifacts to cache.

        var bin_digest: BinDigest = undefined;
        self.hash.hasher.final(&bin_digest);
        return bin_digest;
    }

    /// Returns a hex encoded hash of the inputs.
    pub fn final(self: *Manifest) HexDigest {
        const bin_digest = self.finalBin();
        return binToHex(bin_digest);
    }

    /// If `want_shared_lock` is true, this function automatically downgrades the
    /// lock from exclusive to shared.
    pub fn writeManifest(self: *Manifest) !void {
        assert(self.have_exclusive_lock);

        const manifest_file = self.manifest_file.?;
        if (self.manifest_dirty) {
            self.manifest_dirty = false;

            var buffer: [4000]u8 = undefined;
            var fw = manifest_file.writer(&buffer);
            writeDirtyManifestToStream(self, &fw) catch |err| switch (err) {
                error.WriteFailed => return fw.err.?,
                else => |e| return e,
            };
        }

        if (self.want_shared_lock) {
            try self.downgradeToSharedLock();
        }
    }

    fn writeDirtyManifestToStream(self: *Manifest, fw: *fs.File.Writer) !void {
        try fw.interface.writeAll(manifest_header ++ "\n");
        for (self.files.keys()) |file| {
            try fw.interface.print("{d} {d} {d} {x} {d} {s}\n", .{
                file.stat.size,
                file.stat.inode,
                file.stat.mtime,
                &file.bin_digest,
                file.prefixed_path.prefix,
                file.prefixed_path.sub_path,
            });
        }
        try fw.end();
    }

    fn downgradeToSharedLock(self: *Manifest) !void {
        if (!self.have_exclusive_lock) return;

        // WASI does not currently support flock, so we bypass it here.
        // TODO: If/when flock is supported on WASI, this check should be removed.
        //       See https://github.com/WebAssembly/wasi-filesystem/issues/2
        if (builtin.os.tag != .wasi or std.process.can_spawn or !builtin.single_threaded) {
            const manifest_file = self.manifest_file.?;
            try manifest_file.downgradeLock();
        }

        self.have_exclusive_lock = false;
    }

    fn upgradeToExclusiveLock(self: *Manifest) error{CacheCheckFailed}!bool {
        if (self.have_exclusive_lock) return false;
        assert(self.manifest_file != null);

        // WASI does not currently support flock, so we bypass it here.
        // TODO: If/when flock is supported on WASI, this check should be removed.
        //       See https://github.com/WebAssembly/wasi-filesystem/issues/2
        if (builtin.os.tag != .wasi or std.process.can_spawn or !builtin.single_threaded) {
            const manifest_file = self.manifest_file.?;
            // Here we intentionally have a period where the lock is released, in case there are
            // other processes holding a shared lock.
            manifest_file.unlock();
            manifest_file.lock(.exclusive) catch |err| {
                self.diagnostic = .{ .manifest_lock = err };
                return error.CacheCheckFailed;
            };
        }
        self.have_exclusive_lock = true;
        return true;
    }

    /// Obtain only the data needed to maintain a lock on the manifest file.
    /// The `Manifest` remains safe to deinit.
    /// Don't forget to call `writeManifest` before this!
    pub fn toOwnedLock(self: *Manifest) Lock {
        const lock: Lock = .{
            .manifest_file = self.manifest_file.?,
        };

        self.manifest_file = null;
        return lock;
    }

    /// Releases the manifest file and frees any memory the Manifest was using.
    /// `Manifest.hit` must be called first.
    /// Don't forget to call `writeManifest` before this!
    pub fn deinit(self: *Manifest) void {
        if (self.manifest_file) |file| {
            if (builtin.os.tag == .windows) {
                // See Lock.release for why this is required on Windows
                file.unlock();
            }

            file.close();
        }
        for (self.files.keys()) |*file| {
            file.deinit(self.cache.gpa);
        }
        self.files.deinit(self.cache.gpa);
    }

    pub fn populateFileSystemInputs(man: *Manifest, buf: *std.ArrayListUnmanaged(u8)) Allocator.Error!void {
        assert(@typeInfo(std.zig.Server.Message.PathPrefix).@"enum".fields.len == man.cache.prefixes_len);
        buf.clearRetainingCapacity();
        const gpa = man.cache.gpa;
        const files = man.files.keys();
        if (files.len > 0) {
            for (files) |file| {
                try buf.ensureUnusedCapacity(gpa, file.prefixed_path.sub_path.len + 2);
                buf.appendAssumeCapacity(file.prefixed_path.prefix + 1);
                buf.appendSliceAssumeCapacity(file.prefixed_path.sub_path);
                buf.appendAssumeCapacity(0);
            }
            // The null byte is a separator, not a terminator.
            buf.items.len -= 1;
        }
    }

    pub fn populateOtherManifest(man: *Manifest, other: *Manifest, prefix_map: [4]u8) Allocator.Error!void {
        const gpa = other.cache.gpa;
        assert(@typeInfo(std.zig.Server.Message.PathPrefix).@"enum".fields.len == man.cache.prefixes_len);
        assert(man.cache.prefixes_len == 4);
        for (man.files.keys()) |file| {
            const prefixed_path: PrefixedPath = .{
                .prefix = prefix_map[file.prefixed_path.prefix],
                .sub_path = try gpa.dupe(u8, file.prefixed_path.sub_path),
            };
            errdefer gpa.free(prefixed_path.sub_path);

            const gop = try other.files.getOrPutAdapted(gpa, prefixed_path, FilesAdapter{});
            errdefer _ = other.files.pop();

            if (gop.found_existing) {
                gpa.free(prefixed_path.sub_path);
                continue;
            }

            gop.key_ptr.* = .{
                .prefixed_path = prefixed_path,
                .max_file_size = file.max_file_size,
                .handle = file.handle,
                .stat = file.stat,
                .bin_digest = file.bin_digest,
                .contents = null,
            };

            other.hash.hasher.update(&gop.key_ptr.bin_digest);
        }
    }
}  