hitWithCurrentLock

    fn hitWithCurrentLock(self: *Manifest) HitError!union(enum) {
        hit,
        miss: struct {
            file_digests_populated: usize,
        },
    } {
        const gpa = self.cache.gpa;
        const io = self.cache.io;
        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(io, &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 => |e| return e,
            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(Io.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 = .{ .nanoseconds = 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 = .{ .nanoseconds = 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(io, 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 } };
                },
                error.Canceled => |e| return e,
                else => |e| {
                    self.diagnostic = .{ .file_open = .{
                        .file_index = idx,
                        .err = e,
                    } };
                    return error.CacheCheckFailed;
                },
            };
            defer this_file.close(io);

            const actual_stat = this_file.stat(io) 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.nanoseconds == cache_hash_file.stat.mtime.nanoseconds;
            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 (try self.isProblematicTimestamp(cache_hash_file.stat.mtime)) {
                    // The actual file has an unreliable timestamp, force it to be hashed
                    cache_hash_file.stat.mtime = .zero;
                    cache_hash_file.stat.inode = 0;
                }

                var actual_digest: BinDigest = undefined;
                hashFile(io, 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;
    }