writeManifest

    pub fn writeManifest(self: *Manifest) !void {
        assert(self.have_exclusive_lock);
        const io = self.cache.io;
        const manifest_file = self.manifest_file.?;
        if (self.manifest_dirty) {
            self.manifest_dirty = false;

            var buffer: [4000]u8 = undefined;
            var fw = manifest_file.writer(io, &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();
        }
    }