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Jul 14, 2025
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Improve/simplify memory accounting in worker-fs FileImpl #4511

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71 changes: 43 additions & 28 deletions src/workerd/io/worker-fs.c++
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Original file line number Diff line number Diff line change
Expand Up @@ -623,7 +623,9 @@ class FileImpl final: public File {
FileImpl(kj::ArrayPtr<const kj::byte> data): ownedOrView(data), lastModified(kj::UNIX_EPOCH) {}

// Constructor used to create a writable file.
FileImpl(kj::Array<kj::byte> owned): ownedOrView(kj::mv(owned)), lastModified(kj::UNIX_EPOCH) {}
FileImpl(jsg::Lock& js, kj::Array<kj::byte> owned)
: ownedOrView(Owned(js, kj::mv(owned))),
lastModified(kj::UNIX_EPOCH) {}

kj::Maybe<FsError> setLastModified(jsg::Lock& js, kj::Date date = kj::UNIX_EPOCH) override {
if (isWritable()) {
Expand Down Expand Up @@ -664,23 +666,22 @@ class FileImpl final: public File {
if (!isWritable()) {
return FsError::READ_ONLY;
}
auto& owned = writableView();
size_t end = offset + buffer.size();
if (end > owned.size()) {
if (end > writableView().size()) {
KJ_IF_SOME(err, resize(js, end)) {
return err;
}
}
owned.slice(offset, end).copyFrom(buffer);
writableView().slice(offset, end).copyFrom(buffer);
return static_cast<uint32_t>(buffer.size());
}

kj::Maybe<FsError> resize(jsg::Lock& js, uint32_t size) override {
if (!isWritable()) {
return FsError::READ_ONLY;
}
auto& owned = writableView();
if (size == owned.size()) return kj::none; // Nothing to do.
auto& owned = ownedOrView.get<Owned>();
if (size == owned.data.size()) return kj::none; // Nothing to do.

auto maxSize = Worker::Isolate::from(js).getLimitEnforcer().getBlobSizeLimit();
if (size > maxSize) {
Expand All @@ -689,23 +690,24 @@ class FileImpl final: public File {

auto newData = kj::heapArray<kj::byte>(size);

if (size > owned.size()) {
if (size > owned.data.size()) {
// To grow the file, we need to allocate a new array, copy the old data over,
// and replace the original.
newData.first(owned.size()).copyFrom(owned);
newData.slice(owned.size()).fill(0);
newData.first(owned.data.size()).copyFrom(owned.data);
newData.slice(owned.data.size()).fill(0);
} else {
newData.asPtr().copyFrom(owned.first(size));
newData.asPtr().copyFrom(owned.data.first(size));
}
ownedOrView = newData.attach(js.getExternalMemoryAdjustment(newData.size()));
owned.adjustment.setNow(js, newData.size());
owned.data = kj::mv(newData);
return kj::none;
}

kj::Maybe<FsError> fill(jsg::Lock& js, kj::byte value, kj::Maybe<uint32_t> offset) override {
if (!isWritable()) {
return FsError::READ_ONLY;
}
auto& view = writableView();
auto view = writableView();
int actualOffset = offset.orDefault(0);
if (actualOffset >= view.size() || view.size() == 0) return kj::none;
view.slice(actualOffset).fill(value);
Expand All @@ -723,8 +725,8 @@ class FileImpl final: public File {
void jsgGetMemoryInfo(jsg::MemoryTracker& tracker) const override {
// We only track the memory if we own the data.
KJ_SWITCH_ONEOF(ownedOrView) {
KJ_CASE_ONEOF(owned, kj::Array<kj::byte>) {
tracker.trackField("owned", owned);
KJ_CASE_ONEOF(owned, Owned) {
tracker.trackField("owned", owned.data);
return;
}
KJ_CASE_ONEOF(view, kj::ArrayPtr<const kj::byte>) {
Expand All @@ -736,11 +738,11 @@ class FileImpl final: public File {
kj::OneOf<FsError, kj::Rc<File>> clone(jsg::Lock& js) override {
auto maxSize = Worker::Isolate::from(js).getLimitEnforcer().getBlobSizeLimit();
KJ_SWITCH_ONEOF(ownedOrView) {
KJ_CASE_ONEOF(owned, kj::Array<kj::byte>) {
if (owned.size() > maxSize) [[unlikely]] {
KJ_CASE_ONEOF(owned, Owned) {
if (owned.data.size() > maxSize) [[unlikely]] {
return FsError::FILE_SIZE_LIMIT_EXCEEDED;
}
kj::Rc<File> file = kj::rc<FileImpl>(kj::heapArray<kj::byte>(owned));
kj::Rc<File> file = kj::rc<FileImpl>(js, kj::heapArray<kj::byte>(owned.data));
return kj::mv(file);
}
KJ_CASE_ONEOF(view, kj::ArrayPtr<const kj::byte>) {
Expand All @@ -760,10 +762,11 @@ class FileImpl final: public File {
}

auto stat = file->stat(js);
auto buffer =
kj::heapArray<kj::byte>(stat.size).attach(js.getExternalMemoryAdjustment(stat.size));
auto buffer = kj::heapArray<kj::byte>(stat.size);
file->read(js, 0, buffer.asPtr());
ownedOrView = kj::mv(buffer);
auto& owned = ownedOrView.get<Owned>();
owned.adjustment.setNow(js, buffer.size());
owned.data = kj::mv(buffer);
lastModified = stat.lastModified;
return kj::none;
}
Expand All @@ -780,26 +783,39 @@ class FileImpl final: public File {
}

private:
kj::OneOf<kj::Array<kj::byte>, kj::ArrayPtr<const kj::byte>> ownedOrView;
struct Owned {
kj::Array<kj::byte> data;
jsg::ExternalMemoryAdjustment adjustment;
Owned(jsg::Lock& js, kj::Array<kj::byte>&& data)
: data(kj::mv(data)),
adjustment(js.getExternalMemoryAdjustment(0)) {
adjustment.adjustNow(js, this->data.size());
}
};
kj::OneOf<Owned, kj::ArrayPtr<const kj::byte>> ownedOrView;
kj::Date lastModified;
mutable kj::Maybe<kj::String> maybeUniqueId;

bool isWritable() const {
// Our file is only writable if it owns the actual data buffer.
return ownedOrView.is<kj::Array<kj::byte>>();
return ownedOrView.is<Owned>();
}

kj::ArrayPtr<kj::byte> writableView() {
return KJ_REQUIRE_NONNULL(ownedOrView.tryGet<Owned>()).data.asPtr();
}

kj::Array<kj::byte>& writableView() {
return KJ_REQUIRE_NONNULL(ownedOrView.tryGet<kj::Array<kj::byte>>());
jsg::ExternalMemoryAdjustment& getAdjustment() {
return KJ_REQUIRE_NONNULL(ownedOrView.tryGet<Owned>()).adjustment;
}

kj::ArrayPtr<const kj::byte> readableView() const {
KJ_SWITCH_ONEOF(ownedOrView) {
KJ_CASE_ONEOF(view, kj::ArrayPtr<const kj::byte>) {
return view;
}
KJ_CASE_ONEOF(owned, kj::Array<kj::byte>) {
return owned.asPtr().asConst();
KJ_CASE_ONEOF(owned, Owned) {
return owned.data.asPtr().asConst();
}
}
KJ_UNREACHABLE;
Expand Down Expand Up @@ -1093,8 +1109,7 @@ kj::Rc<File> File::newWritable(jsg::Lock& js, kj::Maybe<uint32_t> size) {
auto actualSize = kj::min(size.orDefault(0), maxSize);
auto data = kj::heapArray<kj::byte>(actualSize);
if (actualSize > 0) data.asPtr().fill(0);
auto owned = data.attach(js.getExternalMemoryAdjustment(actualSize));
return kj::rc<FileImpl>(kj::mv(owned));
return kj::rc<FileImpl>(js, kj::mv(data));
}

kj::Rc<File> File::newReadable(kj::ArrayPtr<const kj::byte> data) {
Expand Down