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node:zlib: account Zstd native allocations against external memory #6210

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dknecht wants to merge 1 commit into from
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node:zlib: account Zstd native allocations against external memory #6210

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31 changes: 25 additions & 6 deletions src/workerd/api/node/zlib-util.c++
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Original file line number Diff line number Diff line change
Expand Up @@ -812,10 +812,19 @@ void zstdFreeDCtx(ZSTD_DCtx* dctx) {
} // namespace

ZstdEncoderContext::ZstdEncoderContext(ZlibMode _mode)
: ZstdContext(_mode),
cctx_(kj::disposeWith<zstdFreeCCtx>(ZSTD_createCCtx())) {}
: ZstdContext(_mode) {}

kj::Maybe<CompressionError> ZstdEncoderContext::initialize(uint64_t pledgedSrcSize) {
KJ_DASSERT(allocator_ != nullptr, "Zstd encoder allocator should not be null");
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@ask-bonk ask-bonk Bot Feb 27, 2026

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KJ_DASSERT is stripped in release builds. If allocator_ is ever null here (e.g. a future caller forgets setAllocator), this will silently pass through and crash inside the ZSTD allocation callback when it dereferences the null opaque pointer. Use KJ_ASSERT so the invariant is enforced in all build modes:

Suggested change
KJ_DASSERT(allocator_ != nullptr, "Zstd encoder allocator should not be null");
KJ_ASSERT(allocator_ != nullptr, "Zstd encoder allocator should not be null");

if (cctx_.get() == nullptr) {
ZSTD_customMem customMem = {
.customAlloc = CompressionAllocator::AllocForBrotli,
.customFree = CompressionAllocator::FreeForZlib,
.opaque = allocator_,
};
cctx_ = kj::disposeWith<zstdFreeCCtx>(ZSTD_createCCtx_advanced(customMem));
}

if (cctx_.get() == nullptr) {
return CompressionError(
"Could not initialize Zstd instance"_kj, "ERR_ZLIB_INITIALIZATION_FAILED"_kj, -1);
Expand Down Expand Up @@ -878,12 +887,19 @@ kj::Maybe<CompressionError> ZstdEncoderContext::getError() const {
}

ZstdDecoderContext::ZstdDecoderContext(ZlibMode _mode)
: ZstdContext(_mode),
dctx_(kj::disposeWith<zstdFreeDCtx>(ZSTD_createDCtx())) {}
: ZstdContext(_mode) {}

kj::Maybe<CompressionError> ZstdDecoderContext::initialize() {
// dctx_ is created in the constructor. It can only be nullptr if ZSTD_createDCtx()
// failed due to memory allocation failure.
KJ_DASSERT(allocator_ != nullptr, "Zstd decoder allocator should not be null");
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@ask-bonk ask-bonk Bot Feb 27, 2026

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Same issue as the encoder path — KJ_DASSERT is a no-op in release builds:

Suggested change
KJ_DASSERT(allocator_ != nullptr, "Zstd decoder allocator should not be null");
KJ_ASSERT(allocator_ != nullptr, "Zstd decoder allocator should not be null");

if (dctx_.get() == nullptr) {
ZSTD_customMem customMem = {
.customAlloc = CompressionAllocator::AllocForBrotli,
.customFree = CompressionAllocator::FreeForZlib,
.opaque = allocator_,
};
dctx_ = kj::disposeWith<zstdFreeDCtx>(ZSTD_createDCtx_advanced(customMem));
}

if (dctx_.get() == nullptr) {
return CompressionError(
"Could not initialize Zstd instance"_kj, "ERR_ZLIB_INITIALIZATION_FAILED"_kj, -1);
Expand Down Expand Up @@ -957,6 +973,7 @@ bool ZlibUtil::ZstdCompressionStream<CompressionContext>::initialize(jsg::Lock&
jsg::Function<void()> writeCallback,
jsg::Optional<uint64_t> pledgedSrcSize) {
this->initializeStream(kj::mv(writeResult), kj::mv(writeCallback));
this->context()->setAllocator(&this->allocator);

uint64_t srcSize = pledgedSrcSize.orDefault(ZSTD_CONTENTSIZE_UNKNOWN);

Expand Down Expand Up @@ -1168,7 +1185,9 @@ void ZlibUtil::brotliWithCallback(

template <typename Context>
kj::Array<kj::byte> ZlibUtil::zstdSync(jsg::Lock& js, InputSource data, ZstdContext::Options opts) {
CompressionAllocator allocator(js.getExternalMemoryTarget());
Context ctx(Context::Mode);
ctx.setAllocator(&allocator);

auto chunkSize = opts.chunkSize.orDefault(ZLIB_PERFORMANT_CHUNK_SIZE);
auto maxOutputLength = opts.maxOutputLength.orDefault(Z_MAX_CHUNK);
Expand Down
5 changes: 5 additions & 0 deletions src/workerd/api/node/zlib-util.h
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Original file line number Diff line number Diff line change
Expand Up @@ -292,6 +292,10 @@ class ZstdContext {
explicit ZstdContext(ZlibMode _mode): mode(_mode) {}
KJ_DISALLOW_COPY(ZstdContext);

void setAllocator(CompressionAllocator* allocator) {
allocator_ = allocator;
}

void setBuffers(kj::ArrayPtr<kj::byte> input, kj::ArrayPtr<kj::byte> output);
void setInputBuffer(kj::ArrayPtr<const kj::byte> input);
void setOutputBuffer(kj::ArrayPtr<kj::byte> output);
Expand All @@ -313,6 +317,7 @@ class ZstdContext {
};

protected:
CompressionAllocator* allocator_ = nullptr;
ZlibMode mode;
ZSTD_inBuffer input_{nullptr, 0, 0};
ZSTD_outBuffer output_{nullptr, 0, 0};
Expand Down
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