src: release memory for zstd contexts in Close()

This aligns zstd streams with other compression libraries in this
regard, and enables releasing memory early when the stream ends in
JS instead of waiting for GC to clean up the wrapper object (which
is a problem that is exacerbated in the zstd context because we
do not track memory and report memory pressure to V8 yet).

Author: addaleax

src/node_zlib.cc

@@ -298,7 +298,6 @@ class ZstdContext : public MemoryRetainer {
   ZstdContext() = default;
 
   // Streaming-related, should be available for all compression libraries:
-  void Close();
   void SetBuffers(const char* in, uint32_t in_len, char* out, uint32_t out_len);
   void SetFlush(int flush);
   void GetAfterWriteOffsets(uint32_t* avail_in, uint32_t* avail_out) const;
@@ -323,6 +322,7 @@ class ZstdCompressContext final : public ZstdContext {
   ZstdCompressContext() = default;
 
   // Streaming-related, should be available for all compression libraries:
+  void Close();
   void DoThreadPoolWork();
   CompressionError ResetStream();
 
@@ -349,6 +349,7 @@ class ZstdDecompressContext final : public ZstdContext {
   ZstdDecompressContext() = default;
 
   // Streaming-related, should be available for all compression libraries:
+  void Close();
   void DoThreadPoolWork();
   CompressionError ResetStream();
 
@@ -1488,8 +1489,6 @@ CompressionError BrotliDecoderContext::GetErrorInfo() const {
   }
 }
 
-void ZstdContext::Close() {}
-
 void ZstdContext::SetBuffers(const char* in,
                              uint32_t in_len,
                              char* out,
@@ -1533,6 +1532,10 @@ CompressionError ZstdCompressContext::SetParameter(int key, int value) {
   return {};
 }
 
+void ZstdCompressContext::Close() {
+  cctx_.reset();
+}
+
 CompressionError ZstdCompressContext::Init(uint64_t pledged_src_size,
                                            std::string_view dictionary) {
   pledged_src_size_ = pledged_src_size;
@@ -1585,6 +1588,10 @@ CompressionError ZstdDecompressContext::SetParameter(int key, int value) {
   return {};
 }
 
+void ZstdDecompressContext::Close() {
+  dctx_.reset();
+}
+
 CompressionError ZstdDecompressContext::Init(uint64_t pledged_src_size,
                                              std::string_view dictionary) {
   dctx_.reset(ZSTD_createDCtx());