[cdc_rsync] Fix issue in UnzstdStream (#59)

Fixes an issue in UnzstdStream where the Read() method always tries to
read new input data if no input data is available, instead of first
trying to uncompress. Since zstd maintains internal buffers,
uncompression might succeed even without reading more input, so this
is faster. This bug can lead to pipeline stalls in cdc_rsync.

cdc_rsync/base/fake_socket.cc

@@ -39,7 +39,8 @@ absl::Status FakeSocket::Receive(void* buffer, size_t size,
   *bytes_received = 0;
   std::unique_lock<std::mutex> lock(data_mutex_);
   data_cv_.wait(lock, [this, size, allow_partial_read]() {
-    return allow_partial_read || data_.size() >= size || shutdown_;
+    size_t min_size = allow_partial_read ? 1 : size;
+    return data_.size() >= min_size || shutdown_;
   });
   if (shutdown_) {
     return absl::UnavailableError("Pipe is shut down");

cdc_rsync/cdc_rsync_client.cc

@@ -779,9 +779,9 @@ absl::Status CdcRsyncClient::StopCompressionStream() {
   message_pump_.FlushOutgoingQueue();
   message_pump_.RedirectOutput(nullptr);
 
-  // Flush compression stream and reset.
-  RETURN_IF_ERROR(compression_stream_->Flush(),
-                  "Failed to flush compression stream");
+  // Finish compression stream and reset.
+  RETURN_IF_ERROR(compression_stream_->Finish(),
+                  "Failed to finish compression stream");
   compression_stream_.reset();
 
   // Wait for the server ack. This must be done before sending more data.

cdc_rsync/zstd_stream.cc

@@ -27,22 +27,19 @@ namespace {
 // trigger a flush. This happens when files with no changes are diff'ed (this
 // produces very low volume data). Flushing prevents that the server gets stale
 // and becomes overwhelmed later.
-constexpr absl::Duration kMinCompressPeriod = absl::Milliseconds(500);
+constexpr absl::Duration kDefaultAutoFlushPeriod = absl::Milliseconds(500);
 
 }  // namespace
 
 ZstdStream::ZstdStream(Socket* socket, int level, uint32_t num_threads)
-    : socket_(socket), cctx_(nullptr) {
+    : socket_(socket),
+      cctx_(nullptr),
+      auto_flush_period_(kDefaultAutoFlushPeriod) {
   status_ = WrapStatus(Initialize(level, num_threads),
                        "Failed to initialize stream compressor");
 }
 
 ZstdStream::~ZstdStream() {
-  if (cctx_) {
-    ZSTD_freeCCtx(cctx_);
-    cctx_ = nullptr;
-  }
-
   {
     absl::MutexLock lock(&mutex_);
     shutdown_ = true;
@@ -50,6 +47,11 @@ ZstdStream::~ZstdStream() {
   if (compressor_thread_.joinable()) {
     compressor_thread_.join();
   }
+
+  if (cctx_) {
+    ZSTD_freeCCtx(cctx_);
+    cctx_ = nullptr;
+  }
 }
 
 absl::Status ZstdStream::Write(const void* data, size_t size) {
@@ -79,7 +81,7 @@ absl::Status ZstdStream::Write(const void* data, size_t size) {
   return absl::OkStatus();
 }
 
-absl::Status ZstdStream::Flush() {
+absl::Status ZstdStream::Finish() {
   absl::MutexLock lock(&mutex_);
   if (!status_.ok()) return status_;
 
@@ -134,7 +136,7 @@ void ZstdStream::ThreadCompressorMain() {
              in_buffer_.size() == in_buffer_.capacity();
     };
     bool flush =
-        !mutex_.AwaitWithTimeout(absl::Condition(&cond), kMinCompressPeriod);
+        !mutex_.AwaitWithTimeout(absl::Condition(&cond), auto_flush_period_);
     if (shutdown_) {
       return;
     }

cdc_rsync/zstd_stream.h

@@ -36,8 +36,13 @@ class ZstdStream {
   // Sends the given |data| to the compressor.
   absl::Status Write(const void* data, size_t size) ABSL_LOCKS_EXCLUDED(mutex_);
 
-  // Flushes all remaining data and sends the compressed data to the socket.
-  absl::Status Flush() ABSL_LOCKS_EXCLUDED(mutex_);
+  // Finishes the stream and flushes all remaining data.
+  absl::Status Finish() ABSL_LOCKS_EXCLUDED(mutex_);
+
+  // Flushes internal buffers if no new data is written for longer than this
+  // time. This makes sure that no data is stuck in the pipeline if no new input
+  // is available. Default is 500 ms.
+  void AutoFlushAfter(absl::Duration dur) { auto_flush_period_ = dur; }
 
  private:
   // Initializes the compressor and related data.
@@ -58,6 +63,8 @@ class ZstdStream {
   bool last_chunk_sent_ ABSL_GUARDED_BY(mutex_) = false;
   absl::Status status_ ABSL_GUARDED_BY(mutex_);
   std::thread compressor_thread_;
+
+  absl::Duration auto_flush_period_;
 };
 
 }  // namespace cdc_ft

cdc_rsync/zstd_stream_test.cc

@@ -32,7 +32,7 @@ class ZstdStreamTest : public ::testing::Test {
 TEST_F(ZstdStreamTest, Small) {
   const std::string want = "Lorem ipsum gibberisulum foobarberis";
   EXPECT_OK(cstream_.Write(want.data(), want.size()));
-  EXPECT_OK(cstream_.Flush());
+  EXPECT_OK(cstream_.Finish());
 
   Buffer buff(1024);
   size_t bytes_read;
@@ -43,6 +43,52 @@ TEST_F(ZstdStreamTest, Small) {
   EXPECT_EQ(got, want);
 }
 
+TEST_F(ZstdStreamTest, AutoFlushesAfterTimeout) {
+  const std::string want = "Lorem ipsum gibberisulum foobarberis";
+  cstream_.AutoFlushAfter(absl::Milliseconds(10));
+  EXPECT_OK(cstream_.Write(want.data(), want.size()));
+  // Note: No flush! cstream_ will compress and send the data after 10 ms.
+
+  // Only read as much data as we have written, or else dstream_.Read() will
+  // expect more data.
+  Buffer buff(want.size());
+  size_t bytes_read;
+  bool eof = false;
+  EXPECT_OK(dstream_.Read(buff.data(), buff.size(), &bytes_read, &eof));
+  EXPECT_FALSE(eof);
+  std::string got(buff.data(), bytes_read);
+  EXPECT_EQ(got, want);
+}
+
+// Regression test for an issue in UnzstdStream, where the reader tried to read
+// from the socket even though the output data was still available in internal
+// buffers.
+TEST_F(ZstdStreamTest, DeliversOutputBeforeReadingNewData) {
+  const std::string want1 = "I want";
+  const std::string want2 = "to eat cookies";
+  cstream_.AutoFlushAfter(absl::Milliseconds(10));
+  EXPECT_OK(cstream_.Write(want1.data(), want1.size()));
+  EXPECT_OK(cstream_.Write(want2.data(), want2.size()));
+  // Note: No flush! cstream_ will compress and send the data after 10 ms.
+
+  Buffer buff1(want1.size());
+  Buffer buff2(want2.size());
+  size_t bytes_read1, bytes_read2;
+  bool eof1 = false, eof2 = false;
+  EXPECT_OK(dstream_.Read(buff1.data(), buff1.size(), &bytes_read1, &eof1));
+
+  // There was a bug in dstream_.Read(), where the method would first try to
+  // read new input before uncompressing data, even though the data was already
+  // present in internal buffers.
+  EXPECT_OK(dstream_.Read(buff2.data(), buff2.size(), &bytes_read2, &eof2));
+  EXPECT_FALSE(eof1);
+  EXPECT_FALSE(eof2);
+  std::string got1(buff1.data(), bytes_read1);
+  std::string got2(buff2.data(), bytes_read2);
+  EXPECT_EQ(got1, want1);
+  EXPECT_EQ(got2, want2);
+}
+
 TEST_F(ZstdStreamTest, Large) {
   Buffer want(1024 * 1024 * 10 + 12345);
   constexpr uint64_t prime = 919393;
@@ -55,7 +101,7 @@ TEST_F(ZstdStreamTest, Large) {
     size_t size = std::min<size_t>(kChunkSize, want.size() - pos);
     EXPECT_OK(cstream_.Write(want.data() + pos, size));
   }
-  EXPECT_OK(cstream_.Flush());
+  EXPECT_OK(cstream_.Finish());
 
   bool eof = false;
   Buffer buff(128 * 1024);