[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_server/unzstd_stream.cc

@@ -41,20 +41,6 @@ absl::Status UnzstdStream::Read(void* out_buffer, size_t out_size,
 
   ZSTD_outBuffer output = {out_buffer, out_size, 0};
   while (output.pos < output.size && !*eof) {
-    if (input_.pos == input_.size) {
-      // Read more compressed input data.
-      // Allow partial reads since the stream could end any time.
-      size_t in_size;
-      absl::Status status =
-          socket_->Receive(in_buffer_.data(), in_buffer_.size(),
-                           /*allow_partial_read=*/true, &in_size);
-      if (!status.ok()) {
-        return WrapStatus(status, "socket_->ReceiveEx() failed");
-      }
-      input_.pos = 0;
-      input_.size = in_size;
-    }
-
     // Decompress.
     size_t ret = ZSTD_decompressStream(dctx_, &output, &input_);
     if (ZSTD_isError(ret)) {
@@ -67,6 +53,20 @@ absl::Status UnzstdStream::Read(void* out_buffer, size_t out_size,
       return MakeStatus("EOF with %u bytes input data available",
                         input_.size - input_.pos);
     }
+
+    if (input_.pos == input_.size && output.pos < output.size && !*eof) {
+      // Read more compressed input data.
+      // Allow partial reads since the stream could end any time.
+      size_t in_size;
+      absl::Status status =
+          socket_->Receive(in_buffer_.data(), in_buffer_.size(),
+                           /*allow_partial_read=*/true, &in_size);
+      if (!status.ok()) {
+        return WrapStatus(status, "socket_->ReceiveEx() failed");
+      }
+      input_.pos = 0;
+      input_.size = in_size;
+    }
   }
 
   // Output buffer is full or eof.