[bytestream] drain any remaining data after Put returns

io.PipeWriter Write calls block until all the data written in the call
is read from the corresponding io.PipeReader. If we don't read all that
data, then the writing goroutine will block forever.

This PipeWriter is intended to be consumed by disk.Put(), but if that
returns early then there will be blocked writes. To un-block them, we
can ensure that any remaining data is drained after disk.Put returns.

This might fix buchgr#473

server/grpc_bytestream.go

@@ -447,6 +447,7 @@ func (s *grpcServer) Write(srv bytestream.ByteStream_WriteServer) error {
 
 				go func() {
 					err := s.cache.Put(cache.CAS, hash, size, rc)
+					_, _ = io.Copy(io.Discard, rc) // Ensure that the writing goroutine is unblocked.
 					putResult <- err
 				}()
 
@@ -460,7 +461,7 @@ func (s *grpcServer) Write(srv bytestream.ByteStream_WriteServer) error {
 				}
 			}
 
-			n, err := pw.Write(req.Data)
+			n, err := pw.Write(req.Data) // This blocks until all the data written here is read from pr.
 			if err != nil {
 				recvResult <- status.Error(codes.Internal, err.Error())
 				return