[bytestream] be careful not to hang goroutines writing to putResult

If this goroutine blocks trying to write to putResult after Write
exits then it will leak.

We can avoid this by making putResult a buffered channel with
length 1, since a Write call will send to this channel at most
once. Then that single send will never block, and the goroutine
can exit.

While we're at it, let's make recvResult buffered too, just in case.

Fixes #444.

server/grpc_bytestream.go

@@ -234,8 +234,8 @@ func (s *grpcServer) Write(srv bytestream.ByteStream_WriteServer) error {
 	var resp bytestream.WriteResponse
 	pr, pw := io.Pipe()
 
-	putResult := make(chan error)
-	recvResult := make(chan error)
+	putResult := make(chan error, 1)
+	recvResult := make(chan error, 1)
 	resourceNameChan := make(chan string, 1)
 
 	go func() {
@@ -297,7 +297,8 @@ func (s *grpcServer) Write(srv bytestream.ByteStream_WriteServer) error {
 				}
 
 				go func() {
-					putResult <- s.cache.Put(cache.CAS, hash, size, pr)
+					err := s.cache.Put(cache.CAS, hash, size, pr)
+					putResult <- err
 				}()
 
 				firstIteration = false