fix docker logs error handling

Previously, `docker logs` (and by extension, `docker service logs`) had
an error in the way they returned errors that occurred after a stream
had already been started. Specifically, the errors were added verbatim
to the HTTP response, which caused StdCopy to fail with an esoteric
error. This change updates StdCopy to accept errors from the source
stream and then return those errors when copying to the destination
stream.

Signed-off-by: Drew Erny <drew.erny@docker.com>

pkg/stdcopy/stdcopy_test.go

@@ -246,6 +246,35 @@ func TestStdCopyDetectsNotFullyWrittenFrames(t *testing.T) {
 	}
 }
 
+// TestStdCopyReturnsErrorFromSystem tests that StdCopy correctly returns an
+// error, when that error is muxed into the Systemerr stream.
+func TestStdCopyReturnsErrorFromSystem(t *testing.T) {
+	// write in the basic messages, just so there's some fluff in there
+	stdOutBytes := []byte(strings.Repeat("o", startingBufLen))
+	stdErrBytes := []byte(strings.Repeat("e", startingBufLen))
+	buffer, err := getSrcBuffer(stdOutBytes, stdErrBytes)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// add in an error message on the Systemerr stream
+	systemErrBytes := []byte(strings.Repeat("S", startingBufLen))
+	systemWriter := NewStdWriter(buffer, Systemerr)
+	_, err = systemWriter.Write(systemErrBytes)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// now copy and demux. we should expect an error containing the string we
+	// wrote out
+	_, err = StdCopy(ioutil.Discard, ioutil.Discard, buffer)
+	if err == nil {
+		t.Fatal("expected error, got none")
+	}
+	if !strings.Contains(err.Error(), string(systemErrBytes)) {
+		t.Fatal("expected error to contain message")
+	}
+}
+
 func BenchmarkWrite(b *testing.B) {
 	w := NewStdWriter(ioutil.Discard, Stdout)
 	data := []byte("Test line for testing stdwriter performance\n")