Add StdCopy happy path test: both reading and writing

Signed-off-by: Sergey Evstifeev <sergey.evstifeev@gmail.com>

pkg/stdcopy/stdcopy.go

@@ -12,6 +12,8 @@ const (
 	stdWriterPrefixLen = 8
 	stdWriterFdIndex   = 0
 	stdWriterSizeIndex = 4
+
+	startingBufLen = 32*1024 + stdWriterPrefixLen + 1
 )
 
 // StdType prefixes type and length to standard stream.
@@ -80,7 +82,7 @@ var errInvalidStdHeader = errors.New("Unrecognized input header")
 // `written` will hold the total number of bytes written to `dstout` and `dsterr`.
 func StdCopy(dstout, dsterr io.Writer, src io.Reader) (written int64, err error) {
 	var (
-		buf       = make([]byte, 32*1024+stdWriterPrefixLen+1)
+		buf       = make([]byte, startingBufLen)
 		bufLen    = len(buf)
 		nr, nw    int
 		er, ew    error

pkg/stdcopy/stdcopy_test.go

@@ -85,6 +85,30 @@ func TestWriteDoesNotReturnNegativeWrittenBytes(t *testing.T) {
 	}
 }
 
+func TestStdCopyWriteAndRead(t *testing.T) {
+	buffer := new(bytes.Buffer)
+	stdOutBytes := []byte(strings.Repeat("o", startingBufLen))
+	dstOut := NewStdWriter(buffer, Stdout)
+	_, err := dstOut.Write(stdOutBytes)
+	if err != nil {
+		t.Fatal(err)
+	}
+	stdErrBytes := []byte(strings.Repeat("e", startingBufLen))
+	dstErr := NewStdWriter(buffer, Stderr)
+	_, err = dstErr.Write(stdErrBytes)
+	if err != nil {
+		t.Fatal(err)
+	}
+	written, err := StdCopy(ioutil.Discard, ioutil.Discard, buffer)
+	if err != nil {
+		t.Fatal(err)
+	}
+	expectedTotalWritten := len(stdOutBytes) + len(stdErrBytes)
+	if written != int64(expectedTotalWritten) {
+		t.Fatalf("Expected to have total of %d bytes written, got %d", expectedTotalWritten, written)
+	}
+}
+
 func TestStdCopyWithInvalidInputHeader(t *testing.T) {
 	dstOut := NewStdWriter(ioutil.Discard, Stdout)
 	dstErr := NewStdWriter(ioutil.Discard, Stderr)