Added tests for StdWriter.Write.

The tests added cover the case when the Writer field returns and error
and, related to that, when the number of written bytes is less than 0.

Signed-off-by: Federico Gimenez <fgimenez@coit.es>

stdcopy/stdcopy_test.go

@@ -2,6 +2,7 @@ package stdcopy
 
 import (
 	"bytes"
+	"errors"
 	"io/ioutil"
 	"strings"
 	"testing"
@@ -49,6 +50,41 @@ func TestWrite(t *testing.T) {
 	}
 }
 
+type errWriter struct {
+	n   int
+	err error
+}
+
+func (f *errWriter) Write(buf []byte) (int, error) {
+	return f.n, f.err
+}
+
+func TestWriteWithWriterError(t *testing.T) {
+	expectedError := errors.New("expected")
+	expectedReturnedBytes := 10
+	writer := NewStdWriter(&errWriter{
+		n:   stdWriterPrefixLen + expectedReturnedBytes,
+		err: expectedError}, Stdout)
+	data := []byte("This won't get written, sigh")
+	n, err := writer.Write(data)
+	if err != expectedError {
+		t.Fatalf("Didn't get expected error.")
+	}
+	if n != expectedReturnedBytes {
+		t.Fatalf("Didn't get expected writen bytes %d, got %d.",
+			expectedReturnedBytes, n)
+	}
+}
+
+func TestWriteDoesNotReturnNegativeWrittenBytes(t *testing.T) {
+	writer := NewStdWriter(&errWriter{n: -1}, Stdout)
+	data := []byte("This won't get written, sigh")
+	actual, _ := writer.Write(data)
+	if actual != 0 {
+		t.Fatalf("Expected returned written bytes equal to 0, got %d", actual)
+	}
+}
+
 func TestStdCopyWithInvalidInputHeader(t *testing.T) {
 	dstOut := NewStdWriter(ioutil.Discard, Stdout)
 	dstErr := NewStdWriter(ioutil.Discard, Stderr)