Fix env.WriteTo count return

Some calls like json.Encoder.Encode mask the number of bytes written to
an io.Writer. The solution provides a wrapper io.Writer around the
actual io.Writer that allows multiple calls to Write to be considered as
one and allow access to this count.

Signed-off-by: Pierre Wacrenier <pierre.wacrenier@gmail.com>

engine/env.go

@@ -7,6 +7,8 @@ import (
 	"io"
 	"strconv"
 	"strings"
+
+	"github.com/docker/docker/utils"
 )
 
 type Env []string
@@ -242,9 +244,10 @@ func (env *Env) Encode(dst io.Writer) error {
 	return nil
 }
 
-func (env *Env) WriteTo(dst io.Writer) (n int64, err error) {
-	// FIXME: return the number of bytes written to respect io.WriterTo
-	return 0, env.Encode(dst)
+func (env *Env) WriteTo(dst io.Writer) (int64, error) {
+	wc := utils.NewWriteCounter(dst)
+	err := env.Encode(wc)
+	return wc.Count, err
 }
 
 func (env *Env) Import(src interface{}) (err error) {

utils/utils.go

@@ -516,3 +516,24 @@ func ReadDockerIgnore(path string) ([]string, error) {
 	}
 	return excludes, nil
 }
+
+// Wrap a concrete io.Writer and hold a count of the number
+// of bytes written to the writer during a "session".
+// This can be convenient when write return is masked
+// (e.g., json.Encoder.Encode())
+type WriteCounter struct {
+	Count  int64
+	Writer io.Writer
+}
+
+func NewWriteCounter(w io.Writer) *WriteCounter {
+	return &WriteCounter{
+		Writer: w,
+	}
+}
+
+func (wc *WriteCounter) Write(p []byte) (count int, err error) {
+	count, err = wc.Writer.Write(p)
+	wc.Count += int64(count)
+	return
+}

utils/utils_test.go

@@ -1,7 +1,9 @@
 package utils
 
 import (
+	"bytes"
 	"os"
+	"strings"
 	"testing"
 )
 
@@ -97,3 +99,26 @@ func TestReadSymlinkedDirectoryToFile(t *testing.T) {
 		t.Errorf("failed to remove symlink: %s", err)
 	}
 }
+
+func TestWriteCounter(t *testing.T) {
+	dummy1 := "This is a dummy string."
+	dummy2 := "This is another dummy string."
+	totalLength := int64(len(dummy1) + len(dummy2))
+
+	reader1 := strings.NewReader(dummy1)
+	reader2 := strings.NewReader(dummy2)
+
+	var buffer bytes.Buffer
+	wc := NewWriteCounter(&buffer)
+
+	reader1.WriteTo(wc)
+	reader2.WriteTo(wc)
+
+	if wc.Count != totalLength {
+		t.Errorf("Wrong count: %d vs. %d", wc.Count, totalLength)
+	}
+
+	if buffer.String() != dummy1+dummy2 {
+		t.Error("Wrong message written")
+	}
+}