Add container attach support.

Signed-off-by: Lantao Liu <lantaol@google.com>

pkg/ioutil/writer_group.go

@@ -0,0 +1,103 @@
+/*
+Copyright 2017 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package ioutil
+
+import (
+	"errors"
+	"io"
+	"sync"
+
+	"github.com/golang/glog"
+)
+
+// WriterGroup is a group of writers. Writer could be dynamically
+// added and removed.
+type WriterGroup struct {
+	mu      sync.Mutex
+	writers map[string]io.WriteCloser
+	closed  bool
+}
+
+var _ io.Writer = &WriterGroup{}
+
+// NewWriterGroup creates an empty writer group.
+func NewWriterGroup() *WriterGroup {
+	return &WriterGroup{
+		writers: make(map[string]io.WriteCloser),
+	}
+}
+
+// Add adds a writer into the group, returns an error when writer
+// group is closed.
+func (g *WriterGroup) Add(key string, w io.WriteCloser) error {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	if g.closed {
+		return errors.New("wait group closed")
+	}
+	g.writers[key] = w
+	return nil
+}
+
+// Remove removes a writer from the group.
+func (g *WriterGroup) Remove(key string) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	w, ok := g.writers[key]
+	if !ok {
+		return
+	}
+	w.Close()
+	delete(g.writers, key)
+}
+
+// Write writes data into each writer. If a writer returns error,
+// it will be closed and removed from the writer group. It returns
+// error if writer group is empty.
+func (g *WriterGroup) Write(p []byte) (int, error) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	for k, w := range g.writers {
+		n, err := w.Write(p)
+		if err != nil {
+			glog.Errorf("Writer %q write error: %v", k, err)
+		} else if len(p) != n {
+			glog.Errorf("Writer %q short write error", k)
+		} else {
+			continue
+		}
+		// The writer is closed or in bad state, remove it.
+		w.Close()
+		delete(g.writers, k)
+	}
+	if len(g.writers) == 0 {
+		return 0, errors.New("writer group is empty")
+	}
+	return len(p), nil
+}
+
+// Close closes the writer group. Write or Add will return error
+// after closed.
+func (g *WriterGroup) Close() {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	for _, w := range g.writers {
+		w.Close()
+	}
+	g.writers = nil
+	g.closed = true
+}