Move types from progressreader and broadcastwriter to broadcaster

progressreader.Broadcaster becomes broadcaster.Buffered and
broadcastwriter.Writer becomes broadcaster.Unbuffered.

The package broadcastwriter is thus renamed to broadcaster.

Signed-off-by: Tibor Vass <tibor@docker.com>

broadcaster/unbuffered.go

@@ -0,0 +1,49 @@
+package broadcaster
+
+import (
+	"io"
+	"sync"
+)
+
+// Unbuffered accumulates multiple io.WriteCloser by stream.
+type Unbuffered struct {
+	mu      sync.Mutex
+	writers []io.WriteCloser
+}
+
+// Add adds new io.WriteCloser.
+func (w *Unbuffered) Add(writer io.WriteCloser) {
+	w.mu.Lock()
+	w.writers = append(w.writers, writer)
+	w.mu.Unlock()
+}
+
+// Write writes bytes to all writers. Failed writers will be evicted during
+// this call.
+func (w *Unbuffered) Write(p []byte) (n int, err error) {
+	w.mu.Lock()
+	var evict []int
+	for i, sw := range w.writers {
+		if n, err := sw.Write(p); err != nil || n != len(p) {
+			// On error, evict the writer
+			evict = append(evict, i)
+		}
+	}
+	for n, i := range evict {
+		w.writers = append(w.writers[:i-n], w.writers[i-n+1:]...)
+	}
+	w.mu.Unlock()
+	return len(p), nil
+}
+
+// Clean closes and removes all writers. Last non-eol-terminated part of data
+// will be saved.
+func (w *Unbuffered) Clean() error {
+	w.mu.Lock()
+	for _, sw := range w.writers {
+		sw.Close()
+	}
+	w.writers = nil
+	w.mu.Unlock()
+	return nil
+}