types: add NewOnceCloser, use it in camget

Change-Id: I465207031468a57f2840d5071fc27713d6184325

cmd/camget/camget.go

@@ -35,6 +35,7 @@ import (
 	"camlistore.org/pkg/index"
 	"camlistore.org/pkg/osutil"
 	"camlistore.org/pkg/schema"
+	"camlistore.org/pkg/types"
 )
 
 var (
@@ -172,7 +173,8 @@ func smartFetch(src blob.Fetcher, targ string, br blob.Ref) error {
 	if err != nil {
 		return err
 	}
-	defer rc.Close()
+	rcc := types.NewOnceCloser(rc)
+	defer rcc.Close()
 
 	sniffer := index.NewBlobSniffer(br)
 	_, err = io.CopyN(sniffer, rc, sniffSize)
@@ -199,7 +201,7 @@ func smartFetch(src blob.Fetcher, targ string, br blob.Ref) error {
 		_, err = io.Copy(f, r)
 		return err
 	}
-	rc.Close()
+	rcc.Close()
 
 	switch b.Type() {
 	case "directory":

pkg/types/types.go

@@ -28,6 +28,7 @@ import (
 	"regexp"
 	"runtime"
 	"strings"
+	"sync"
 	"time"
 )
 
@@ -213,6 +214,28 @@ func U32(n int64) uint32 {
 	return uint32(n)
 }
 
+// NewOnceCloser returns a Closer wrapping c which only calls Close on c
+// once. Subsequent calls to Close return nil.
+func NewOnceCloser(c io.Closer) io.Closer {
+	return &onceCloser{c: c}
+}
+
+type onceCloser struct {
+	mu sync.Mutex
+	c  io.Closer
+}
+
+func (c *onceCloser) Close() error {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if c.c == nil {
+		return nil
+	}
+	err := c.c.Close()
+	c.c = nil
+	return err
+}
+
 // TB is a copy of Go 1.2's testing.TB.
 type TB interface {
 	Error(args ...interface{})