Add write support for the GNU tar binary numeric field extension.

R=rsc
APPROVED=rsc
DELTA=102  (89 added, 1 deleted, 12 changed)
OCL=35321
CL=35327

2 files changed, 39 insertions, 0 deletions

diff --git a/src/pkg/testing/iotest/Makefile b/src/pkg/testing/iotest/Makefile
index b223fb932..a37a9d622 100644
--- a/src/pkg/testing/iotest/Makefile
+++ b/src/pkg/testing/iotest/Makefile
@@ -8,5 +8,6 @@ TARG=testing/iotest
 GOFILES=\
 	logger.go\
 	reader.go\
+	writer.go\
 
 include $(GOROOT)/src/Make.pkg
diff --git a/src/pkg/testing/iotest/writer.go b/src/pkg/testing/iotest/writer.go
new file mode 100644
index 000000000..7bd5ddda6
--- /dev/null
+++ b/src/pkg/testing/iotest/writer.go
@@ -0,0 +1,38 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package iotest
+
+import (
+	"io";
+	"os";
+)
+
+// TruncateWriter returns a Writer that writes to w
+// but stops silently after n bytes.
+func TruncateWriter(w io.Writer, n int64) io.Writer {
+	return &truncateWriter{w, n};
+}
+
+type truncateWriter struct {
+	w io.Writer;
+	n int64;
+}
+
+func (t *truncateWriter) Write(p []byte) (n int, err os.Error) {
+	if t.n <= 0 {
+		return len(p), nil
+	}
+	// real write
+	n = len(p);
+	if int64(n) > t.n {
+		n = int(t.n);
+	}
+	n, err = t.w.Write(p[0:n]);
+	t.n -= int64(n);
+	if err == nil {
+		n = len(p);
+	}
+	return
+}