archive/tar: catch short writes.

Also make error messages consistent throughout.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5777064

src/pkg/archive/tar/reader.go

@@ -18,7 +18,7 @@ import (
 )
 
 var (
-	ErrHeader = errors.New("invalid tar header")
+	ErrHeader = errors.New("archive/tar: invalid tar header")
 )
 
 // A Reader provides sequential access to the contents of a tar archive.

src/pkg/archive/tar/writer.go

@@ -5,18 +5,19 @@
 package tar
 
 // TODO(dsymonds):
-// - catch more errors (no first header, write after close, etc.)
+// - catch more errors (no first header, etc.)
 
 import (
 	"errors"
+	"fmt"
 	"io"
 	"strconv"
 )
 
 var (
-	ErrWriteTooLong    = errors.New("write too long")
-	ErrFieldTooLong    = errors.New("header field too long")
-	ErrWriteAfterClose = errors.New("write after close")
+	ErrWriteTooLong    = errors.New("archive/tar: write too long")
+	ErrFieldTooLong    = errors.New("archive/tar: header field too long")
+	ErrWriteAfterClose = errors.New("archive/tar: write after close")
 )
 
 // A Writer provides sequential writing of a tar archive in POSIX.1 format.
@@ -48,6 +49,11 @@ func NewWriter(w io.Writer) *Writer { return &Writer{w: w} }
 
 // Flush finishes writing the current file (optional).
 func (tw *Writer) Flush() error {
+	if tw.nb > 0 {
+		tw.err = fmt.Errorf("archive/tar: missed writing %d bytes", tw.nb)
+		return tw.err
+	}
+
 	n := tw.nb + tw.pad
 	for n > 0 && tw.err == nil {
 		nr := n
@@ -193,6 +199,9 @@ func (tw *Writer) Close() error {
 	}
 	tw.Flush()
 	tw.closed = true
+	if tw.err != nil {
+		return tw.err
+	}
 
 	// trailer: two zero blocks
 	for i := 0; i < 2; i++ {

src/pkg/archive/tar/writer_test.go

@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
+	"strings"
 	"testing"
 	"testing/iotest"
 	"time"
@@ -95,7 +96,8 @@ var writerTests = []*writerTest{
 					Uname:    "dsymonds",
 					Gname:    "eng",
 				},
-				// no contents
+				// fake contents
+				contents: strings.Repeat("\x00", 4<<10),
 			},
 		},
 	},
@@ -150,7 +152,9 @@ testLoop:
 
 		buf := new(bytes.Buffer)
 		tw := NewWriter(iotest.TruncateWriter(buf, 4<<10)) // only catch the first 4 KB
+		big := false
 		for j, entry := range test.entries {
+			big = big || entry.header.Size > 1<<10
 			if err := tw.WriteHeader(entry.header); err != nil {
 				t.Errorf("test %d, entry %d: Failed writing header: %v", i, j, err)
 				continue testLoop
@@ -160,7 +164,8 @@ testLoop:
 				continue testLoop
 			}
 		}
-		if err := tw.Close(); err != nil {
+		// Only interested in Close failures for the small tests.
+		if err := tw.Close(); err != nil && !big {
 			t.Errorf("test %d: Failed closing archive: %v", i, err)
 			continue testLoop
 		}