archive/tar: avoid empty IO operations

The interfaces for io.Reader and io.Writer permit calling Read/Write
with an empty buffer. However, this condition is often not well tested
and can lead to bugs in various implementations of io.Reader and io.Writer.
For example, see #22028 for buggy io.Reader in the bzip2 package.

We reduce the likelihood of hitting these bugs by adjusting
regFileReader.Read and regFileWriter.Write to avoid performing
Read and Write calls when the buffer is known to be empty.

Fixes #22029

Change-Id: Ie4a26be53cf87bc4d2abd951fa005db5871cc75c
Reviewed-on: https://go-review.googlesource.com/66111
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Giovanni Bajo <rasky@develer.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/archive/tar/reader.go

@@ -649,12 +649,14 @@ type regFileReader struct {
 	nb int64     // Number of remaining bytes to read
 }
 
-func (fr *regFileReader) Read(b []byte) (int, error) {
+func (fr *regFileReader) Read(b []byte) (n int, err error) {
 	if int64(len(b)) > fr.nb {
 		b = b[:fr.nb]
 	}
-	n, err := fr.r.Read(b)
-	fr.nb -= int64(n)
+	if len(b) > 0 {
+		n, err = fr.r.Read(b)
+		fr.nb -= int64(n)
+	}
 	switch {
 	case err == io.EOF && fr.nb > 0:
 		return n, io.ErrUnexpectedEOF

src/archive/tar/reader_test.go

@@ -7,6 +7,7 @@ package tar
 import (
 	"bytes"
 	"crypto/md5"
+	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -1395,6 +1396,17 @@ func TestReadGNUSparsePAXHeaders(t *testing.T) {
 	}
 }
 
+// testNonEmptyReader wraps an io.Reader and ensures that
+// Read is never called with an empty buffer.
+type testNonEmptyReader struct{ io.Reader }
+
+func (r testNonEmptyReader) Read(b []byte) (int, error) {
+	if len(b) == 0 {
+		return 0, errors.New("unexpected empty Read call")
+	}
+	return r.Reader.Read(b)
+}
+
 func TestFileReader(t *testing.T) {
 	type (
 		testRead struct { // Read(cnt) == (wantStr, wantErr)
@@ -1443,7 +1455,6 @@ func TestFileReader(t *testing.T) {
 		maker: makeReg{"", 1},
 		tests: []testFnc{
 			testRemaining{1, 1},
-			testRead{0, "", io.ErrUnexpectedEOF},
 			testRead{5, "", io.ErrUnexpectedEOF},
 			testWriteTo{nil, 0, io.ErrUnexpectedEOF},
 			testRemaining{1, 1},
@@ -1611,14 +1622,14 @@ func TestFileReader(t *testing.T) {
 		var fr fileReader
 		switch maker := v.maker.(type) {
 		case makeReg:
-			r := strings.NewReader(maker.str)
+			r := testNonEmptyReader{strings.NewReader(maker.str)}
 			fr = &regFileReader{r, maker.size}
 		case makeSparse:
 			if !validateSparseEntries(maker.spd, maker.size) {
 				t.Fatalf("invalid sparse map: %v", maker.spd)
 			}
 			sph := invertSparseEntries(maker.spd, maker.size)
-			r := strings.NewReader(maker.makeReg.str)
+			r := testNonEmptyReader{strings.NewReader(maker.makeReg.str)}
 			fr = &regFileReader{r, maker.makeReg.size}
 			fr = &sparseFileReader{fr, sph, 0}
 		default:

src/archive/tar/writer.go

@@ -452,13 +452,15 @@ type regFileWriter struct {
 	nb int64     // Number of remaining bytes to write
 }
 
-func (fw *regFileWriter) Write(b []byte) (int, error) {
+func (fw *regFileWriter) Write(b []byte) (n int, err error) {
 	overwrite := int64(len(b)) > fw.nb
 	if overwrite {
 		b = b[:fw.nb]
 	}
-	n, err := fw.w.Write(b)
-	fw.nb -= int64(n)
+	if len(b) > 0 {
+		n, err = fw.w.Write(b)
+		fw.nb -= int64(n)
+	}
 	switch {
 	case err != nil:
 		return n, err

src/archive/tar/writer_test.go

@@ -7,6 +7,7 @@ package tar
 import (
 	"bytes"
 	"encoding/hex"
+	"errors"
 	"io"
 	"io/ioutil"
 	"os"
@@ -987,6 +988,17 @@ func TestIssue12594(t *testing.T) {
 	}
 }
 
+// testNonEmptyWriter wraps an io.Writer and ensures that
+// Write is never called with an empty buffer.
+type testNonEmptyWriter struct{ io.Writer }
+
+func (w testNonEmptyWriter) Write(b []byte) (int, error) {
+	if len(b) == 0 {
+		return 0, errors.New("unexpected empty Write call")
+	}
+	return w.Writer.Write(b)
+}
+
 func TestFileWriter(t *testing.T) {
 	type (
 		testWrite struct { // Write(str) == (wantCnt, wantErr)
@@ -1225,17 +1237,18 @@ func TestFileWriter(t *testing.T) {
 	for i, v := range vectors {
 		var wantStr string
 		bb := new(bytes.Buffer)
+		w := testNonEmptyWriter{bb}
 		var fw fileWriter
 		switch maker := v.maker.(type) {
 		case makeReg:
-			fw = &regFileWriter{bb, maker.size}
+			fw = &regFileWriter{w, maker.size}
 			wantStr = maker.wantStr
 		case makeSparse:
 			if !validateSparseEntries(maker.sph, maker.size) {
 				t.Fatalf("invalid sparse map: %v", maker.sph)
 			}
 			spd := invertSparseEntries(maker.sph, maker.size)
-			fw = &regFileWriter{bb, maker.makeReg.size}
+			fw = &regFileWriter{w, maker.makeReg.size}
 			fw = &sparseFileWriter{fw, spd, 0}
 			wantStr = maker.makeReg.wantStr
 		default: