compress/zlib: make errors persistent

Ensure that all errors (including io.EOF) are persistent across method
calls on zlib.Reader. Furthermore, ensure that these persistent errors
are properly cleared when Reset is called.

Fixes #14675

Change-Id: I15a20c7e25dc38219e7e0ff255d1ba775a86bb47
Reviewed-on: https://go-review.googlesource.com/20292Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/compress/zlib/reader.go

@@ -84,19 +84,17 @@ func NewReaderDict(r io.Reader, dict []byte) (io.ReadCloser, error) {
 	return z, nil
 }
 
-func (z *reader) Read(p []byte) (n int, err error) {
+func (z *reader) Read(p []byte) (int, error) {
 	if z.err != nil {
 		return 0, z.err
 	}
-	if len(p) == 0 {
-		return 0, nil
-	}
 
-	n, err = z.decompressor.Read(p)
+	var n int
+	n, z.err = z.decompressor.Read(p)
 	z.digest.Write(p[0:n])
-	if n != 0 || err != io.EOF {
-		z.err = err
-		return
+	if z.err != io.EOF {
+		// In the normal case we return here.
+		return n, z.err
 	}
 
 	// Finished file; check checksum.
@@ -105,20 +103,20 @@ func (z *reader) Read(p []byte) (n int, err error) {
 			err = io.ErrUnexpectedEOF
 		}
 		z.err = err
-		return 0, err
+		return n, z.err
 	}
 	// ZLIB (RFC 1950) is big-endian, unlike GZIP (RFC 1952).
 	checksum := uint32(z.scratch[0])<<24 | uint32(z.scratch[1])<<16 | uint32(z.scratch[2])<<8 | uint32(z.scratch[3])
 	if checksum != z.digest.Sum32() {
 		z.err = ErrChecksum
-		return 0, z.err
+		return n, z.err
 	}
-	return
+	return n, io.EOF
 }
 
 // Calling Close does not close the wrapped io.Reader originally passed to NewReader.
 func (z *reader) Close() error {
-	if z.err != nil {
+	if z.err != nil && z.err != io.EOF {
 		return z.err
 	}
 	z.err = z.decompressor.Close()
@@ -126,36 +124,42 @@ func (z *reader) Close() error {
 }
 
 func (z *reader) Reset(r io.Reader, dict []byte) error {
+	*z = reader{decompressor: z.decompressor}
 	if fr, ok := r.(flate.Reader); ok {
 		z.r = fr
 	} else {
 		z.r = bufio.NewReader(r)
 	}
-	_, err := io.ReadFull(z.r, z.scratch[0:2])
-	if err != nil {
-		if err == io.EOF {
-			err = io.ErrUnexpectedEOF
+
+	// Read the header (RFC 1950 section 2.2.).
+	_, z.err = io.ReadFull(z.r, z.scratch[0:2])
+	if z.err != nil {
+		if z.err == io.EOF {
+			z.err = io.ErrUnexpectedEOF
 		}
-		return err
+		return z.err
 	}
 	h := uint(z.scratch[0])<<8 | uint(z.scratch[1])
 	if (z.scratch[0]&0x0f != zlibDeflate) || (h%31 != 0) {
-		return ErrHeader
+		z.err = ErrHeader
+		return z.err
 	}
 	haveDict := z.scratch[1]&0x20 != 0
 	if haveDict {
-		_, err = io.ReadFull(z.r, z.scratch[0:4])
-		if err != nil {
-			if err == io.EOF {
-				err = io.ErrUnexpectedEOF
+		_, z.err = io.ReadFull(z.r, z.scratch[0:4])
+		if z.err != nil {
+			if z.err == io.EOF {
+				z.err = io.ErrUnexpectedEOF
 			}
-			return err
+			return z.err
 		}
 		checksum := uint32(z.scratch[0])<<24 | uint32(z.scratch[1])<<16 | uint32(z.scratch[2])<<8 | uint32(z.scratch[3])
 		if checksum != adler32.Checksum(dict) {
-			return ErrDictionary
+			z.err = ErrDictionary
+			return z.err
 		}
 	}
+
 	if z.decompressor == nil {
 		if haveDict {
 			z.decompressor = flate.NewReaderDict(z.r, dict)

src/compress/zlib/reader_test.go

@@ -127,16 +127,18 @@ func TestDecompressor(t *testing.T) {
 	b := new(bytes.Buffer)
 	for _, tt := range zlibTests {
 		in := bytes.NewReader(tt.compressed)
-		zlib, err := NewReaderDict(in, tt.dict)
+		zr, err := NewReaderDict(in, tt.dict)
 		if err != nil {
 			if err != tt.err {
 				t.Errorf("%s: NewReader: %s", tt.desc, err)
 			}
 			continue
 		}
-		defer zlib.Close()
+		defer zr.Close()
+
+		// Read and verify correctness of data.
 		b.Reset()
-		n, err := io.Copy(b, zlib)
+		n, err := io.Copy(b, zr)
 		if err != nil {
 			if err != tt.err {
 				t.Errorf("%s: io.Copy: %v want %v", tt.desc, err, tt.err)
@@ -147,5 +149,13 @@ func TestDecompressor(t *testing.T) {
 		if s != tt.raw {
 			t.Errorf("%s: got %d-byte %q want %d-byte %q", tt.desc, n, s, len(tt.raw), tt.raw)
 		}
+
+		// Check for sticky errors.
+		if n, err := zr.Read([]byte{0}); n != 0 || err != io.EOF {
+			t.Errorf("%s: Read() = (%d, %v), want (0, io.EOF)", tt.desc, n, err)
+		}
+		if err := zr.Close(); err != nil {
+			t.Errorf("%s: Close() = %v, want nil", tt.desc, err)
+		}
 	}
 }