io: add ErrBadWriteCount

Fixes #39978

Change-Id: Ib41459861ba9f7cf0bf1fc95b1479c358c4bdbd8
GitHub-Last-Rev: 19cbb1461ca04a8eb64f0c4f354d8fb81a70d4f3
GitHub-Pull-Request: golang/go#39989
Reviewed-on: https://go-review.googlesource.com/c/go/+/240740
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

diff --git a/src/io/io.go b/src/io/io.go
index adc0c0d550a01eb9e8db7f9952f83bcc473c72b2..87ebe8c1475d7437730dd6f889bda5abf4400de3 100644 (file)
--- a/src/io/io.go
+++ b/src/io/io.go
@@ -30,6 +30,9 @@ var ErrShortWrite = errors.New("short write")
 // ErrShortBuffer means that a read required a longer buffer than was provided.
 var ErrShortBuffer = errors.New("short buffer")
 
+// ErrBadWriteCount means that a write returned an impossible count.
+var ErrBadWriteCount = errors.New("Write returned impossible count")
+
 // EOF is the error returned by Read when no more input is available.
 // (Read must return EOF itself, not an error wrapping EOF,
 // because callers will test for EOF using ==.)
@@ -411,9 +414,13 @@ func copyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
                nr, er := src.Read(buf)
                if nr > 0 {
                        nw, ew := dst.Write(buf[0:nr])
-                       if nw > 0 {
-                               written += int64(nw)
+                       if nw < 0 || nr < nw {
+                               nw = 0
+                               if ew == nil {
+                                       ew = ErrBadWriteCount
+                               }
                        }
+                       written += int64(nw)
                        if ew != nil {
                                err = ew
                                break

diff --git a/src/io/io_test.go b/src/io/io_test.go
index 170513dcc0d5921b08c120d7ee6c2352332de298..a8399bcac60213271fb63fcdd549e9c04fb9dbb0 100644 (file)
--- a/src/io/io_test.go
+++ b/src/io/io_test.go
@@ -429,3 +429,31 @@ func TestSectionReader_Size(t *testing.T) {
                }
        }
 }
+
+// largeWriter returns an invalid count that is larger than the number
+// of bytes provided (issue 39978).
+type largeWriter struct {
+       err error
+}
+
+func (w largeWriter) Write(p []byte) (int, error) {
+       return len(p) + 1, w.err
+}
+
+func TestCopyLargeWriter(t *testing.T) {
+       want := ErrBadWriteCount
+       rb := new(Buffer)
+       wb := largeWriter{}
+       rb.WriteString("hello, world.")
+       if _, err := Copy(wb, rb); err != want {
+               t.Errorf("Copy error: got %v, want %v", err, want)
+       }
+
+       want = errors.New("largeWriterError")
+       rb = new(Buffer)
+       wb = largeWriter{err: want}
+       rb.WriteString("hello, world.")
+       if _, err := Copy(wb, rb); err != want {
+               t.Errorf("Copy error: got %v, want %v", err, want)
+       }
+}