[release-branch.go1.1] bufio: check buffer availability before reading in ReadFrom

This change was applied by hand, as bufio has seen some refactoring
since 1.1 was branched. The only difference between this and the
original patch is the offset of the change, and s/flush/Flush/.

««« CL 11801043 / 3ffbc06b4874
bufio: check buffer availability before reading in ReadFrom

Fixes  issue 5947 .

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/11801043
»»»

Update #5928

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/12002043

2 files changed, 33 insertions, 6 deletions

diff --git a/src/pkg/bufio/bufio.go b/src/pkg/bufio/bufio.go
index df3501f2ca..10e12d42be 100644
--- a/src/pkg/bufio/bufio.go
+++ b/src/pkg/bufio/bufio.go
@@ -585,23 +585,28 @@ func (b *Writer) ReadFrom(r io.Reader) (n int64, err error) {
 	}
 	var m int
 	for {
+		if b.Available() == 0 {
+			if err1 := b.Flush(); err1 != nil {
+				return n, err1
+			}
+		}
 		m, err = r.Read(b.buf[b.n:])
 		if m == 0 {
 			break
 		}
 		b.n += m
 		n += int64(m)
-		if b.Available() == 0 {
-			if err1 := b.Flush(); err1 != nil {
-				return n, err1
-			}
-		}
 		if err != nil {
 			break
 		}
 	}
 	if err == io.EOF {
-		err = nil
+		// If we filled the buffer exactly, flush pre-emptively.
+		if b.Available() == 0 {
+			err = b.Flush()
+		} else {
+			err = nil
+		}
 	}
 	return n, err
 }
diff --git a/src/pkg/bufio/bufio_test.go b/src/pkg/bufio/bufio_test.go
index 79ed0f178e..9fcbfda68d 100644
--- a/src/pkg/bufio/bufio_test.go
+++ b/src/pkg/bufio/bufio_test.go
@@ -847,6 +847,10 @@ func TestWriterReadFrom(t *testing.T) {
 				t.Errorf("ws[%d],rs[%d]: w.ReadFrom(r) = %d, %v, want %d, nil", wi, ri, n, err, len(input))
 				continue
 			}
+			if err := w.Flush(); err != nil {
+				t.Errorf("Flush returned %v", err)
+				continue
+			}
 			if got, want := b.String(), string(input); got != want {
 				t.Errorf("ws[%d], rs[%d]:\ngot  %q\nwant %q\n", wi, ri, got, want)
 			}
@@ -1003,6 +1007,24 @@ func TestReaderClearError(t *testing.T) {
 	}
 }
 
+// Test for golang.org/issue/5947
+func TestWriterReadFromWhileFull(t *testing.T) {
+	buf := new(bytes.Buffer)
+	w := NewWriterSize(buf, 10)
+
+	// Fill buffer exactly.
+	n, err := w.Write([]byte("0123456789"))
+	if n != 10 || err != nil {
+		t.Fatalf("Write returned (%v, %v), want (10, nil)", n, err)
+	}
+
+	// Use ReadFrom to read in some data.
+	n2, err := w.ReadFrom(strings.NewReader("abcdef"))
+	if n2 != 6 || err != nil {
+		t.Fatalf("ReadFrom returned (%v, %v), want (6, nil)", n, err)
+	}
+}
+
 // An onlyReader only implements io.Reader, no matter what other methods the underlying implementation may have.
 type onlyReader struct {
 	r io.Reader