bufio: fix bug that ReadFrom stops before EOF or error

ReadFrom should not return until it receives a non-nil error
or too many contiguous (0, nil)s from a given reader.
Currently it immediately returns if it receives one (0, nil).
Fixes #7611.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/76400048

src/pkg/bufio/bufio.go

@@ -38,6 +38,7 @@ type Reader struct {
 }
 
 const minReadBufferSize = 16
+const maxConsecutiveEmptyReads = 100
 
 // NewReaderSize returns a new Reader whose buffer has at least the specified
 // size. If the argument io.Reader is already a Reader with large enough
@@ -625,9 +626,16 @@ func (b *Writer) ReadFrom(r io.Reader) (n int64, err error) {
 				return n, err1
 			}
 		}
-		m, err = r.Read(b.buf[b.n:])
+		nr := 0
-		if m == 0 {
+		for nr < maxConsecutiveEmptyReads {
-			break
+			m, err = r.Read(b.buf[b.n:])
+			if m != 0 || err != nil {
+				break
+			}
+			nr++
+		}
+		if nr == maxConsecutiveEmptyReads {
+			return n, io.ErrNoProgress
 		}
 		b.n += m
 		n += int64(m)

src/pkg/bufio/bufio_test.go

@@ -1060,6 +1060,60 @@ func TestWriterReadFromWhileFull(t *testing.T) {
 	}
 }
 
+type emptyThenNonEmptyReader struct {
+	r io.Reader
+	n int
+}
+
+func (r *emptyThenNonEmptyReader) Read(p []byte) (int, error) {
+	if r.n <= 0 {
+		return r.r.Read(p)
+	}
+	r.n--
+	return 0, nil
+}
+
+// Test for golang.org/issue/7611
+func TestWriterReadFromUntilEOF(t *testing.T) {
+	buf := new(bytes.Buffer)
+	w := NewWriterSize(buf, 5)
+
+	// Partially fill buffer
+	n, err := w.Write([]byte("0123"))
+	if n != 4 || err != nil {
+		t.Fatalf("Write returned (%v, %v), want (4, nil)", n, err)
+	}
+
+	// Use ReadFrom to read in some data.
+	r := &emptyThenNonEmptyReader{r: strings.NewReader("abcd"), n: 3}
+	n2, err := w.ReadFrom(r)
+	if n2 != 4 || err != nil {
+		t.Fatalf("ReadFrom returned (%v, %v), want (4, nil)", n2, err)
+	}
+	w.Flush()
+	if got, want := string(buf.Bytes()), "0123abcd"; got != want {
+		t.Fatalf("buf.Bytes() returned %q, want %q", got, want)
+	}
+}
+
+func TestWriterReadFromErrNoProgress(t *testing.T) {
+	buf := new(bytes.Buffer)
+	w := NewWriterSize(buf, 5)
+
+	// Partially fill buffer
+	n, err := w.Write([]byte("0123"))
+	if n != 4 || err != nil {
+		t.Fatalf("Write returned (%v, %v), want (4, nil)", n, err)
+	}
+
+	// Use ReadFrom to read in some data.
+	r := &emptyThenNonEmptyReader{r: strings.NewReader("abcd"), n: 100}
+	n2, err := w.ReadFrom(r)
+	if n2 != 0 || err != io.ErrNoProgress {
+		t.Fatalf("buf.Bytes() returned (%v, %v), want (0, io.ErrNoProgress)", n2, err)
+	}
+}
+
 func TestReaderReset(t *testing.T) {
 	r := NewReader(strings.NewReader("foo foo"))
 	buf := make([]byte, 3)

src/pkg/bufio/scan.go

@@ -172,7 +172,7 @@ func (s *Scanner) Scan() bool {
 				break
 			}
 			loop++
-			if loop > 100 {
+			if loop > maxConsecutiveEmptyReads {
 				s.setErr(io.ErrNoProgress)
 				break
 			}