bufio: do not cache Read errors

Reader previously had cached an error from the underlying reader
and would return it on every subsequent call to Read.  The Reader
will now return the error only once, and subsequent calls will result
in a new Read call to the underlying Reader.

Fixes #1934.

R=bradfitz, rogpeppe, rsc
CC=golang-dev
https://golang.org/cl/4528133

src/pkg/bufio/bufio.go

diff --git a/src/pkg/bufio/bufio.go b/src/pkg/bufio/bufio.go
index 497e770..cb2667b 100644
--- a/src/pkg/bufio/bufio.go
+++ b/src/pkg/bufio/bufio.go
@@ -103,6 +103,12 @@
 	}
 }
 
+func (b *Reader) readErr() os.Error {
+	err := b.err
+	b.err = nil
+	return err
+}
+
 // Peek returns the next n bytes without advancing the reader. The bytes stop
 // being valid at the next read call. If Peek returns fewer than n bytes, it
 // also returns an error explaining why the read is short. The error is
@@ -121,7 +127,7 @@
 	if m > n {
 		m = n
 	}
-	err := b.err
+	err := b.readErr()
 	if m < n && err == nil {
 		err = ErrBufferFull
 	}
@@ -136,11 +142,11 @@
 func (b *Reader) Read(p []byte) (n int, err os.Error) {
 	n = len(p)
 	if n == 0 {
-		return 0, b.err
+		return 0, b.readErr()
 	}
 	if b.w == b.r {
 		if b.err != nil {
-			return 0, b.err
+			return 0, b.readErr()
 		}
 		if len(p) >= len(b.buf) {
 			// Large read, empty buffer.
@@ -150,11 +156,11 @@
 				b.lastByte = int(p[n-1])
 				b.lastRuneSize = -1
 			}
-			return n, b.err
+			return n, b.readErr()
 		}
 		b.fill()
 		if b.w == b.r {
-			return 0, b.err
+			return 0, b.readErr()
 		}
 	}
 
@@ -174,7 +180,7 @@
 	b.lastRuneSize = -1
 	for b.w == b.r {
 		if b.err != nil {
-			return 0, b.err
+			return 0, b.readErr()
 		}
 		b.fill()
 	}
@@ -210,7 +216,7 @@
 	}
 	b.lastRuneSize = -1
 	if b.r == b.w {
-		return 0, 0, b.err
+		return 0, 0, b.readErr()
 	}
 	rune, size = int(b.buf[b.r]), 1
 	if rune >= 0x80 {
@@ -262,7 +268,7 @@
 		if b.err != nil {
 			line := b.buf[b.r:b.w]
 			b.r = b.w
-			return line, b.err
+			return line, b.readErr()
 		}
 
 		n := b.Buffered()

src/pkg/bufio/bufio_test.go

diff --git a/src/pkg/bufio/bufio_test.go b/src/pkg/bufio/bufio_test.go
index 123adac..5709213 100644
--- a/src/pkg/bufio/bufio_test.go
+++ b/src/pkg/bufio/bufio_test.go
@@ -53,11 +53,12 @@
 		if e == os.EOF {
 			break
 		}
-		if e != nil {
+		if e == nil {
+			b[nb] = c
+			nb++
+		} else if e != iotest.ErrTimeout {
 			panic("Data: " + e.String())
 		}
-		b[nb] = c
-		nb++
 	}
 	return string(b[0:nb])
 }
@@ -86,6 +87,7 @@
 	{"byte", iotest.OneByteReader},
 	{"half", iotest.HalfReader},
 	{"data+err", iotest.DataErrReader},
+	{"timeout", iotest.TimeoutReader},
 }
 
 // Call ReadString (which ends up calling everything else)
@@ -97,7 +99,7 @@
 		if e == os.EOF {
 			break
 		}
-		if e != nil {
+		if e != nil && e != iotest.ErrTimeout {
 			panic("GetLines: " + e.String())
 		}
 		s += s1

src/pkg/testing/iotest/reader.go

diff --git a/src/pkg/testing/iotest/reader.go b/src/pkg/testing/iotest/reader.go
index e4003d7..daa6ede 100644
--- a/src/pkg/testing/iotest/reader.go
+++ b/src/pkg/testing/iotest/reader.go
@@ -58,7 +58,7 @@
 			r.unread = r.data[0:n1]
 			err = err1
 		}
-		if n > 0 {
+		if n > 0 || err != nil {
 			break
 		}
 		n = copy(p, r.unread)
@@ -66,3 +66,22 @@
 	}
 	return
 }
+
+var ErrTimeout = os.NewError("timeout")
+
+// TimeoutReader returns ErrTimeout on the second read
+// with no data.  Subsequent calls to read succeed.
+func TimeoutReader(r io.Reader) io.Reader { return &timeoutReader{r, 0} }
+
+type timeoutReader struct {
+	r     io.Reader
+	count int
+}
+
+func (r *timeoutReader) Read(p []byte) (int, os.Error) {
+	r.count++
+	if r.count == 2 {
+		return 0, ErrTimeout
+	}
+	return r.r.Read(p)
+}