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

diff --git a/src/pkg/bufio/bufio.go b/src/pkg/bufio/bufio.go
index ef74471..de81b4d 100644
--- a/src/pkg/bufio/bufio.go
+++ b/src/pkg/bufio/bufio.go
@@ -38,6 +38,7 @@
 }
 
 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 @@
 				return n, err1
 			}
 		}
-		m, err = r.Read(b.buf[b.n:])
-		if m == 0 {
-			break
+		nr := 0
+		for nr < maxConsecutiveEmptyReads {
+			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)