io: clarify Reader returning 0, nil

Fixes issue 8317.

LGTM=bradfitz
R=bradfitz, iant, r
CC=golang-codereviews
https://codereview.appspot.com/143100043

1 files changed, 5 insertions, 2 deletions

diff --git a/src/io/io.go b/src/io/io.go
index e8bbad537..7507a8492 100644
--- a/src/io/io.go
+++ b/src/io/io.go
@@ -62,8 +62,11 @@ var ErrNoProgress = errors.New("multiple Read calls return no data or error")
 // allowed EOF behaviors.
 //
 // Implementations of Read are discouraged from returning a
-// zero byte count with a nil error, and callers should treat
-// that situation as a no-op. Implementations must not retain p.
+// zero byte count with a nil error, except when len(p) == 0.
+// Callers should treat a return of 0 and nil as indicating that
+// nothing happened; in particular it does not indicate EOF.
+//
+// Implementations must not retain p.
 type Reader interface {
 	Read(p []byte) (n int, err error)
 }