io: clarify that Read(nil) can return 0, EOF

Fixes golang#40385

Change-Id: I965b5db985fd4418a992e883073cbc8309b2cb88
Reviewed-on: https://go-review.googlesource.com/c/go/+/498355
Reviewed-by: Brad Fitzpatrick <[email protected]>
Reviewed-by: Rob Pike <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>

Author: ianlancetaylor

src/io/io.go

@@ -74,6 +74,9 @@ var ErrNoProgress = errors.New("multiple Read calls return no data or error")
 // that happen after reading some bytes and also both of the
 // allowed EOF behaviors.
 //
+// If len(p) == 0, Read should always return n == 0. It may return a
+// non-nil error if some error condition is known, such as EOF.
+//
 // Implementations of Read are discouraged from returning a
 // zero byte count with a nil error, except when len(p) == 0.
 // Callers should treat a return of 0 and nil as indicating that