io: make clear that EOF should not be wrapped

For #40827.

Change-Id: Ifd108421abd8d0988dd7b985e4f9e2bd5356964a
Reviewed-on: https://go-review.googlesource.com/c/go/+/258524
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/src/io/io.go b/src/io/io.go
index 3dea70b947..adc0c0d550 100644
--- a/src/io/io.go
+++ b/src/io/io.go
@@ -31,6 +31,8 @@ var ErrShortWrite = errors.New("short write")
 var ErrShortBuffer = errors.New("short buffer")
 
 // EOF is the error returned by Read when no more input is available.
+// (Read must return EOF itself, not an error wrapping EOF,
+// because callers will test for EOF using ==.)
 // Functions should return EOF only to signal a graceful end of input.
 // If the EOF occurs unexpectedly in a structured data stream,
 // the appropriate error is either ErrUnexpectedEOF or some other error