From e9ad52e46dee4b4f9c73ff44f44e1e234815800f Mon Sep 17 00:00:00 2001
From: Ian Lance Taylor <iant@golang.org>
Date: Mon, 24 Aug 2020 21:42:20 -0700
Subject: [PATCH] net: export ErrClosed

This permits programs to reliably detect whether they are using a
closed network connection.

Fixes #4373

Change-Id: Ib4ce8cc82bbb134c4689f0ebc8b9b11bb8b32a22
Reviewed-on: https://go-review.googlesource.com/c/go/+/250357
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Russ Cox <rsc@golang.org>
---
 doc/go1.16.html       | 12 ++++++++++++
 src/net/error_test.go | 17 +++++++++++------
 src/net/net.go        | 12 ++++++++++++
 3 files changed, 35 insertions(+), 6 deletions(-)

diff --git a/doc/go1.16.html b/doc/go1.16.html
index b11af7fd99..c82b3b9276 100644
--- a/doc/go1.16.html
+++ b/doc/go1.16.html
@@ -99,6 +99,18 @@ Do not send CLs removing the interior tags from such phrases.
   TODO
 </p>
 
+<h3 id="net"><a href="/pkg/net/">net</a></h3>
+
+<p><!-- CL -->
+  The case of I/O on a closed network connection, or I/O on a network
+  connection that is closed before any of the I/O completes, can now
+  be detected using the new <a href="/pkg/net/#ErrClosed">ErrClosed</a> error.
+  A typical use would be <code>errors.Is(err, net.ErrClosed)</code>.
+  In earlier releases the only way to reliably detect this case was to
+  match the string returned by the <code>Error</code> method
+  with <code>"use of closed network connection"</code>.
+</p>
+
 <h3 id="unicode"><a href="/pkg/unicode/">unicode</a></h3>
 
 <p><!-- CL 248765 -->
diff --git a/src/net/error_test.go b/src/net/error_test.go
index 8d4a7ffb3d..62dfb9c15d 100644
--- a/src/net/error_test.go
+++ b/src/net/error_test.go
@@ -8,6 +8,7 @@ package net
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"internal/poll"
 	"io"
@@ -101,7 +102,7 @@ second:
 		goto third
 	}
 	switch nestedErr {
-	case errCanceled, poll.ErrNetClosing, errMissingAddress, errNoSuitableAddress,
+	case errCanceled, ErrClosed, errMissingAddress, errNoSuitableAddress,
 		context.DeadlineExceeded, context.Canceled:
 		return nil
 	}
@@ -436,7 +437,7 @@ second:
 		goto third
 	}
 	switch nestedErr {
-	case poll.ErrNetClosing, errTimeout, poll.ErrNotPollable, os.ErrDeadlineExceeded:
+	case ErrClosed, errTimeout, poll.ErrNotPollable, os.ErrDeadlineExceeded:
 		return nil
 	}
 	return fmt.Errorf("unexpected type on 2nd nested level: %T", nestedErr)
@@ -478,7 +479,7 @@ second:
 		goto third
 	}
 	switch nestedErr {
-	case errCanceled, poll.ErrNetClosing, errMissingAddress, errTimeout, os.ErrDeadlineExceeded, ErrWriteToConnected, io.ErrUnexpectedEOF:
+	case errCanceled, ErrClosed, errMissingAddress, errTimeout, os.ErrDeadlineExceeded, ErrWriteToConnected, io.ErrUnexpectedEOF:
 		return nil
 	}
 	return fmt.Errorf("unexpected type on 2nd nested level: %T", nestedErr)
@@ -508,6 +509,10 @@ func parseCloseError(nestedErr error, isShutdown bool) error {
 		return fmt.Errorf("error string %q does not contain expected string %q", nestedErr, want)
 	}
 
+	if !isShutdown && !errors.Is(nestedErr, ErrClosed) {
+		return fmt.Errorf("errors.Is(%v, errClosed) returns false, want true", nestedErr)
+	}
+
 	switch err := nestedErr.(type) {
 	case *OpError:
 		if err := err.isValid(); err != nil {
@@ -531,7 +536,7 @@ second:
 		goto third
 	}
 	switch nestedErr {
-	case poll.ErrNetClosing:
+	case ErrClosed:
 		return nil
 	}
 	return fmt.Errorf("unexpected type on 2nd nested level: %T", nestedErr)
@@ -627,7 +632,7 @@ second:
 		goto third
 	}
 	switch nestedErr {
-	case poll.ErrNetClosing, errTimeout, poll.ErrNotPollable, os.ErrDeadlineExceeded:
+	case ErrClosed, errTimeout, poll.ErrNotPollable, os.ErrDeadlineExceeded:
 		return nil
 	}
 	return fmt.Errorf("unexpected type on 2nd nested level: %T", nestedErr)
@@ -706,7 +711,7 @@ second:
 		goto third
 	}
 	switch nestedErr {
-	case poll.ErrNetClosing:
+	case ErrClosed:
 		return nil
 	}
 	return fmt.Errorf("unexpected type on 2nd nested level: %T", nestedErr)
diff --git a/src/net/net.go b/src/net/net.go
index 2e61a7c02e..4b4ed129cc 100644
--- a/src/net/net.go
+++ b/src/net/net.go
@@ -81,6 +81,7 @@ package net
 import (
 	"context"
 	"errors"
+	"internal/poll"
 	"io"
 	"os"
 	"sync"
@@ -632,6 +633,17 @@ func (e *DNSError) Timeout() bool { return e.IsTimeout }
 // error and return a DNSError for which Temporary returns false.
 func (e *DNSError) Temporary() bool { return e.IsTimeout || e.IsTemporary }
 
+// errClosed exists just so that the docs for ErrClosed don't mention
+// the internal package poll.
+var errClosed = poll.ErrNetClosing
+
+// ErrClosed is the error returned by an I/O call on a network
+// connection that has already been closed, or that is closed by
+// another goroutine before the I/O is completed. This may be wrapped
+// in another error, and should normally be tested using
+// errors.Is(err, net.ErrClosed).
+var ErrClosed = errClosed
+
 type writerOnly struct {
 	io.Writer
 }
-- 
2.50.0