check for net.ErrClosed instead of "use of closed network connection"

The infamous "use of closed network connection" error was added in
[cl-5649076] as a non-exported error. This made it not possible to
write code to handle it as a sentinel error, other than through string-
matching.

Commit [moby@cc851db] (docker v0.6.4) added a [`IsClosedError`] utility
for this (as [[email protected]] did not yet export this error).
The `IsClosedError` was later moved to the `go-connections` module, but
various other places in our code used similar matching.

There was a feature-request [go-4373] to export it, which
got accepted and implemented in [CL 5649076], so starting with go1.16
we now have [[email protected]], so can remove the string matching.

[CL 5649076]: https://golang.org/cl/5649076
[moby@cc851db]: moby/moby@cc851db
[`IsClosedError`]: https://github.com/moby/moby/blob/cc851dbb3f4725562729f12663e246690a61c620/utils/utils.go#L1032-L1040
[[email protected]]: https://github.com/golang/go/blob/go1.1.2/src/pkg/net/net.go#L341
[go-4373]: golang/go#4373
[[email protected]]: https://github.com/golang/go/blob/go1.16/src/net/net.go#L636-L645

Signed-off-by: Sebastiaan van Stijn <[email protected]>

Author: thaJeztah

proxy/udp_proxy.go

@@ -2,8 +2,8 @@ package proxy
 
 import (
 	"encoding/binary"
+	"errors"
 	"net"
-	"strings"
 	"sync"
 	"syscall"
 	"time"
@@ -115,10 +115,9 @@ func (proxy *UDPProxy) Run() {
 	for {
 		read, from, err := proxy.listener.ReadFromUDP(readBuf)
 		if err != nil {
-			// NOTE: Apparently ReadFrom doesn't return
-			// ECONNREFUSED like Read do (see comment in
-			// UDPProxy.replyLoop)
-			if !isClosedError(err) {
+			// NOTE: Apparently ReadFrom doesn't return ECONNREFUSED like
+			// Read does (see comment in [UDPProxy.replyLoop]).
+			if !errors.Is(err, net.ErrClosed) {
 				proxy.Logger.Printf("Stopping proxy on udp/%v for udp/%v (%s)", proxy.frontendAddr, proxy.backendAddr, err)
 			}
 			break
@@ -164,13 +163,3 @@ func (proxy *UDPProxy) FrontendAddr() net.Addr { return proxy.frontendAddr }
 
 // BackendAddr returns the proxied UDP address.
 func (proxy *UDPProxy) BackendAddr() net.Addr { return proxy.backendAddr }
-
-func isClosedError(err error) bool {
-	/* This comparison is ugly, but unfortunately, net.go doesn't export errClosing.
-	 * See:
-	 * http://golang.org/src/pkg/net/net.go
-	 * https://code.google.com/p/go/issues/detail?id=4337
-	 * https://groups.google.com/forum/#!msg/golang-nuts/0_aaCvBmOcM/SptmDyX1XJMJ
-	 */
-	return strings.HasSuffix(err.Error(), "use of closed network connection")
-}

sockets/inmem_socket.go

@@ -1,13 +1,10 @@
 package sockets
 
 import (
-	"errors"
 	"net"
 	"sync"
 )
 
-var errClosed = errors.New("use of closed network connection")
-
 // InmemSocket implements net.Listener using in-memory only connections.
 type InmemSocket struct {
 	chConn  chan net.Conn
@@ -36,13 +33,15 @@ func (s *InmemSocket) Addr() net.Addr {
 	return dummyAddr(s.addr)
 }
 
-// Accept implements the Accept method in the Listener interface; it waits for the next call and returns a generic Conn.
+// Accept implements the Accept method in the Listener interface; it waits
+// for the next call and returns a generic Conn. It returns a [net.ErrClosed]
+// if the connection is already closed.
 func (s *InmemSocket) Accept() (net.Conn, error) {
 	select {
 	case conn := <-s.chConn:
 		return conn, nil
 	case <-s.chClose:
-		return nil, errClosed
+		return nil, net.ErrClosed
 	}
 }
 
@@ -58,13 +57,14 @@ func (s *InmemSocket) Close() error {
 	return nil
 }
 
-// Dial is used to establish a connection with the in-mem server
+// Dial is used to establish a connection with the in-mem server.
+// It returns a [net.ErrClosed] if the connection is already closed.
 func (s *InmemSocket) Dial(network, addr string) (net.Conn, error) {
 	srvConn, clientConn := net.Pipe()
 	select {
 	case s.chConn <- srvConn:
 	case <-s.chClose:
-		return nil, errClosed
+		return nil, net.ErrClosed
 	}
 
 	return clientConn, nil

sockets/inmem_socket_test.go

@@ -1,6 +1,10 @@
 package sockets
 
-import "testing"
+import (
+	"errors"
+	"net"
+	"testing"
+)
 
 func TestInmemSocket(t *testing.T) {
 	l := NewInmemSocket("test", 0)
@@ -33,7 +37,7 @@ func TestInmemSocket(t *testing.T) {
 
 	_ = l.Close()
 	_, err = l.Dial("test", "test")
-	if err != errClosed {
-		t.Fatalf("expected `errClosed` error, got %v", err)
+	if !errors.Is(err, net.ErrClosed) {
+		t.Fatalf(`expected "net.ErrClosed" error, got %[1]v (%[1]T)`, err)
 	}
 }