net: map context errors from aborted dial to internal net package errors

Map the error returned when a dial is aborted from the context package
error to the internal net package error. For example, context.Canceled
errors map to errCanceled, and context.DeadlineExceeded errors map to
poll.ErrTimeout.

Fixes #23648

Change-Id: Idf9d3d08052d540740c0b054503aaed931dc5b1e
Reviewed-on: https://go-review.googlesource.com/103518
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Author: benburkert

src/net/dial_unix_test.go

@@ -102,7 +102,8 @@ func TestDialContextCancelRace(t *testing.T) {
 	if !ok || oe.Op != "dial" {
 		t.Fatalf("Dial error = %#v; want dial *OpError", err)
 	}
-	if oe.Err != ctx.Err() {
-		t.Errorf("DialContext = (%v, %v); want OpError with error %v", c, err, ctx.Err())
+
+	if oe.Err != errCanceled {
+		t.Errorf("DialContext = (%v, %v); want OpError with error %v", c, err, errCanceled)
 	}
 }

src/net/fd_unix.go

@@ -121,7 +121,7 @@ func (fd *netFD) connect(ctx context.Context, la, ra syscall.Sockaddr) (rsa sysc
 				// == nil). Because we've now poisoned the connection
 				// by making it unwritable, don't return a successful
 				// dial. This was issue 16523.
-				ret = ctxErr
+				ret = mapErr(ctxErr)
 				fd.Close() // prevent a leak
 			}
 		}()