net: use Done rather than comparing with context.Background

Fixes #49023

Change-Id: I3de70f8a25f4ba8a0fb8bb96581371e33fde2f7a
GitHub-Last-Rev: b7ec9405ad
GitHub-Pull-Request: golang/go#49024
Reviewed-on: https://go-review.googlesource.com/c/go/+/356471
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Damien Neil <dneil@google.com>

src/net/fd_unix.go

@@ -91,12 +91,11 @@ func (fd *netFD) connect(ctx context.Context, la, ra syscall.Sockaddr) (rsa sysc
 	}
 
 	// Start the "interrupter" goroutine, if this context might be canceled.
-	// (The background context cannot)
 	//
 	// The interrupter goroutine waits for the context to be done and
 	// interrupts the dial (by altering the fd's write deadline, which
 	// wakes up waitWrite).
-	if ctx != context.Background() {
+	if ctxDone := ctx.Done(); ctxDone != nil {
 		// Wait for the interrupter goroutine to exit before returning
 		// from connect.
 		done := make(chan struct{})
@@ -116,7 +115,7 @@ func (fd *netFD) connect(ctx context.Context, la, ra syscall.Sockaddr) (rsa sysc
 		}()
 		go func() {
 			select {
-			case <-ctx.Done():
+			case <-ctxDone:
 				// Force the runtime's poller to immediately give up
 				// waiting for writability, unblocking waitWrite
 				// below.