net: add Example for Unix dialer with context

Showing users how to accomplish this more easily is a compromise
between doing nothing and adding a new API to the net package.

Fixes #38506.

Change-Id: I43f831cf94951c987cf3c8c1aa55f0012ee8034e
Reviewed-on: https://go-review.googlesource.com/c/go/+/263417
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Brad Fitzpatrick <bradfitz@golang.org>

src/net/example_test.go

@@ -55,6 +55,27 @@ func ExampleDialer() {
 	}
 }
 
+func ExampleDialer_unix() {
+	// DialUnix does not take a context.Context parameter. This example shows
+	// how to dial a Unix socket with a Context. Note that the Context only
+	// applies to the dial operation; it does not apply to the connection once
+	// it has been established.
+	var d net.Dialer
+	ctx, cancel := context.WithTimeout(context.Background(), time.Minute)
+	defer cancel()
+
+	d.LocalAddr = nil // if you have a local addr, add it here
+	raddr := net.UnixAddr{Name: "/path/to/unix.sock", Net: "unix"}
+	conn, err := d.DialContext(ctx, "unix", raddr.String())
+	if err != nil {
+		log.Fatalf("Failed to dial: %v", err)
+	}
+	defer conn.Close()
+	if _, err := conn.Write([]byte("Hello, socket!")); err != nil {
+		log.Fatal(err)
+	}
+}
+
 func ExampleIPv4() {
 	fmt.Println(net.IPv4(8, 8, 8, 8))