crypto/tls: use context.AfterFunc in handshakeContext

This saves a goroutine when ctx can be canceled but is not canceled
during the handshakeContext call.

Use ctx consistently, because in this path (c.quic == nil) handshakeCtx
will only be canceled when ctx is canceled.

Change-Id: I7f4565119f30d589dce026b0d7ef3c324220525a
Reviewed-on: https://go-review.googlesource.com/c/go/+/699895
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Michael Pratt <mpratt@google.com>

conn.go

@@ -1591,7 +1591,7 @@ func (c *Conn) handshakeContext(ctx context.Context) (ret error) {
 	}
 
 	handshakeCtx, cancel := context.WithCancel(ctx)
-	// Note: defer this before starting the "interrupter" goroutine
+	// Note: defer this before calling context.AfterFunc
 	// so that we can tell the difference between the input being canceled and
 	// this cancellation. In the former case, we need to close the connection.
 	defer cancel()
@@ -1600,28 +1600,14 @@ func (c *Conn) handshakeContext(ctx context.Context) (ret error) {
 		c.quic.cancelc = handshakeCtx.Done()
 		c.quic.cancel = cancel
 	} else if ctx.Done() != nil {
-		// Start the "interrupter" goroutine, if this context might be canceled.
-		// (The background context cannot).
-		//
-		// The interrupter goroutine waits for the input context to be done and
-		// closes the connection if this happens before the function returns.
-		done := make(chan struct{})
-		interruptRes := make(chan error, 1)
+		// Close the connection if ctx is canceled before the function returns.
+		stop := context.AfterFunc(ctx, func() {
+			_ = c.conn.Close()
+		})
 		defer func() {
-			close(done)
-			if ctxErr := <-interruptRes; ctxErr != nil {
+			if !stop() {
 				// Return context error to user.
-				ret = ctxErr
-			}
-		}()
-		go func() {
-			select {
-			case <-handshakeCtx.Done():
-				// Close the connection, discarding the error
-				_ = c.conn.Close()
-				interruptRes <- handshakeCtx.Err()
-			case <-done:
-				interruptRes <- nil
+				ret = ctx.Err()
 			}
 		}()
 	}