net: fix testHookCanceledDial race

Loading and calling testHookCanceledDial in the function passed to
context.AfterFunc is racey, because the function runs in a separate
goroutine, and is not synchronized with the test code that restores
the original testHookCanceledDial value.

We could add a channel and wait for the "AfterFunc" to return in
the deferred function, but that's a lot of synchronization overhead
just for a bit of test code. Instead we simply load
testHookCanceledDial into a local variable synchronously. This fixes
the race without introducing any overhead.

Fixes golang#75474

Change-Id: If8fbd0f5f65375577c2ded64a13a15b406c45ecc
Reviewed-on: https://go-review.googlesource.com/c/go/+/704455
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
Auto-Submit: Damien Neil <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: database64128

src/net/fd_unix.go

@@ -82,6 +82,9 @@ func (fd *netFD) connect(ctx context.Context, la, ra syscall.Sockaddr) (rsa sysc
 			defer fd.pfd.SetWriteDeadline(noDeadline)
 		}
 
+		// Load the hook function synchronously to prevent a race
+		// with test code that restores the old value.
+		testHookCanceledDial := testHookCanceledDial
 		stop := context.AfterFunc(ctx, func() {
 			// Force the runtime's poller to immediately give up
 			// waiting for writability, unblocking waitWrite