runtime: make cgocallback wait on package init

With the new buildmodes c-archive and c-shared, it is possible for a
cgo call to come in early in the lifecycle of a Go program. Calls
before the runtime has been initialized are caught by
_cgo_wait_runtime_init_done. However a call can come in after the
runtime has initialized, but before the program's package init
functions have finished running.

To avoid this cgocallback checks m.ncgo to see if we are on a thread
running Go. If not, we may be a foreign thread and it blocks until
main_init is complete.

Change-Id: I7a9f137fa2a40c322a0b93764261f9aa17fcf5b8
Reviewed-on: https://go-review.googlesource.com/8897
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>

src/runtime/proc.go

@@ -12,6 +12,12 @@ func runtime_init()
 //go:linkname main_init main.init
 func main_init()
 
+// main_init_done is a signal used by cgocallbackg that initialization
+// has been completed. It is made before _cgo_notify_runtime_init_done,
+// so all cgo calls can rely on it existing. When main_init is complete,
+// it is closed, meaning cgocallbackg can reliably receive from it.
+var main_init_done chan bool
+
 //go:linkname main_main main.main
 func main_main()
 
@@ -70,6 +76,7 @@ func main() {
 		// Allocate new M as main_main() is expected to block forever.
 		systemstack(newextram)
 	}
+	main_init_done = make(chan bool)
 	if iscgo {
 		if _cgo_thread_start == nil {
 			throw("_cgo_thread_start missing")
@@ -95,6 +102,7 @@ func main() {
 	}
 
 	main_init()
+	close(main_init_done)
 
 	needUnlock = false
 	unlockOSThread()