runtime: work around false negative in deadlock detection

Not a complete fix for issue 3342, but fixes the trivial case.
There may still be a race in the instants before and after
a scavenger-induced garbage collection.

Intended to be "obviously safe": a call to runtime·gosched
before main.main is no different than a call to runtime.Gosched
at the beginning of main.main, and it is (or had better be)
safe to call runtime.Gosched at any point during main.

Update #3342.

R=iant
CC=golang-dev
https://golang.org/cl/5919052

src/pkg/runtime/proc.c

@@ -236,6 +236,11 @@ runtime·main(void)
 	if(!runtime·sched.lockmain)
 		runtime·UnlockOSThread();
 
+	// The deadlock detection has false negatives.
+	// Let scvg start up, to eliminate the false negative
+	// for the trivial program func main() { select{} }.
+	runtime·gosched();
+
 	main·main();
 	runtime·exit(0);
 	for(;;)
@@ -591,6 +596,20 @@ top:
 	}
 
 	// Look for deadlock situation.
+	// There is a race with the scavenger that causes false negatives:
+	// if the scavenger is just starting, then we have
+	//	scvg != nil && grunning == 0 && gwait == 0
+	// and we do not detect a deadlock.  It is possible that we should
+	// add that case to the if statement here, but it is too close to Go 1
+	// to make such a subtle change.  Instead, we work around the
+	// false negative in trivial programs by calling runtime.gosched
+	// from the main goroutine just before main.main.
+	// See runtime·main above.
+	//
+	// On a related note, it is also possible that the scvg == nil case is
+	// wrong and should include gwait, but that does not happen in
+	// standard Go programs, which all start the scavenger.
+	//
 	if((scvg == nil && runtime·sched.grunning == 0) ||
 	   (scvg != nil && runtime·sched.grunning == 1 && runtime·sched.gwait == 0 &&
 	    (scvg->status == Grunning || scvg->status == Gsyscall))) {

test/fixedbugs/bug429.go

@@ -0,0 +1,13 @@
+// $G $D/$F.go && $L $F.$A && ! ./$A.out || echo BUG: bug429
+
+// Copyright 2012 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Should print deadlock message, not hang.
+
+package main
+
+func main() {
+	select {}
+}

test/golden.out

@@ -15,6 +15,9 @@
 
 == fixedbugs/
 
+=========== fixedbugs/bug429.go
+throw: all goroutines are asleep - deadlock!
+
 == bugs/
 
 =========== bugs/bug395.go