runtime: restore deadlock detection in the simplest case.

Fixes #3342.

R=iant, r, dave, rsc
CC=golang-dev, remy
https://golang.org/cl/5844051

src/pkg/runtime/mheap.c

@@ -358,6 +358,9 @@ runtime·MHeap_Scavenger(void)
 
 	h = &runtime·mheap;
 	for(k=0;; k++) {
+		// Return to the scheduler in case the rest of the world is deadlocked.
+		runtime·gosched();
+
 		runtime·noteclear(&note);
 		runtime·entersyscall();
 		runtime·notetsleep(&note, tick);

src/pkg/runtime/proc.c

@@ -521,6 +521,16 @@ mnextg(M *m, G *g)
 	}
 }
 
+// Check for a deadlock situation.
+static void
+checkdeadlock(void) {
+	if((scvg == nil && runtime·sched.grunning == 0) ||
+	   (scvg != nil && runtime·sched.grunning == 1 && runtime·sched.gwait == 0 &&
+	    (scvg->status == Grunnable || scvg->status == Grunning || scvg->status == Gsyscall))) {
+		runtime·throw("all goroutines are asleep - deadlock!");
+	}
+}
+
 // Get the next goroutine that m should run.
 // Sched must be locked on entry, is unlocked on exit.
 // Makes sure that at most $GOMAXPROCS g's are
@@ -570,6 +580,9 @@ top:
 				continue;
 			}
 			runtime·sched.grunning++;
+			// The work could actually have been the sole scavenger
+			// goroutine. Look for deadlock situation.
+			checkdeadlock();
 			schedunlock();
 			return gp;
 		}
@@ -591,11 +604,7 @@ top:
 	}
 
 	// Look for deadlock situation.
-	if((scvg == nil && runtime·sched.grunning == 0) ||
-	   (scvg != nil && runtime·sched.grunning == 1 && runtime·sched.gwait == 0 &&
-	    (scvg->status == Grunning || scvg->status == Gsyscall))) {
-		runtime·throw("all goroutines are asleep - deadlock!");
-	}
+	checkdeadlock();
 
 	m->nextg = nil;
 	m->waitnextg = 1;

test/fixedbugs/bug429.go

+// $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