runtime: fix bad status throw

when garbage collector sees recovering goroutine

Fixes #711.

R=r
CC=golang-dev
https://golang.org/cl/869045

src/pkg/runtime/mgc0.c

@@ -135,6 +135,7 @@ mark(void)
 		case Grunnable:
 		case Gsyscall:
 		case Gwaiting:
+		case Grecovery:
 			scanstack(gp);
 			break;
 		}

src/pkg/runtime/proc.c

@@ -461,9 +461,7 @@ scheduler(void)
 			
 			// unwind to the stack frame with d->sp in it.
 			unwindstack(gp, d->sp);
-			if(d->sp < gp->stackguard || gp->stackbase < d->sp)
-				throw("bad stack in recovery");
-			
+
 			// make the deferproc for this d return again,
 			// this time returning 1.  function will jump to
 			// standard return epilogue.
@@ -930,6 +928,11 @@ unwindstack(G *gp, byte *sp)
 		gp->stackguard = top->stackguard;
 		free(stk);
 	}
+
+	if(sp != nil && (sp < gp->stackguard - StackGuard || gp->stackbase < sp)) {
+		printf("recover: %p not in [%p, %p]\n", sp, gp->stackguard - StackGuard, gp->stackbase);
+		throw("bad unwindstack");
+	}
 }
 
 static void

src/pkg/runtime/runtime.h

@@ -92,6 +92,10 @@ extern	register	M*	m;
 enum
 {
 	// G status
+	//
+	// If you add to this list, add to the list
+	// of "okay during garbage collection" status
+	// in mgc0.c too.
 	Gidle,
 	Grunnable,
 	Grunning,