runtime: fix traceback in cgo programs

Fixes #6061.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/12609043

src/pkg/runtime/crash_cgo_test.go

@@ -26,6 +26,14 @@ func TestCgoSignalDeadlock(t *testing.T) {
 	}
 }
 
+func TestCgoTraceback(t *testing.T) {
+	got := executeTest(t, cgoTracebackSource, nil)
+	want := "OK\n"
+	if got != want {
+		t.Fatalf("expected %q, but got %q", want, got)
+	}
+}
+
 const cgoSignalDeadlockSource = `
 package main
 
@@ -90,3 +98,22 @@ func main() {
 	fmt.Printf("OK\n")
 }
 `
+
+const cgoTracebackSource = `
+package main
+
+/* void foo(void) {} */
+import "C"
+
+import (
+	"fmt"
+	"runtime"
+)
+
+func main() {
+	C.foo()
+	buf := make([]byte, 1)
+	runtime.Stack(buf, true)
+	fmt.Printf("OK\n")
+}
+`

src/pkg/runtime/panic.c

@@ -415,6 +415,8 @@ runtime·startpanic(void)
 		runtime·exit(3);
 	}
 	m->dying = 1;
+	if(g != nil)
+		g->writebuf = nil;
 	runtime·xadd(&runtime·panicking, 1);
 	runtime·lock(&paniclk);
 }

src/pkg/runtime/proc.c

@@ -712,11 +712,18 @@ runtime·newextram(void)
 	gp->sched.sp = gp->stackbase;
 	gp->sched.lr = 0;
 	gp->sched.g = gp;
+	gp->syscallpc = gp->sched.pc;
+	gp->syscallsp = gp->sched.sp;
+	gp->syscallstack = gp->stackbase;
+	gp->syscallguard = gp->stackguard;
 	gp->status = Gsyscall;
 	mp->curg = gp;
 	mp->locked = LockInternal;
 	mp->lockedg = gp;
 	gp->lockedm = mp;
+	gp->goid = runtime·xadd64(&runtime·sched.goidgen, 1);
+	if(raceenabled)
+		gp->racectx = runtime·racegostart(runtime·newextram);
 	// put on allg for garbage collector
 	runtime·lock(&runtime·sched);
 	if(runtime·lastg == nil)
@@ -725,9 +732,6 @@ runtime·newextram(void)
 		runtime·lastg->alllink = gp;
 	runtime·lastg = gp;
 	runtime·unlock(&runtime·sched);
-	gp->goid = runtime·xadd64(&runtime·sched.goidgen, 1);
-	if(raceenabled)
-		gp->racectx = runtime·racegostart(runtime·newextram);
 
 	// Add m to the extra list.
 	mnext = lockextra(true);