runtime: crash when func main calls Goexit and all other goroutines exit

This has typically crashed in the past, although usually with
an 'all goroutines are asleep - deadlock!' message that shows
no goroutines (because there aren't any).

Previous discussion at:
https://groups.google.com/d/msg/golang-nuts/uCT_7WxxopQ/BoSBlLFzUTkJ
https://groups.google.com/d/msg/golang-dev/KUojayEr20I/u4fp_Ej5PdUJ
http://golang.org/issue/7711

There is general agreement that runtime.Goexit terminates the
main goroutine, so that main cannot return, so the program does
not exit.

The interpretation that all other goroutines exiting causes an
exit(0) is relatively new and was not part of those discussions.
That is what this CL changes.

Thankfully, even though the exit(0) has been there for a while,
some other accounting bugs made it very difficult to trigger,
so it is reasonable to replace. In particular, see golang.org/issue/7711#c10
for an examination of the behavior across past releases.

Fixes #7711.

LGTM=iant, r
R=golang-codereviews, iant, dvyukov, r
CC=golang-codereviews
https://golang.org/cl/88210044

doc/go1.3.html

@@ -379,6 +379,15 @@ In particular, it only calls <a href="/pkg/os/exec/#LookPath"><code>LookPath</co
 when the binary's file name contains no path separators.
 </li>
 
+<li>
+If the main goroutine calls 
+<a href="/pkg/runtime/#Goexit"><code>runtime.Goexit</code>
+and all other goroutines finish execution, the program now always crashes,
+reporting a detected deadlock.
+Earlier versions of Go handled this situation inconsistently: most instances
+were reported as deadlocks, but some trivial cases exited cleanly instead.
+</li>
+
 <li>
 The <a href="/pkg/strconv/#CanBackquote"><code>CanBackquote</code></a>
 function in the <a href="/pkg/strconv/"><code>strconv</code></a> package

src/pkg/runtime/crash_test.go

@@ -111,8 +111,9 @@ func TestLockedDeadlock2(t *testing.T) {
 
 func TestGoexitDeadlock(t *testing.T) {
 	output := executeTest(t, goexitDeadlockSource, nil)
-	if output != "" {
-		t.Fatalf("expected no output, got:\n%s", output)
+	want := "no goroutines (main called runtime.Goexit) - deadlock!"
+	if !strings.Contains(output, want) {
+		t.Fatalf("output:\n%s\n\nwant output containing: %s", output, want)
 	}
 }
 
@@ -144,13 +145,12 @@ panic: again
 
 }
 
-func TestGoexitExit(t *testing.T) {
+func TestGoexitCrash(t *testing.T) {
 	output := executeTest(t, goexitExitSource, nil)
-	want := ""
-	if output != want {
-		t.Fatalf("output:\n%s\n\nwanted:\n%s", output, want)
+	want := "no goroutines (main called runtime.Goexit) - deadlock!"
+	if !strings.Contains(output, want) {
+		t.Fatalf("output:\n%s\n\nwant output containing: %s", output, want)
 	}
-
 }
 
 const crashSource = `

src/pkg/runtime/extern.go

@@ -79,6 +79,11 @@ func Gosched()
 
 // Goexit terminates the goroutine that calls it.  No other goroutine is affected.
 // Goexit runs all deferred calls before terminating the goroutine.
+//
+// Calling Goexit from the main goroutine terminates that goroutine
+// without func main returning. Since func main has not returned,
+// the program continues execution of other goroutines.
+// If all other goroutines exit, the program crashes.
 func Goexit()
 
 // Caller reports file and line number information about function invocations on

src/pkg/runtime/proc.c

@@ -2501,7 +2501,7 @@ checkdead(void)
 	}
 	runtime·unlock(&allglock);
 	if(grunning == 0)  // possible if main goroutine calls runtime·Goexit()
-		runtime·exit(0);
+		runtime·throw("no goroutines (main called runtime.Goexit) - deadlock!");
 	m->throwing = -1;  // do not dump full stacks
 	runtime·throw("all goroutines are asleep - deadlock!");
 }