runtime: mark extra M's G as dead when not in use

Currently the extra Ms created for cgo callbacks have a corresponding
G that's kept in syscall state with only a call to goexit on its
stack. This leads to confusing output from runtime.NumGoroutines and
in tracebacks:

goroutine 17 [syscall, locked to thread]:
runtime.goexit()
	.../src/runtime/asm_amd64.s:2197 +0x1

Fix this by putting this goroutine into state _Gdead when it's not in
use instead of _Gsyscall. To keep the goroutine counts correct, we
also add one to sched.ngsys while the goroutine is in _Gdead. The
effect of this is as if the goroutine simply doesn't exist when it's
not in use.

Fixes #16631.
Fixes #16714.

Change-Id: Ieae08a2febd4b3d00bef5c23fd6ca88fb2bb0087
Reviewed-on: https://go-review.googlesource.com/45030
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 97 insertions, 0 deletions

diff --git a/src/runtime/testdata/testprogcgo/numgoroutine.go b/src/runtime/testdata/testprogcgo/numgoroutine.go
new file mode 100644
index 0000000000..c1ac3eff8a
--- /dev/null
+++ b/src/runtime/testdata/testprogcgo/numgoroutine.go
@@ -0,0 +1,97 @@
+// Copyright 2017 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.
+
+package main
+
+/*
+#include <stddef.h>
+#include <pthread.h>
+
+extern void CallbackNumGoroutine();
+
+static void* thread2(void* arg __attribute__ ((unused))) {
+	CallbackNumGoroutine();
+	return NULL;
+}
+
+static void CheckNumGoroutine() {
+	pthread_t tid;
+	pthread_create(&tid, NULL, thread2, NULL);
+	pthread_join(tid, NULL);
+}
+*/
+import "C"
+
+import (
+	"fmt"
+	"runtime"
+	"strings"
+)
+
+var baseGoroutines int
+
+func init() {
+	register("NumGoroutine", NumGoroutine)
+}
+
+func NumGoroutine() {
+	// Test that there are just the expected number of goroutines
+	// running. Specifically, test that the spare M's goroutine
+	// doesn't show up.
+	//
+	// On non-Windows platforms there's a signal handling thread
+	// started by os/signal.init in addition to the main
+	// goroutine.
+	if runtime.GOOS != "windows" {
+		baseGoroutines = 1
+	}
+	if _, ok := checkNumGoroutine("first", 1+baseGoroutines); !ok {
+		return
+	}
+
+	// Test that the goroutine for a callback from C appears.
+	if C.CheckNumGoroutine(); !callbackok {
+		return
+	}
+
+	// Make sure we're back to the initial goroutines.
+	if _, ok := checkNumGoroutine("third", 1+baseGoroutines); !ok {
+		return
+	}
+
+	fmt.Println("OK")
+}
+
+func checkNumGoroutine(label string, want int) (string, bool) {
+	n := runtime.NumGoroutine()
+	if n != want {
+		fmt.Printf("%s NumGoroutine: want %d; got %d\n", label, want, n)
+		return "", false
+	}
+
+	sbuf := make([]byte, 32<<10)
+	sbuf = sbuf[:runtime.Stack(sbuf, true)]
+	n = strings.Count(string(sbuf), "goroutine ")
+	if n != want {
+		fmt.Printf("%s Stack: want %d; got %d:\n%s\n", label, want, n, string(sbuf))
+		return "", false
+	}
+	return string(sbuf), true
+}
+
+var callbackok bool
+
+//export CallbackNumGoroutine
+func CallbackNumGoroutine() {
+	stk, ok := checkNumGoroutine("second", 2+baseGoroutines)
+	if !ok {
+		return
+	}
+	if !strings.Contains(stk, "CallbackNumGoroutine") {
+		fmt.Printf("missing CallbackNumGoroutine from stack:\n%s\n", stk)
+		return
+	}
+
+	callbackok = true
+}