cgo, runtime: diagnose callback on non-Go thread

Before:
$ go run x.go
signal 11 (core dumped)
$

After:
$ go run x.go
runtime: cgo callback on thread not created by Go.
signal 11 (core dumped)
$

For issue 3068.
Not a fix, but as much of a fix as we can do before Go 1.

R=golang-dev, rogpeppe, gri
CC=golang-dev
https://golang.org/cl/5781047

1 files changed, 10 insertions, 0 deletions

diff --git a/src/pkg/runtime/thread_linux.c b/src/pkg/runtime/thread_linux.c
index 6b428440e0..af765d53b9 100644
--- a/src/pkg/runtime/thread_linux.c
+++ b/src/pkg/runtime/thread_linux.c
@@ -255,3 +255,13 @@ runtime·setprof(bool on)
 {
 	USED(on);
 }
+
+static int8 badcallback[] = "runtime: cgo callback on thread not created by Go.\n";
+
+// This runs on a foreign stack, without an m or a g.  No stack split.
+#pragma textflag 7
+void
+runtime·badcallback(void)
+{
+	runtime·write(2, badcallback, sizeof badcallback - 1);
+}