runtime: co-exist with NPTL's pthread_cancel.

NPTL uses SIGRTMIN (signal 32) to effect thread cancellation.
Go's runtime replaces NPTL's signal handler with its own, and
ends up aborting if a C library that ends up calling
pthread_cancel is used.

This patch prevents runtime from replacing NPTL's handler.

Fixes #6997.

R=golang-codereviews, iant, dvyukov
CC=golang-codereviews
https://golang.org/cl/47540043

misc/cgo/test/cgo_linux_test.go

@@ -7,3 +7,4 @@ package cgotest
 import "testing"
 
 func TestSetgid(t *testing.T) { testSetgid(t) }
+func Test6997(t *testing.T)   { test6997(t) }

misc/cgo/test/issue6997_linux.c

@@ -0,0 +1,26 @@
+// Copyright 2014 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.
+
+#include <pthread.h>
+#include <stdio.h>
+#include <unistd.h>
+
+static pthread_t thread;
+
+static void* threadfunc(void* dummy) {
+	while(1) {
+		sleep(1);
+	}
+}
+
+int StartThread() {
+	return pthread_create(&thread, NULL, &threadfunc, NULL);
+}
+
+int CancelThread() {
+	void *r;
+	pthread_cancel(thread);
+	pthread_join(thread, &r);
+	return (r == PTHREAD_CANCELED);
+}

misc/cgo/test/issue6997_linux.go

@@ -0,0 +1,40 @@
+// Copyright 2014 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.
+
+// Test that pthread_cancel works as expected
+// (NPTL uses SIGRTMIN to implement thread cancellation)
+// See http://golang.org/issue/6997
+package cgotest
+
+/*
+#cgo CFLAGS: -pthread
+#cgo LDFLAGS: -pthread
+extern int StartThread();
+extern int CancelThread();
+*/
+import "C"
+
+import "testing"
+import "time"
+
+func test6997(t *testing.T) {
+	r := C.StartThread()
+	if r != 0 {
+		t.Error("pthread_create failed")
+	}
+	c := make(chan C.int)
+	go func() {
+		time.Sleep(500 * time.Millisecond)
+		c <- C.CancelThread()
+	}()
+
+	select {
+	case r = <-c:
+		if r == 0 {
+			t.Error("pthread finished but wasn't cancelled??")
+		}
+	case <-time.After(5 * time.Second):
+		t.Error("hung in pthread_cancel/pthread_join")
+	}
+}

src/pkg/runtime/signals_linux.h

@@ -41,7 +41,7 @@ SigTab runtime·sigtab[] = {
 	/* 29 */	N, "SIGIO: i/o now possible",
 	/* 30 */	N, "SIGPWR: power failure restart",
 	/* 31 */	N, "SIGSYS: bad system call",
-	/* 32 */	N, "signal 32",
+	/* 32 */	0, "signal 32", /* SIGCANCEL; see issue 6997 */
 	/* 33 */	0, "signal 33", /* SIGSETXID; see issue 3871 */
 	/* 34 */	N, "signal 34",
 	/* 35 */	N, "signal 35",