misc/cgo/test: add test for issue 3871: cgo setgid hang on GNU/Linux

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/6445049

misc/cgo/test/basic.go

@@ -11,6 +11,7 @@ package cgotest
 #include <stdlib.h>
 #include <sys/stat.h>
 #include <errno.h>
+#include <unistd.h>
 
 #define SHIFT(x, y)  ((x)<<(y))
 #define KILO SHIFT(1, 10)
@@ -57,6 +58,7 @@ import "C"
 import (
 	"syscall"
 	"testing"
+	"time"
 	"unsafe"
 )
 
@@ -124,6 +126,20 @@ func testMultipleAssign(t *testing.T) {
 	C.free(unsafe.Pointer(p))
 }
 
+func testSetgid(t *testing.T) {
+	// Issue 3871.
+	c := make(chan bool)
+	go func() {
+		C.setgid(0)
+		c <- true
+	}()
+	select {
+	case <-c:
+	case <-time.After(5 * time.Second):
+		t.Error("setgid hung")
+	}
+}
+
 var (
 	cuint  = (C.uint)(0)
 	culong C.ulong

misc/cgo/test/cgo_test.go

@@ -27,5 +27,6 @@ func Test1328(t *testing.T)                { test1328(t) }
 func TestParallelSleep(t *testing.T)       { testParallelSleep(t) }
 func TestSetEnv(t *testing.T)              { testSetEnv(t) }
 func TestHelpers(t *testing.T)             { testHelpers(t) }
+func TestSetgid(t *testing.T)              { testSetgid(t) }
 
 func BenchmarkCgoCall(b *testing.B) { benchCgoCall(b) }