os/user: don't create C function mygetgrouplist

Instead of exporting the C function mygetgrouplist as a global symbol to
conflict with other symbols of the same name, use trivial Go code and a
static C function.

Change-Id: I98dd667814d0a0ed8f7b1d4cfc6483d5a6965b26
Reviewed-on: https://go-review.googlesource.com/23008
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/user/getgrouplist_darwin.c → src/os/user/getgrouplist_darwin.go

@@ -2,13 +2,14 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
+package user
 
+/*
 #include <unistd.h>
 #include <sys/types.h>
 #include <stdlib.h>
 
-int mygetgrouplist(const char* user, gid_t group, gid_t* groups, int* ngroups) {
+static int mygetgrouplist(const char* user, gid_t group, gid_t* groups, int* ngroups) {
 	int* buf = malloc(*ngroups * sizeof(int));
 	int rv = getgrouplist(user, (int) group, buf, ngroups);
 	int i;
@@ -20,3 +21,9 @@ int mygetgrouplist(const char* user, gid_t group, gid_t* groups, int* ngroups) {
 	free(buf);
 	return rv;
 }
+*/
+import "C"
+
+func getGroupList(name *C.char, userGID C.gid_t, gids *C.gid_t, n *C.int) C.int {
+	return C.mygetgrouplist(name, userGID, gids, n)
+}

src/os/user/getgrouplist_unix.c → src/os/user/getgrouplist_unix.go

@@ -2,13 +2,21 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
 // +build dragonfly freebsd !android,linux netbsd openbsd
 
+package user
+
+/*
 #include <unistd.h>
 #include <sys/types.h>
 #include <grp.h>
 
-int mygetgrouplist(const char* user, gid_t group, gid_t* groups, int* ngroups) {
+static int mygetgrouplist(const char* user, gid_t group, gid_t* groups, int* ngroups) {
 	return getgrouplist(user, group, groups, ngroups);
 }
+*/
+import "C"
+
+func getGroupList(name *C.char, userGID C.gid_t, gids *C.gid_t, n *C.int) C.int {
+	return C.mygetgrouplist(name, userGID, gids, n)
+}

src/os/user/listgroups_unix.go

@@ -16,8 +16,6 @@ import (
 #include <unistd.h>
 #include <sys/types.h>
 #include <stdlib.h>
-
-extern int mygetgrouplist(const char* user, gid_t group, gid_t* groups, int* ngroups);
 */
 import "C"
 
@@ -32,7 +30,7 @@ func listGroups(u *User) ([]string, error) {
 
 	n := C.int(256)
 	gidsC := make([]C.gid_t, n)
-	rv := C.mygetgrouplist(nameC, userGID, &gidsC[0], &n)
+	rv := getGroupList(nameC, userGID, &gidsC[0], &n)
 	if rv == -1 {
 		// More than initial buffer, but now n contains the correct size.
 		const maxGroups = 2048
@@ -40,7 +38,7 @@ func listGroups(u *User) ([]string, error) {
 			return nil, fmt.Errorf("user: list groups for %s: member of more than %d groups", u.Username, maxGroups)
 		}
 		gidsC = make([]C.gid_t, n)
-		rv := C.mygetgrouplist(nameC, userGID, &gidsC[0], &n)
+		rv := getGroupList(nameC, userGID, &gidsC[0], &n)
 		if rv == -1 {
 			return nil, fmt.Errorf("user: list groups for %s failed (changed groups?)", u.Username)
 		}