ctdb: Add new helper ctdb_mutex_fcntl_helper

This implements the type of fcntl locking that the recovery lock uses.
The intent is to use it for multiple locks and allow the choice of
helper to be configured.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>

ctdb/packaging/RPM/ctdb.spec.in

@@ -204,6 +204,7 @@ rm -rf $RPM_BUILD_ROOT
 %{_libexecdir}/ctdb/ctdb_lock_helper
 %{_libexecdir}/ctdb/ctdb_event_helper
 %{_libexecdir}/ctdb/ctdb_recovery_helper
+%{_libexecdir}/ctdb/ctdb_mutex_fcntl_helper
 %{_libexecdir}/ctdb/ctdb_natgw
 %{_libexecdir}/ctdb/ctdb_lvs
 %{_libexecdir}/ctdb/ctdb_killtcp

ctdb/server/ctdb_mutex_fcntl_helper.c

@@ -0,0 +1,90 @@
+/*
+   CTDB mutex fcntl lock file helper
+
+   Copyright (C) Martin Schwenke 2015
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, see <http://www.gnu.org/licenses/>.
+*/
+
+#include "replace.h"
+#include "system/filesys.h"
+#include "system/network.h"
+
+/* protocol.h is just needed for ctdb_sock_addr, which is used in system.h */
+#include "protocol/protocol.h"
+#include "common/system.h"
+
+static char *progname = NULL;
+
+static char fcntl_lock(const char *file)
+{
+	int fd;
+	struct flock lock;
+
+	fd = open(file, O_RDWR|O_CREAT, 0600);
+	if (fd == -1) {
+		fprintf(stderr, "%s: Unable to open %s - (%s)\n",
+			progname, file, strerror(errno));
+		return '3';
+	}
+
+	lock.l_type = F_WRLCK;
+	lock.l_whence = SEEK_SET;
+	lock.l_start = 0;
+	lock.l_len = 1;
+	lock.l_pid = 0;
+
+	if (fcntl(fd, F_SETLK, &lock) != 0) {
+		int saved_errno = errno;
+		close(fd);
+		fd = -1;
+		if (saved_errno == EACCES ||
+		    saved_errno == EAGAIN) {
+			/* Lock contention, fail silently */
+			return '1';
+		}
+
+		/* Log an error for any other failure */
+		fprintf(stderr,
+			"%s: Failed to get lock on '%s' - (%s)\n",
+			progname, file, strerror(saved_errno));
+		return '3';
+	}
+
+	return '0';
+}
+
+int main(int argc, char *argv[])
+{
+	char result;
+	int ppid;
+	const char *file = NULL;
+
+	progname = argv[0];
+
+	if (argc != 2) {
+		fprintf(stderr, "Usage: %s <file>\n", progname);
+		exit(1);
+	}
+
+	ppid = getppid();
+	file = argv[1];
+
+	result = fcntl_lock(file);
+	sys_write(STDOUT_FILENO, &result, 1);
+
+	ctdb_wait_for_process_to_exit(ppid);
+
+	return 0;
+}

ctdb/wscript

@@ -451,6 +451,12 @@ def build(bld):
                              samba-util replace tdb''',
                      install_path='${CTDB_HELPER_BINDIR}')
 
+    bld.SAMBA_BINARY('ctdb_mutex_fcntl_helper',
+                     source='server/ctdb_mutex_fcntl_helper.c',
+                     deps='ctdb-system',
+                     includes='include',
+                     install_path='${CTDB_HELPER_BINDIR}')
+
     bld.SAMBA_GENERATOR('ctdb-smnotify-h',
                         source='utils/smnotify/smnotify.x',
                         target='utils/smnotify/smnotify.h',