libshare: nfs: commonify nfs_exports_[un]lock(), FILE_HEADER

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Wilson <gwilson@delphix.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11886

lib/libshare/nfs.c

@@ -0,0 +1,73 @@
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License (the "License").
+ * You may not use this file except in compliance with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or http://www.opensolaris.org/os/licensing.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include "nfs.h"
+
+
+static int nfs_lock_fd = -1;
+
+
+/*
+ * nfs_exports_[lock|unlock] are used to guard against conconcurrent
+ * updates to the exports file. Each protocol is responsible for
+ * providing the necessary locking to ensure consistency.
+ */
+__attribute__((visibility("hidden"))) int
+nfs_exports_lock(const char *name)
+{
+	int err;
+
+	nfs_lock_fd = open(name, O_RDWR | O_CREAT | O_CLOEXEC, 0600);
+	if (nfs_lock_fd == -1) {
+		err = errno;
+		fprintf(stderr, "failed to lock %s: %s\n", name, strerror(err));
+		return (err);
+	}
+
+	if (flock(nfs_lock_fd, LOCK_EX) != 0) {
+		err = errno;
+		fprintf(stderr, "failed to lock %s: %s\n", name, strerror(err));
+		(void) close(nfs_lock_fd);
+		nfs_lock_fd = -1;
+		return (err);
+	}
+
+	return (0);
+}
+
+__attribute__((visibility("hidden"))) void
+nfs_exports_unlock(const char *name)
+{
+	verify(nfs_lock_fd > 0);
+
+	if (flock(nfs_lock_fd, LOCK_UN) != 0) {
+		fprintf(stderr, "failed to unlock %s: %s\n",
+		    name, strerror(errno));
+	}
+
+	(void) close(nfs_lock_fd);
+	nfs_lock_fd = -1;
+}