WIP: Concurrent modifications to "/etc/dfs/sharetab" does not work

Signed-off-by: Prakash Surya <prakash.surya@delphix.com>

Requires-builders: all

include/sys/fs/zfs.h

@@ -1072,6 +1072,7 @@ typedef struct ddt_histogram {
 #define	ZFS_DRIVER	"zfs"
 #define	ZFS_DEV		"/dev/zfs"
 #define	ZFS_SHARETAB	"/etc/dfs/sharetab"
+#define	ZFS_SHARETAB_LOCK	"/etc/dfs/sharetab.lck"
 
 #define	ZFS_SUPER_MAGIC	0x2fc12fc1
 

lib/libshare/libshare.c

@@ -26,6 +26,7 @@
 
 #include <stdio.h>
 #include <stdlib.h>
+#include <fcntl.h>
 #include <errno.h>
 #include <strings.h>
 #include <libintl.h>
@@ -165,6 +166,58 @@ parse_sharetab(sa_handle_impl_t impl_handle)
 	fclose(fp);
 }
 
+static int sharetab_fd = -1;
+
+int
+sharetab_lock(void)
+{
+	struct flock lock;
+
+	assert(sharetab_fd == -1);
+
+	if (mkdir("/etc/dfs", 0755) < 0 && errno != EEXIST) {
+		perror("sharetab_lock: failed to create /etc/dfs");
+		return (-1);
+	}
+
+	sharetab_fd = open(ZFS_SHARETAB_LOCK, (O_RDWR | O_CREAT), 0600);
+
+	if (sharetab_fd < 0) {
+		perror("sharetab_lock: failed to open");
+		return (-1);
+	}
+
+	lock.l_type = F_WRLCK;
+	lock.l_whence = SEEK_SET;
+	lock.l_start = 0;
+	lock.l_len = 0;
+	if (fcntl(sharetab_fd, F_SETLKW, &lock) < 0) {
+		perror("sharetab_lock: failed to lock");
+		return (-1);
+	}
+	return (0);
+}
+
+int
+sharetab_unlock(void)
+{
+	struct flock lock;
+	int retval = -1;
+
+	if (sharetab_fd < 0)
+		return (-1);
+	lock.l_type = F_UNLCK;
+	lock.l_whence = SEEK_SET;
+	lock.l_start = 0;
+	lock.l_len = 0;
+	if (fcntl(sharetab_fd, F_SETLK, &lock) == 0)
+		retval = 0;
+	close(sharetab_fd);
+	sharetab_fd = -1;
+	return (retval);
+}
+
+
 static void
 update_sharetab(sa_handle_impl_t impl_handle)
 {
@@ -175,9 +228,6 @@ update_sharetab(sa_handle_impl_t impl_handle)
 	sa_fstype_t *fstype;
 	const char *resource;
 
-	if (mkdir("/etc/dfs", 0755) < 0 && errno != EEXIST) {
-		return;
-	}
 
 	temp_fd = mkstemp(tempfile);
 

lib/libspl/include/libshare.h

@@ -83,6 +83,9 @@ extern sa_share_t sa_find_share(sa_handle_t, char *);
 extern int sa_enable_share(sa_group_t, char *);
 extern int sa_disable_share(sa_share_t, char *);
 
+extern int sharetab_lock(void);
+extern int sharetab_unlock(void);
+
 /* protocol specific interfaces */
 extern int sa_parse_legacy_options(sa_group_t, char *, char *);
 

lib/libzfs/libzfs_mount.c

@@ -864,6 +864,7 @@ zfs_share_proto(zfs_handle_t *zhp, zfs_share_proto_t *proto)
 	if (!zfs_is_mountable(zhp, mountpoint, sizeof (mountpoint), NULL))
 		return (0);
 
+	verify(sharetab_lock() == 0);
 	for (curr_proto = proto; *curr_proto != PROTO_END; curr_proto++) {
 		/*
 		 * Return success if there are no share options.
@@ -879,6 +880,7 @@ zfs_share_proto(zfs_handle_t *zhp, zfs_share_proto_t *proto)
 			(void) zfs_error_fmt(hdl, EZFS_SHARENFSFAILED,
 			    dgettext(TEXT_DOMAIN, "cannot share '%s': %s"),
 			    zfs_get_name(zhp), sa_errorstr(ret));
+			verify(sharetab_unlock() == 0);
 			return (-1);
 		}
 
@@ -910,6 +912,7 @@ zfs_share_proto(zfs_handle_t *zhp, zfs_share_proto_t *proto)
 				    proto_table[*curr_proto].p_share_err,
 				    dgettext(TEXT_DOMAIN, "cannot share '%s'"),
 				    zfs_get_name(zhp));
+				verify(sharetab_unlock() == 0);
 				return (-1);
 			}
 			hdl->libzfs_shareflags |= ZFSSHARE_MISS;
@@ -925,17 +928,20 @@ zfs_share_proto(zfs_handle_t *zhp, zfs_share_proto_t *proto)
 				    proto_table[*curr_proto].p_share_err,
 				    dgettext(TEXT_DOMAIN, "cannot share '%s'"),
 				    zfs_get_name(zhp));
+				verify(sharetab_unlock() == 0);
 				return (-1);
 			}
 		} else {
 			(void) zfs_error_fmt(hdl,
 			    proto_table[*curr_proto].p_share_err,
 			    dgettext(TEXT_DOMAIN, "cannot share '%s'"),
 			    zfs_get_name(zhp));
+			verify(sharetab_unlock() == 0);
 			return (-1);
 		}
 
 	}
+	verify(sharetab_unlock() == 0);
 	return (0);
 }