fs-util: introduce open_parent() helper

We often open the parent directory of a path. Let's add a common helper
for that, that shortens our code a bit and adds some extra safety
checks, for example it will fail if used on the root directory (which
doesn't really have a parent).

The helper is actually generalized from a function in btrfs-util.[ch]
which already existed for this purpose.

src/basic/btrfs-util.c

@@ -28,6 +28,7 @@
 #include "device-nodes.h"
 #include "fd-util.h"
 #include "fileio.h"
+#include "fs-util.h"
 #include "io-util.h"
 #include "macro.h"
 #include "missing.h"
@@ -59,23 +60,6 @@ static int validate_subvolume_name(const char *name) {
         return 0;
 }
 
-static int open_parent(const char *path, int flags) {
-        _cleanup_free_ char *parent = NULL;
-        int fd;
-
-        assert(path);
-
-        parent = dirname_malloc(path);
-        if (!parent)
-                return -ENOMEM;
-
-        fd = open(parent, flags);
-        if (fd < 0)
-                return -errno;
-
-        return fd;
-}
-
 static int extract_subvolume_name(const char *path, const char **subvolume) {
         const char *fn;
         int r;
@@ -144,7 +128,7 @@ int btrfs_subvol_make(const char *path) {
         if (r < 0)
                 return r;
 
-        fd = open_parent(path, O_RDONLY|O_NOCTTY|O_CLOEXEC|O_DIRECTORY);
+        fd = open_parent(path, O_CLOEXEC, 0);
         if (fd < 0)
                 return fd;
 
@@ -1283,7 +1267,7 @@ int btrfs_subvol_remove(const char *path, BtrfsRemoveFlags flags) {
         if (r < 0)
                 return r;
 
-        fd = open_parent(path, O_RDONLY|O_NOCTTY|O_CLOEXEC|O_DIRECTORY);
+        fd = open_parent(path, O_CLOEXEC, 0);
         if (fd < 0)
                 return fd;
 
@@ -1723,7 +1707,7 @@ int btrfs_subvol_snapshot_fd(int old_fd, const char *new_path, BtrfsSnapshotFlag
         if (r < 0)
                 return r;
 
-        new_fd = open_parent(new_path, O_RDONLY|O_NOCTTY|O_CLOEXEC|O_DIRECTORY);
+        new_fd = open_parent(new_path, O_CLOEXEC, 0);
         if (new_fd < 0)
                 return new_fd;