mountpoint-util: add new helper name_to_handle_at_try_fid()

Newer kernels support a new flag for name_to_handle_at(): AT_HANDLE_FID.
This flag is supposed to return an identifier for an inode that we can
use for checking inode identity. It's supposed to be a replacement for
checking .st_ino which doesn't work anymore today because inode numbers
are no longer unique on file systems (not on overlayfs, and not on btrfs
for example). Hence, be a good citizen and add infrastructure to support
AT_HANDLE_FID. Unfortunately that doesn't work for old kernels, hence
add a fallback logic: if we can use the flag, use it. If we cannot use
name_to_handle_at() without it, which might give us a good ID too. But
of course tha tcan fail as well, which callers have to check.

src/basic/missing_fcntl.h

@@ -92,3 +92,7 @@
 #define RAW_O_LARGEFILE 00100000
 #endif
 #endif
+
+#ifndef AT_HANDLE_FID
+#define AT_HANDLE_FID AT_REMOVEDIR
+#endif

src/basic/mountpoint-util.c

@@ -13,6 +13,7 @@
 #include "fileio.h"
 #include "filesystems.h"
 #include "fs-util.h"
+#include "missing_fcntl.h"
 #include "missing_fs.h"
 #include "missing_mount.h"
 #include "missing_stat.h"
@@ -62,7 +63,8 @@ int name_to_handle_at_loop(
 
         size_t n = ORIGINAL_MAX_HANDLE_SZ;
 
-        assert((flags & ~(AT_SYMLINK_FOLLOW|AT_EMPTY_PATH)) == 0);
+        assert(fd >= 0 || fd == AT_FDCWD);
+        assert((flags & ~(AT_SYMLINK_FOLLOW|AT_EMPTY_PATH|AT_HANDLE_FID)) == 0);
 
         /* We need to invoke name_to_handle_at() in a loop, given that it might return EOVERFLOW when the specified
          * buffer is too small. Note that in contrast to what the docs might suggest, MAX_HANDLE_SZ is only good as a
@@ -119,6 +121,30 @@ int name_to_handle_at_loop(
         }
 }
 
+int name_to_handle_at_try_fid(
+                int fd,
+                const char *path,
+                struct file_handle **ret_handle,
+                int *ret_mnt_id,
+                int flags) {
+
+        int r;
+
+        assert(fd >= 0 || fd == AT_FDCWD);
+
+        /* First issues name_to_handle_at() with AT_HANDLE_FID. If this fails and this is not a fatal error
+         * we'll try without the flag, in order to support older kernels that didn't have AT_HANDLE_FID
+         * (i.e. older than Linux 6.5). */
+
+        r = name_to_handle_at_loop(fd, path, ret_handle, ret_mnt_id, flags | AT_HANDLE_FID);
+        if (r >= 0)
+                return r;
+        if (is_name_to_handle_at_fatal_error(r))
+                return r;
+
+        return name_to_handle_at_loop(fd, path, ret_handle, ret_mnt_id, flags & ~AT_HANDLE_FID);
+}
+
 static int fd_fdinfo_mnt_id(int fd, const char *filename, int flags, int *ret_mnt_id) {
         char path[STRLEN("/proc/self/fdinfo/") + DECIMAL_STR_MAX(int)];
         _cleanup_free_ char *fdinfo = NULL;

src/basic/mountpoint-util.h

@@ -39,6 +39,7 @@
 bool is_name_to_handle_at_fatal_error(int err);
 
 int name_to_handle_at_loop(int fd, const char *path, struct file_handle **ret_handle, int *ret_mnt_id, int flags);
+int name_to_handle_at_try_fid(int fd, const char *path, struct file_handle **ret_handle, int *ret_mnt_id, int flags);
 
 bool file_handle_equal(const struct file_handle *a, const struct file_handle *b);