dtar: fix creation with symlink and --preserve-* (#629)

Fix creation of corrupted archive with symlink when
--preserve-{xattrs,acl,flags} command line option is used.

When it is provided a valid file descriptor, libarchive
archive_read_disk_entry_from_file() calls fstat() which follows
symlinks. This commit introduces an exception for symlinks to not
provide a file descriptor to archive_read_disk_entry_from_file() which
makes it call lstat() on entry path and create a valid symlink archive
entry eventually.

fix #628

Signed-off-by: Rémi Palancher <[email protected]>

Author: rezib

src/common/mfu_flist_archive.c

@@ -427,7 +427,17 @@ static int encode_header(
             }
             archive_read_disk_set_behavior(source, flags);
 
-            /* build the entry by querying the item associated with the open file descriptor */
+            /* build the entry by querying the item associated with the open
+             * file descriptor, on which libarchive calls fstat(). For symlinks,
+             * ignore this file description to make libarchive() call lstat() on
+             * the file instead and define a symlink entry eventually.
+             */
+            mfu_filetype type = mfu_flist_file_get_type(flist, idx);
+            if (type == MFU_TYPE_LINK) {
+                mfu_close(fname, fd);
+                fd = -1;
+            }
+
             int r = archive_read_disk_entry_from_file(source, entry, fd, NULL);
             if (r != ARCHIVE_OK) {
                 MFU_LOG(MFU_LOG_ERR, "Failed to define entry for '%s': archive_read_disk_entry_from_file(): %s",