Fix O(n^2) time bug in --delay-directory-restore

delayed_set_stat avoids inserting duplicate entries into
delayed_set_stat_head. It was doing this by scanning the entire
list.

Normally this list is small, but if --delay-directory-restore is
used (including automatically for incremental archives), this list
grows with the total number of directories in the archive.

The entire scan takes O(n) time. Extracting an archive with n
directories could therefore take O(n^2) time.

The included test uses AT_SKIP_LARGE_FILES, allowing it to optionally be
skipped. It may execute slowly on certain filesystems or disks, as it
creates thousands of directories.

There are still potentially problematic O(n) scans in
find_direct_ancestor and remove_delayed_set_stat, which this patch does
not attempt to fix.

* NEWS: Update.
* src/extract.c (delayed_set_stat_table): Create a table for O(1)
lookups of entries in the delayed_set_stat_head list. The list
remains, as tracking insertion order is important.
(dl_hash, dl_compare): New hash table helper functions.
(delay_set_stat): Create the hash table, replace the O(n) list scan
with a hash_lookup, insert new entries into the hash table.
(remove_delayed_set_stat): Also remove entry from hash table.
(apply_nonancestor_delayed_set_stat): Also remove entry from hash
table.
(extract_finish): Free the (empty) hash table.
* tests/extrac26.at: New file.
* tests/Makefile.am (TESTSUITE_AT): Include extrac26.at.
* tests/testsuite.at: Include extrac26.at.

NEWS

@@ -1,4 +1,4 @@
-GNU tar NEWS - User visible changes. 2023-08-02
+GNU tar NEWS - User visible changes. 2023-08-21
 Please send GNU tar bug reports to <bug-tar@gnu.org>
 
 version TBD
@@ -26,6 +26,9 @@ used, command output will be parsed using strptime(3).
 
 * Bug fixes
 
+** Fixed O(n^2) time complexity bug for large numbers of directories when
+   extracting with --delay-directory-restore or reading incremental archives.
+
 ** tar no longer uses alloca, fixing an unlikely stack overflow.
 
 

src/extract.c

@@ -130,6 +130,9 @@ struct delayed_set_stat
 
 static struct delayed_set_stat *delayed_set_stat_head;
 
+/* Table of delayed stat updates hashed by path; null if none.  */
+static Hash_table *delayed_set_stat_table;
+
 /* A link whose creation we have delayed.  */
 struct delayed_link
   {
@@ -214,6 +217,20 @@ dl_compare (void const *a, void const *b)
   return (da->dev == db->dev) & (da->ino == db->ino);
 }
 
+static size_t
+ds_hash (void const *entry, size_t table_size)
+{
+  struct delayed_set_stat const *ds = entry;
+  return hash_string (ds->file_name, table_size);
+}
+
+static bool
+ds_compare (void const *a, void const *b)
+{
+  struct delayed_set_stat const *dsa = a, *dsb = b;
+  return strcmp (dsa->file_name, dsb->file_name) == 0;
+}
+
 /*  Set up to extract files.  */
 void
 extr_init (void)
@@ -513,11 +530,14 @@ delay_set_stat (char const *file_name, struct tar_stat_info const *st,
   size_t file_name_len = strlen (file_name);
   struct delayed_set_stat *data;
 
-  for (data = delayed_set_stat_head; data; data = data->next)
-    if (strcmp (data->file_name, file_name) == 0)
-      break;
+  if (! (delayed_set_stat_table
+	 || (delayed_set_stat_table = hash_initialize (0, 0, ds_hash,
+	                                               ds_compare, NULL))))
+    xalloc_die ();
+
+  const struct delayed_set_stat key = { .file_name = (char*) file_name };
 
-  if (data)
+  if ((data = hash_lookup (delayed_set_stat_table, &key)) != NULL)
     {
       if (data->interdir)
 	{
@@ -541,6 +561,8 @@ delay_set_stat (char const *file_name, struct tar_stat_info const *st,
       delayed_set_stat_head = data;
       data->file_name_len = file_name_len;
       data->file_name = xstrdup (file_name);
+      if (! hash_insert (delayed_set_stat_table, data))
+	xalloc_die ();
       data->after_links = false;
       if (st)
 	{
@@ -652,6 +674,7 @@ remove_delayed_set_stat (const char *fname)
       if (chdir_current == data->change_dir
 	  && strcmp (data->file_name, fname) == 0)
 	{
+	  hash_remove (delayed_set_stat_table, data);
 	  free_delayed_set_stat (data);
 	  if (prev)
 	    prev->next = next;
@@ -1000,6 +1023,7 @@ apply_nonancestor_delayed_set_stat (char const *file_name, bool after_links)
 	}
 
       delayed_set_stat_head = data->next;
+      hash_remove (delayed_set_stat_table, data);
       free_delayed_set_stat (data);
     }
 }
@@ -1962,6 +1986,13 @@ extract_finish (void)
   /* Finally, fix the status of directories that are ancestors
      of delayed links.  */
   apply_nonancestor_delayed_set_stat ("", 1);
+
+  /* This table should be empty after apply_nonancestor_delayed_set_stat  */
+  if (delayed_set_stat_table != NULL)
+    {
+      hash_free (delayed_set_stat_table);
+      delayed_set_stat_table = NULL;
+    }
 }
 
 bool

tests/Makefile.am

@@ -133,6 +133,7 @@ TESTSUITE_AT = \
  extrac23.at\
  extrac24.at\
  extrac25.at\
+ extrac26.at\
  filerem01.at\
  filerem02.at\
  grow.at\

tests/extrac26.at

@@ -0,0 +1,43 @@
+# Test suite for GNU tar.                             -*- autotest -*-
+# Copyright 2022-2023 Free Software Foundation, Inc.
+#
+# This file is part of GNU tar.
+#
+# GNU tar is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# GNU tar is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+AT_SETUP([extract a large directory tree with --delay-directory-restore])
+AT_KEYWORDS([extract extrac26])
+
+AT_TAR_CHECK([
+AT_SKIP_LARGE_FILES
+AT_TIMEOUT_PREREQ
+
+echo Creating dirtree
+awk 'BEGIN { for (j = 0; j < 300; j++) for (k = 0; k < 300; k++) print "dirtree/" j "/" k }' | \
+  xargs mkdir -p
+
+echo Creating archive
+tar -cf archive.tar dirtree
+
+echo Extracting archive
+mkdir output
+timeout 15 tar -xf archive.tar --delay-directory-restore -C output
+],
+[0],
+[Creating dirtree
+Creating archive
+Extracting archive
+],
+[],[],[],[gnu])
+
+AT_CLEANUP

tests/testsuite.at

@@ -349,6 +349,7 @@ m4_include([extrac22.at])
 m4_include([extrac23.at])
 m4_include([extrac24.at])
 m4_include([extrac25.at])
+m4_include([extrac26.at])
 
 m4_include([backup01.at])