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.

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])