find: ignore more vanished subdirectories with -ignore_readdir_race

Similar to commit 889d001ab750 which handles vanished files better,
also fix the race for subdirectories (FTS_DNR).

Reproducer:
In average, the following produced 6-10 failures out of 1000 runs:
  $ mkdir testdir
  $ while :; do mkdir testdir/foo; rmdir testdir/foo; done &
  $ for f in $(seq 1000); do \
      find testdir -ignore_readdir_race -ls ; done >/dev/null
  find: 'testdir/foo': No such file or directory
  find: 'testdir/foo': No such file or directory

* find/ftsfind.c (consider_visiting): Return when FTS returned ENOENT
for FTS_DNR, i.e., unreadable directory, with -ignore_readdir_race.
* tests/find/readdir_race.sh: Add test.
* tests/local.mk (sh_tests): Reference it.
* NEWS (Bug Fixes): Amend and improve description of the previous fix.

See also https://savannah.gnu.org/bugs/?45930

NEWS

@@ -4,8 +4,9 @@ GNU findutils NEWS - User visible changes.      -*- outline -*- (allout)
 
 ** Bug Fixes
 
-  'find -ignore_readdir_race' now has a race between FTS read and the visiting
-  of the entry when the file was removed. [#45930]
+  'find -ignore_readdir_race' now better handles races between FTS reading a
+  directory and visiting its entries when the file or directory was meanwhile
+  removed. [#45930]
 
   To fix a POSIX compatibility bug, -exec foo Z{} + is no longer a
   complete predicate, because '+' is only a terminator when it follows

find/ftsfind.c

@@ -304,6 +304,9 @@ consider_visiting (FTS *p, FTSENT *ent)
     }
   if (ent->fts_info == FTS_DNR)
     {
+      /* Ignore ENOENT error for vanished directories.  */
+      if (ENOENT == ent->fts_errno && options.ignore_readdir_race)
+        return;
       nonfatal_target_file_error (ent->fts_errno, ent->fts_path);
       if (options.do_dir_first)
         {
@@ -355,7 +358,7 @@ consider_visiting (FTS *p, FTSENT *ent)
             }
           else
             {
-             /* Ignore unlink() error for vanished files.  */
+             /* Ignore ENOENT error for vanished files.  */
              if (ENOENT == ent->fts_errno && options.ignore_readdir_race)
                  return;
 

tests/find/readdir_race.sh

@@ -0,0 +1,48 @@
+#!/bin/sh
+# Verify that -ignore_readdir_race properly handles vanished files/directories.
+
+# Copyright (C) 2024 Free Software Foundation, Inc.
+
+# This program 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.
+
+# This program 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 <https://www.gnu.org/licenses/>.
+
+. "${srcdir=.}/tests/init.sh"; fu_path_prepend_
+print_ver_ find
+
+# Require seq(1) for this test - which may not be available
+# on some systems, e.g on some *BSDs.
+seq 2 >/dev/null 2>&1 \
+  || skip_ "required utility 'seq' missing"
+
+mkdir testdir || framework_failure_
+
+# Constantly create and remove a subdirectory in the background.
+# Disable shell debugging for this part.
+# Terminate the background process later again.
+endless_mkdir_rmdir() {
+  { set +x; } 2>/dev/null
+  while :; do mkdir testdir/foo; rmdir testdir/foo; done
+}
+endless_mkdir_rmdir & pid=$!
+cleanup_() { kill $pid 2>/dev/null && wait $pid; }
+
+# Now run find(1) many times.
+> err
+for f in $(seq 1000); do \
+  find testdir -ignore_readdir_race -ls 2>> err || fail=1; \
+done > out
+
+test 1000 -le $( wc -l < out ) || fail=1
+compare /dev/null err || fail=1
+
+Exit $fail

tests/local.mk

@@ -131,6 +131,7 @@ sh_tests = \
   tests/find/newer.sh \
   tests/find/opt-numeric-arg.sh \
   tests/find/sv-bug-66365-exec.sh \
+  tests/find/readdir_race.sh \
   tests/find/user-group-max.sh \
   tests/xargs/conflicting_opts.sh \
   tests/xargs/verbose-quote.sh \