find: fix -used predicate

* find/pred.c (pred_used): Reverse the check for the delta between
the atime and ctime of the file to accomodate to how pred_timewindow
is working.
Also return false if atime < ctime.
Bug introduced in FINDUTILS_4_3_3-1 in commit a4718831e17.
* tests/find/used.sh: Add test.
* tests/local.mk (all_tests): Reference the test.
* NEWS (Bug Fixes): Mention the fix.

Reported by Martin Schulte in
<https://lists.gnu.org/r/bug-findutils/2019-11/msg00010.html>

NEWS

@@ -12,6 +12,10 @@ of each constructed command line to be executed.  [#57291]
 find no longer crashes when an XFS filesystem is heavily changed during the run.
 Discussed at: <https://lists.gnu.org/r/bug-gnulib/2020-04/msg00068.html>
 
+find -used works again.  This predicate was not working properly since adding
+the support for sub-second timestamp resolution for various predicates in
+FINDUTILS_4_3_3-1 back in 2007.
+
 #54730: Add additional valuable example of find -quit
 
 #48135: Fix testsuite error on Hurd and BSD related to ln

find/pred.c

@@ -1163,11 +1163,15 @@ pred_used (const char *pathname, struct stat *stat_buf, struct predicate *pred_p
 
   (void) pathname;
 
-  /* TODO: this needs to be retested carefully (manually, if necessary) */
   at = get_stat_atime (stat_buf);
   ct = get_stat_ctime (stat_buf);
-  delta.tv_sec  = at.tv_sec  - ct.tv_sec;
-  delta.tv_nsec = at.tv_nsec - ct.tv_nsec;
+
+  /* Always evaluate to false if atime < ctime.  */
+  if (compare_ts (at, ct) < 0)
+    return false;
+
+  delta.tv_sec  = ct.tv_sec  - at.tv_sec;
+  delta.tv_nsec = ct.tv_nsec - at.tv_nsec;
   if (delta.tv_nsec < 0)
     {
       delta.tv_nsec += 1000000000;

tests/find/used.sh

@@ -0,0 +1,94 @@
+#!/bin/sh
+# Verify that find -used works.
+
+# Copyright (C) 2020 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/>.
+
+. "${srcdir=.}/tests/init.sh"; fu_path_prepend_
+print_ver_ find
+
+# Create sample files with the access date D=1,3,5,7 days in the future.
+for d in 1 3 5 7; do
+  touch -a -d "$(date -d "$d day" '+%Y-%m-%d %H:%M:%S')" t$d \
+    || skip_ "creating files with future timestamp failed"
+
+  # Check with stat(1) the access (%X) vs. the status change time (%z).
+  exp="$( expr 86400 '*' $d )" \
+    && x="$( stat -c "%X" t$d )" \
+    && z="$( stat -c "%Z" t$d )" \
+    && tdiff="$( expr "$x" - "$z" )" \
+    && test "$tdiff" -ge "$exp" \
+    || skip_ "cannot verify timestamps of sample files"
+done
+# Create another sample file with timestamp now.
+touch t0 \
+  || skip_ "creating sample file failed"
+
+stat -c "Name: %n  Access: %x  Change: %z" t? || : # ignore error.
+
+# Verify the output for "find -used $d".  Use even number of days to avoid
+# possibly strange effects due to atime/ctime precision etc.
+for d in -8 -6 -4 -2 -0 0 2 4 6 8 +0 +2 +4 +6 +8; do
+  echo "== testing: find -used $d"
+  find . -type f -name 't*' -used $d > out2 \
+    || fail=1
+  LC_ALL=C sort out2 || framework_failure_
+done > out
+
+cat <<\EOF > exp || framework_failure_
+== testing: find -used -8
+./t0
+./t1
+./t3
+./t5
+./t7
+== testing: find -used -6
+./t0
+./t1
+./t3
+./t5
+== testing: find -used -4
+./t0
+./t1
+./t3
+== testing: find -used -2
+./t0
+./t1
+== testing: find -used -0
+== testing: find -used 0
+== testing: find -used 2
+== testing: find -used 4
+== testing: find -used 6
+== testing: find -used 8
+== testing: find -used +0
+./t1
+./t3
+./t5
+./t7
+== testing: find -used +2
+./t3
+./t5
+./t7
+== testing: find -used +4
+./t5
+./t7
+== testing: find -used +6
+./t7
+== testing: find -used +8
+EOF
+
+compare exp out || { fail=1; cat out; }
+
+Exit $fail

tests/local.mk

@@ -116,6 +116,7 @@ all_tests = \
   tests/find/exec-plus-last-file.sh \
   tests/find/refuse-noop.sh \
   tests/find/debug-missing-arg.sh \
+  tests/find/used.sh \
   tests/xargs/verbose-quote.sh \
   $(all_root_tests)