Implement FAKETIME_FOLLOW_ABSOLUTE feature

See README for feature intent.
Simple regression test at test_follow_absolute.sh.

README

@@ -420,6 +420,15 @@ LD_PRELOAD=/path/to/libfaketime.so.1 \
 # (in a different terminal window or whatever)
 touch -t 2002290123.45 /tmp/my-demo-file.tmp
 
+Setting the environment variable FAKETIME_FOLLOW_ABSOLUTE=1 enables a submode
+of FAKETIME_FOLLOW_FILE behavior where fake time ONLY advances when the follow
+file's timestamp advances. In this mode an application that is not subject to
+libfaketime LD_PRELOAD intercept can absolutely control time for applications
+that are hooked by libfaketime. For example, a host application can control the
+timestamp of a follow file mapped into a container to implement (relatively)
+clean pause/resume behavior for fake time applications running within the
+container.
+
 
 Changing the 'x' modifier during run-time
 -----------------------------------------

src/libfaketime.c

@@ -2701,12 +2701,33 @@ static void parse_ft_string(const char *user_faked_time)
         else
         {
           user_faked_time_timespec.tv_sec = master_file_stats.st_mtime;
-          user_faked_time_timespec.tv_nsec = 0;
+          if (NULL == getenv("FAKETIME_FOLLOW_ABSOLUTE"))
+          {
+            /* Normal FAKETIME_FOLLOW_FILE behavior; fake time coasts between mtime updates */
+            user_faked_time_timespec.tv_nsec = 0;
+          }
+          else
+          {
+            /* Set fake time to nanosecond-precision mtime */
+            user_faked_time_timespec.tv_nsec = master_file_stats.st_mtim.tv_nsec;
+
+            /* Freeze fake time (mtime is absolute truth in this mode) */
+            if (ft_mode != FT_NOOP)
+            {
+              ft_mode = FT_FREEZE;
+            }
+
+            /* Bypass parse_modifiers since we have absolute time */
+            user_faked_time_set = true;
+          }
         }
       }
       if (NULL == getenv("FAKETIME_DONT_RESET"))
         reset_time();
-      goto parse_modifiers;
+      if (!user_faked_time_set)
+      {
+        goto parse_modifiers;
+      }
       break;
 
     case 'i':

test/functests/test_follow_absolute.sh

@@ -0,0 +1,79 @@
+# Tests for FAKETIME_FOLLOW_ABSOLUTE feature.
+#
+# When FAKETIME_FOLLOW_ABSOLUTE=1 is set alongside FAKETIME="%" and
+# FAKETIME_FOLLOW_FILE, time freezes at the follow file's mtime
+# and only advances when the file's mtime changes.
+
+FOLLOW_FILE=".follow_absolute_test_file"
+
+init()
+{
+	typeset testsuite="$1"
+	PLATFORM=$(platform)
+	if [ -z "$PLATFORM" ]; then
+		echo "$testsuite: unknown platform! quitting"
+		return 1
+	fi
+	echo "# PLATFORM=$PLATFORM"
+	return 0
+}
+
+run()
+{
+	init
+
+	run_testcase follow_absolute_basic
+	run_testcase follow_absolute_freeze
+	run_testcase follow_absolute_tracks_mtime
+
+	rm -f "$FOLLOW_FILE"
+}
+
+# Helper to run a command with follow-absolute configuration
+follow_absolute_cmd()
+{
+	FAKETIME_FOLLOW_FILE="$FOLLOW_FILE" \
+	FAKETIME_FOLLOW_ABSOLUTE=1 \
+	fakecmd "%" "$@"
+}
+
+# Test that time matches the follow file's mtime
+follow_absolute_basic()
+{
+	touch -d "2020-03-15 10:30:00 UTC" "$FOLLOW_FILE"
+	typeset actual
+	actual=$(follow_absolute_cmd date -u +"%Y-%m-%d %H:%M:%S")
+	asserteq "$actual" "2020-03-15 10:30:00" \
+		"time should match follow file mtime"
+}
+
+# Test that time stays frozen (does not advance with real time)
+follow_absolute_freeze()
+{
+	touch -d "2020-03-15 10:30:00 UTC" "$FOLLOW_FILE"
+	typeset timestamps
+	timestamps=$(follow_absolute_cmd \
+		perl -e 'print time(), "\n"; sleep(2); print time(), "\n"')
+	typeset first second
+	first=$(echo "$timestamps" | head -1)
+	second=$(echo "$timestamps" | tail -1)
+	asserteq "$first" "$second" \
+		"time should stay frozen within a single process"
+}
+
+# Test that time tracks file mtime changes at millisecond precision
+follow_absolute_tracks_mtime()
+{
+	touch -d "2020-03-15 10:30:00.000 UTC" "$FOLLOW_FILE"
+	typeset first
+	first=$(follow_absolute_cmd \
+		perl -MTime::HiRes=time -e 'printf "%.3f\n", time()')
+
+	touch -d "2020-03-15 10:30:00.005 UTC" "$FOLLOW_FILE"
+	typeset second
+	second=$(follow_absolute_cmd \
+		perl -MTime::HiRes=time -e 'printf "%.3f\n", time()')
+
+	assertneq "$first" "$second" \
+		"time should advance with file mtime (ms precision)"
+}