shared/calendarspec: abort calculation after 1000 iterations

We have a bug where we seem to enter an infinite loop when running in the
Europe/Dublin timezone. The timezone is "special" because it has negative SAVE
values. The handling of this should obviously be fixed, but let's use a
belt-and-suspenders approach, and gracefully fail if we fail to find an answer
within a specific number of attempts. The code in this function is rather
complex, and it's hard to rule out another bug in the future.

src/shared/calendarspec.c

@@ -1210,6 +1210,10 @@ static bool matches_weekday(int weekdays_bits, const struct tm *tm, bool utc) {
         return (weekdays_bits & (1 << k));
 }
 
+/* A safety valve: if we get stuck in the calculation, return an error.
+ * C.f. https://bugzilla.redhat.com/show_bug.cgi?id=1941335. */
+#define MAX_CALENDAR_ITERATIONS 1000
+
 static int find_next(const CalendarSpec *spec, struct tm *tm, usec_t *usec) {
         struct tm c;
         int tm_usec;
@@ -1223,7 +1227,7 @@ static int find_next(const CalendarSpec *spec, struct tm *tm, usec_t *usec) {
         c = *tm;
         tm_usec = *usec;
 
-        for (;;) {
+        for (unsigned iteration = 0; iteration < MAX_CALENDAR_ITERATIONS; iteration++) {
                 /* Normalize the current date */
                 (void) mktime_or_timegm(&c, spec->utc);
                 c.tm_isdst = spec->dst;
@@ -1320,6 +1324,14 @@ static int find_next(const CalendarSpec *spec, struct tm *tm, usec_t *usec) {
                 *usec = tm_usec;
                 return 0;
         }
+
+        /* It seems we entered an infinite loop. Let's gracefully return an error instead of hanging or
+         * aborting. This code is also exercised when timers.target is brought up during early boot, so
+         * aborting here is problematic and hard to diagnose for users. */
+        _cleanup_free_ char *s = NULL;
+        (void) calendar_spec_to_string(spec, &s);
+        return log_warning_errno(SYNTHETIC_ERRNO(EDEADLK),
+                                 "Infinite loop in calendar calculation: %s", strna(s));
 }
 
 static int calendar_spec_next_usec_impl(const CalendarSpec *spec, usec_t usec, usec_t *ret_next) {