shared/calendarspec: fix normalization when DST is negative

When trying to calculate the next firing of 'hourly', we'd lose the
tm_isdst value on the next iteration.

On most systems in Europe/Dublin it would cause a 100% cpu hang due to
timers restarting.

This happens in Europe/Dublin because Ireland defines the Irish Standard Time
as UTC+1, so winter time is encoded in tzdata as negative 1 hour of daylight
saving.

Before this patch:
$ env TZ=IST-1GMT-0,M10.5.0/1,M3.5.0/1 systemd-analyze calendar --base-time='Sat 2025-03-29 22:00:00 UTC' --iterations=5 'hourly'
  Original form: hourly
Normalized form: *-*-* *:00:00
    Next elapse: Sat 2025-03-29 23:00:00 GMT
       (in UTC): Sat 2025-03-29 23:00:00 UTC
       From now: 13h ago
   Iteration #2: Sun 2025-03-30 00:00:00 GMT
       (in UTC): Sun 2025-03-30 00:00:00 UTC
       From now: 12h ago
   Iteration #3: Sun 2025-03-30 00:00:00 GMT  <-- note every next iteration having the same firing time
       (in UTC): Sun 2025-03-30 00:00:00 UTC
       From now: 12h ago
...

With this patch:
$ env TZ=IST-1GMT-0,M10.5.0/1,M3.5.0/1 systemd-analyze calendar --base-time='Sat 2025-03-29 22:00:00 UTC' --iterations=5 'hourly'
  Original form: hourly
Normalized form: *-*-* *:00:00
    Next elapse: Sat 2025-03-29 23:00:00 GMT
       (in UTC): Sat 2025-03-29 23:00:00 UTC
       From now: 13h ago
   Iteration #2: Sun 2025-03-30 00:00:00 GMT
       (in UTC): Sun 2025-03-30 00:00:00 UTC
       From now: 12h ago
   Iteration #3: Sun 2025-03-30 02:00:00 IST  <-- the expected 1 hour jump
       (in UTC): Sun 2025-03-30 01:00:00 UTC
       From now: 11h ago
...

This bug isn't reproduced on Debian and Ubuntu because they mitigate it by
using the rearguard version of tzdata. ArchLinux and NixOS don't, so it would
cause pid1 to spin during DST transition.

This is how the affected tzdata looks like:
$ zdump -V -c 2024,2025 Europe/Dublin
Europe/Dublin  Sun Mar 31 00:59:59 2024 UT = Sun Mar 31 00:59:59 2024 GMT isdst=1 gmtoff=0
Europe/Dublin  Sun Mar 31 01:00:00 2024 UT = Sun Mar 31 02:00:00 2024 IST isdst=0 gmtoff=3600
Europe/Dublin  Sun Oct 27 00:59:59 2024 UT = Sun Oct 27 01:59:59 2024 IST isdst=0 gmtoff=3600
Europe/Dublin  Sun Oct 27 01:00:00 2024 UT = Sun Oct 27 01:00:00 2024 GMT isdst=1 gmtoff=0

Compare it to Europe/London:
$ zdump -V -c 2024,2025 Europe/London
Europe/London  Sun Mar 31 00:59:59 2024 UT = Sun Mar 31 00:59:59 2024 GMT isdst=0 gmtoff=0
Europe/London  Sun Mar 31 01:00:00 2024 UT = Sun Mar 31 02:00:00 2024 BST isdst=1 gmtoff=3600
Europe/London  Sun Oct 27 00:59:59 2024 UT = Sun Oct 27 01:59:59 2024 BST isdst=1 gmtoff=3600
Europe/London  Sun Oct 27 01:00:00 2024 UT = Sun Oct 27 01:00:00 2024 GMT isdst=0 gmtoff=0

Fixes #32039.

src/shared/calendarspec.c

@@ -1235,14 +1235,43 @@ static bool matches_weekday(int weekdays_bits, const struct tm *tm, bool utc) {
         return (weekdays_bits & (1 << k));
 }
 
+static int tm_compare(const struct tm *t1, const struct tm *t2) {
+        int r;
+
+        assert(t1);
+        assert(t2);
+
+        r = CMP(t1->tm_year, t2->tm_year);
+        if (r != 0)
+                return r;
+
+        r = CMP(t1->tm_mon, t2->tm_mon);
+        if (r != 0)
+                return r;
+
+        r = CMP(t1->tm_mday, t2->tm_mday);
+        if (r != 0)
+                return r;
+
+        r = CMP(t1->tm_hour, t2->tm_hour);
+        if (r != 0)
+                return r;
+
+        r = CMP(t1->tm_min, t2->tm_min);
+        if (r != 0)
+                return r;
+
+        return CMP(t1->tm_sec, t2->tm_sec);
+}
+
 /* 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;
+        int tm_usec, r;
-        int r;
+        bool invalidate_dst = false;
 
         /* Returns -ENOENT if the expression is not going to elapse anymore */
 
@@ -1255,7 +1284,8 @@ static int find_next(const CalendarSpec *spec, struct tm *tm, usec_t *usec) {
         for (unsigned iteration = 0; iteration < MAX_CALENDAR_ITERATIONS; iteration++) {
                 /* Normalize the current date */
                 (void) mktime_or_timegm_usec(&c, spec->utc, /* ret= */ NULL);
-                c.tm_isdst = spec->dst;
+                if (!invalidate_dst)
+                        c.tm_isdst = spec->dst;
 
                 c.tm_year += 1900;
                 r = find_matching_component(spec, spec->year, &c, &c.tm_year);
@@ -1345,6 +1375,18 @@ static int find_next(const CalendarSpec *spec, struct tm *tm, usec_t *usec) {
                 if (r == 0)
                         continue;
 
+                r = tm_compare(tm, &c);
+                if (r == 0) {
+                        assert(tm_usec + 1 <= 1000000);
+                        r = CMP(*usec, (usec_t) tm_usec + 1);
+                }
+                if (r >= 0) {
+                        /* We're stuck - advance, let mktime determine DST transition and try again. */
+                        invalidate_dst = true;
+                        c.tm_hour++;
+                        continue;
+                }
+
                 *tm = c;
                 *usec = tm_usec;
                 return 0;

src/test/test-calendarspec.c

@@ -47,7 +47,7 @@ static void _test_next(int line, const char *input, const char *new_tz, usec_t a
         if (old_tz)
                 old_tz = strdupa_safe(old_tz);
 
-        if (!isempty(new_tz))
+        if (!isempty(new_tz) && !strchr(new_tz, ','))
                 new_tz = strjoina(":", new_tz);
 
         assert_se(set_unset_env("TZ", new_tz, true) == 0);
@@ -219,6 +219,8 @@ TEST(calendar_spec_next) {
         /* Check that we don't start looping if mktime() moves us backwards */
         test_next("Sun *-*-* 01:00:00 Europe/Dublin", "", 1616412478000000, 1617494400000000);
         test_next("Sun *-*-* 01:00:00 Europe/Dublin", "IST", 1616412478000000, 1617494400000000);
+        /* Europe/Dublin TZ that moves DST backwards */
+        test_next("hourly", "IST-1GMT-0,M10.5.0/1,M3.5.0/1", 1743292800000000, 1743296400000000);
 }
 
 TEST(calendar_spec_from_string) {