calendarspec: warn on weekday/date conflict in systemd-analyze and systemd-run

When a fixed date (e.g. 2027-01-01) is paired with a weekday constraint
(e.g. Thu) that does not match, the timer silently never elapses.

Add calendar_spec_from_string_full(..., warn_on_weekday_mismatch) so
user-facing tools can opt in to a log_warning() at parse time:
- systemd-analyze calendar: uses _full(true)
- systemd-run --on-calendar: uses _full(true)
- .timer OnCalendar=: uses log_syntax() with file/line context

Add test_calendar_spec_weekday_conflict(): forks a child with stderr
captured in a memfd via pidref_safe_fork_full(), verifies the warning
is emitted for conflicting specs and suppressed for valid ones.

Fixes: #40350
Signed-off-by: dongshengyuan <dongshengyuan@uniontech.com>

src/basic/time-util.c

@@ -321,24 +321,26 @@ struct timeval *timeval_store(struct timeval *tv, usec_t u) {
         return tv;
 }
 
+/* Returns the abbreviated English weekday name for wd in Mon=0 … Sun=6 order.
+ * We use non-localized (English) form so that timestamps can be parsed with
+ * parse_timestamp() and always read the same regardless of locale. */
+static const char *const weekday_table[_WEEKDAY_MAX] = {
+        [WEEKDAY_MON] = "Mon",
+        [WEEKDAY_TUE] = "Tue",
+        [WEEKDAY_WED] = "Wed",
+        [WEEKDAY_THU] = "Thu",
+        [WEEKDAY_FRI] = "Fri",
+        [WEEKDAY_SAT] = "Sat",
+        [WEEKDAY_SUN] = "Sun",
+};
+
+DEFINE_STRING_TABLE_LOOKUP_TO_STRING(weekday, int);
+
 char* format_timestamp_style(
                 char *buf,
                 size_t l,
                 usec_t t,
                 TimestampStyle style) {
-
-        /* The weekdays in non-localized (English) form. We use this instead of the localized form, so that
-         * our generated timestamps may be parsed with parse_timestamp(), and always read the same. */
-        static const char * const weekdays[] = {
-                [0] = "Sun",
-                [1] = "Mon",
-                [2] = "Tue",
-                [3] = "Wed",
-                [4] = "Thu",
-                [5] = "Fri",
-                [6] = "Sat",
-        };
-
         struct tm tm;
         bool utc, us;
         size_t n;
@@ -387,8 +389,9 @@ char* format_timestamp_style(
                 return NULL;
 
         /* Start with the week day */
-        assert((size_t) tm.tm_wday < ELEMENTSOF(weekdays));
-        memcpy(buf, weekdays[tm.tm_wday], 4);
+        const char *weekday = weekday_to_string(tm.tm_wday == 0 ? 6 : tm.tm_wday - 1);
+        assert(weekday);
+        memcpy(buf, weekday, 4);
 
         if (style == TIMESTAMP_DATE) {
                 /* Special format string if only date should be shown. */

src/basic/time-util.h

@@ -56,6 +56,18 @@ typedef enum TimestampStyle {
 #define USEC_PER_YEAR ((usec_t) (31557600ULL*USEC_PER_SEC))
 #define NSEC_PER_YEAR ((nsec_t) (31557600ULL*NSEC_PER_SEC))
 
+enum {
+        WEEKDAY_MON,
+        WEEKDAY_TUE,
+        WEEKDAY_WED,
+        WEEKDAY_THU,
+        WEEKDAY_FRI,
+        WEEKDAY_SAT,
+        WEEKDAY_SUN,
+        _WEEKDAY_MAX,
+        _WEEKDAY_INVALID = -EINVAL,
+};
+
 /* We assume a maximum timezone length of 6. TZNAME_MAX is not defined on Linux, but glibc internally initializes this
  * to 6. Let's rely on that. */
 #define FORMAT_TIMESTAMP_MAX (3U+1U+10U+1U+8U+1U+6U+1U+6U+1U)
@@ -125,6 +137,10 @@ char* format_timestamp_style(char *buf, size_t l, usec_t t, TimestampStyle style
 char* format_timestamp_relative_full(char *buf, size_t l, usec_t t, clockid_t clock, bool implicit_left) _warn_unused_result_;
 char* format_timespan(char *buf, size_t l, usec_t t, usec_t accuracy) _warn_unused_result_;
 
+/* Returns the abbreviated English weekday name for wd in Mon=0 … Sun=6 order
+ * (matching systemd's weekdays_bits layout). */
+const char* weekday_to_string(int i);
+
 _warn_unused_result_
 static inline char* format_timestamp_relative(char *buf, size_t l, usec_t t) {
         return format_timestamp_relative_full(buf, l, t, CLOCK_REALTIME, /* implicit_left= */ false);