fuzz-calendarspec: increase coverage by calculating occurences

Coverage data shows that we didn't test calendar_spec_next_usec() and
associated functions at all.

The input samples so far were only used until the first NUL. We take advantage
of that by using the part until the second NUL as the starting timestamp,
retaining backwards compatibility for how the first part is used.

src/fuzz/fuzz-calendarspec.c

@@ -4,19 +4,54 @@
 #include "calendarspec.h"
 #include "fd-util.h"
 #include "fuzz.h"
+#include "string-util.h"
+#include "time-util.h"
 
 int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         _cleanup_(calendar_spec_freep) CalendarSpec *cspec = NULL;
-        _cleanup_free_ char *str = NULL, *p = NULL;
+        _cleanup_free_ char *str = NULL;
+        int r;
 
         if (!getenv("SYSTEMD_LOG_LEVEL"))
                 log_set_max_level(LOG_CRIT);
 
         str = memdup_suffix0(data, size);
 
-        if (calendar_spec_from_string(str, &cspec) >= 0) {
-                (void) calendar_spec_valid(cspec);
-                (void) calendar_spec_to_string(cspec, &p);
+        size_t l1 = strlen(str);
+        const char* usecs = l1 < size ? str + l1 + 1 : "";
+
+        r = calendar_spec_from_string(str, &cspec);
+        if (r < 0) {
+                log_debug_errno(r, "Failed to parse \"%s\": %m", str);
+                return 0;
+        }
+
+        _cleanup_free_ char *p = NULL;
+        assert_se(calendar_spec_valid(cspec));
+        assert_se(calendar_spec_to_string(cspec, &p) == 0);
+        assert(p);
+
+        log_debug("spec: %s → %s", str, p);
+
+        _cleanup_(calendar_spec_freep) CalendarSpec *cspec2 = NULL;
+        assert_se(calendar_spec_from_string(p, &cspec2) >= 0);
+        assert_se(calendar_spec_valid(cspec2));
+
+        usec_t usec = 0;
+        (void) parse_time(usecs, &usec, 1);
+
+        /* If timezone is set, calendar_spec_next_usec() would fork, bleh :(
+         * Let's not try that. */
+        cspec->timezone = mfree(cspec->timezone);
+
+        log_debug("00: %s", strna(FORMAT_TIMESTAMP(usec)));
+        for (unsigned i = 1; i <= 20; i++) {
+                r = calendar_spec_next_usec(cspec, usec, &usec);
+                if (r < 0) {
+                        log_debug_errno(r, "%02u: %m", i);
+                        break;
+                }
+                log_debug("%02u: %s", i, FORMAT_TIMESTAMP(usec));
         }
 
         return 0;