fix(cron): prevent spin loop when job completes within firing second (#17821)

When a cron job fires at 13:00:00.014 and completes at 13:00:00.021,
computeNextRunAtMs was flooring nowMs to 13:00:00.000 and asking croner
for the next occurrence from that exact boundary. Croner could return
13:00:00.000 (same second) since it uses >= semantics, causing the job
to be immediately re-triggered hundreds of times.

Fix: Ask croner for the next occurrence starting from the NEXT second
(e.g., 13:00:01.000). This ensures we always skip the current/elapsed
second and correctly return the next day's occurrence.

This also correctly handles the before-match case: if nowMs is
11:59:59.500, we ask from 12:00:00.000, and croner returns today's
12:00:00.000 match.

Added regression tests for the spin loop scenario.

src/cron/schedule.test.ts

@@ -66,5 +66,23 @@ describe("cron schedule", () => {
       const next = computeNextRunAtMs(dailyNoon, noonMs - 500);
       expect(next).toBe(noonMs);
     });
+
+    it("advances to next day when job completes within same second it fired (#17821)", () => {
+      // Regression test for #17821: cron jobs that fire and complete within
+      // the same second (e.g., fire at 12:00:00.014, complete at 12:00:00.021)
+      // were getting nextRunAtMs set to the same second, causing a spin loop.
+      //
+      // Simulating: job scheduled for 12:00:00, fires at .014, completes at .021
+      const completedAtMs = noonMs + 21; // 12:00:00.021
+      const next = computeNextRunAtMs(dailyNoon, completedAtMs);
+      expect(next).toBe(noonMs + 86_400_000); // must be next day, NOT noonMs
+    });
+
+    it("advances to next day when job completes just before second boundary (#17821)", () => {
+      // Edge case: job completes at .999, still within the firing second
+      const completedAtMs = noonMs + 999; // 12:00:00.999
+      const next = computeNextRunAtMs(dailyNoon, completedAtMs);
+      expect(next).toBe(noonMs + 86_400_000); // next day
+    });
   });
 });

src/cron/schedule.ts

@@ -49,19 +49,23 @@ export function computeNextRunAtMs(schedule: CronSchedule, nowMs: number): numbe
     timezone: resolveCronTimezone(schedule.tz),
     catch: false,
   });
-  // Cron operates at second granularity, so floor nowMs to the start of the
-  // current second.  We ask croner for the next occurrence strictly *after*
-  // nowSecondMs so that a job whose schedule matches the current second is
-  // never re-scheduled into the same (already-elapsed) second.
+  // Ask croner for the next occurrence starting from the NEXT second.
+  // This prevents re-scheduling into the current second when a job fires
+  // at 13:00:00.014 and completes at 13:00:00.021 — without this fix,
+  // croner could return 13:00:00.000 (same second) causing a spin loop
+  // where the job fires hundreds of times per second (see #17821).
   //
-  // Previous code used `nowSecondMs - 1` which caused croner to return the
-  // current second as a valid next-run, leading to rapid duplicate fires when
-  // multiple jobs triggered simultaneously (see #14164).
-  const nowSecondMs = Math.floor(nowMs / 1000) * 1000;
-  const next = cron.nextRun(new Date(nowSecondMs));
+  // By asking from the next second (e.g., 13:00:01.000), we ensure croner
+  // returns the following day's occurrence (e.g., 13:00:00.000 tomorrow).
+  //
+  // This also correctly handles the "before match" case: if nowMs is
+  // 11:59:59.500, we ask from 12:00:00.000, and croner returns 12:00:00.000
+  // (today's match) since it uses >= semantics for the start time.
+  const askFromNextSecondMs = Math.floor(nowMs / 1000) * 1000 + 1000;
+  const next = cron.nextRun(new Date(askFromNextSecondMs));
   if (!next) {
     return undefined;
   }
   const nextMs = next.getTime();
-  return Number.isFinite(nextMs) && nextMs > nowSecondMs ? nextMs : undefined;
+  return Number.isFinite(nextMs) ? nextMs : undefined;
 }