fix double firing of schedules at initial interval (#3624)

.changeset/blue-comics-prove.md

@@ -0,0 +1,5 @@
+---
+"effect": patch
+---
+
+Fixed double firing of cron schedules in cases where the current time matched the initial interval.

packages/effect/src/internal/schedule.ts

@@ -433,21 +433,24 @@ export const cron = (expression: string | Cron.Cron): Schedule.Schedule<[number,
       const cron = parsed.right
       const date = new Date(now)
 
-      let next: number
       if (initial && Cron.match(cron, date)) {
-        next = now
-      } else {
-        const result = Cron.next(cron, date)
-        next = result.getTime()
+        const next = now
+        const start = beginningOfMinute(next)
+        const end = endOfMinute(next)
+        return core.succeed([
+          [false, [next, start, end]],
+          [start, end],
+          ScheduleDecision.continueWith(Interval.make(start + 60000, end + 60000))
+        ])
       }
 
+      const next = Cron.next(cron, date).getTime()
       const start = beginningOfMinute(next)
       const end = endOfMinute(next)
-      const interval = Interval.make(start, end)
       return core.succeed([
         [false, [next, start, end]],
         [start, end],
-        ScheduleDecision.continueWith(interval)
+        ScheduleDecision.continueWith(Interval.make(start, end))
       ])
     }
   )

packages/effect/test/Schedule.test.ts

@@ -572,6 +572,30 @@ describe("Schedule", () => {
       }))
   })
   describe("cron-like scheduling - repeats at point of time (minute of hour, day of week, ...)", () => {
+    it.effect("recur every minute after initial interval using cron", () =>
+      Effect.gen(function*($) {
+        const ref = yield* $(Ref.make<ReadonlyArray<string>>([]))
+        yield* $(TestClock.setTime(new Date(2024, 0, 1, 0, 0, 35).getTime()))
+        const schedule = Schedule.cron("* * * * *")
+        yield* $(
+          TestClock.currentTimeMillis,
+          Effect.tap((instant) => Ref.update(ref, Array.append(format(instant)))),
+          Effect.repeat(schedule),
+          Effect.fork
+        )
+        yield* $(TestClock.adjust("5 minutes"))
+        const result = yield* $(Ref.get(ref))
+        const expected = [
+          "Mon Jan 01 2024 00:00:35",
+          "Mon Jan 01 2024 00:01:00",
+          "Mon Jan 01 2024 00:02:00",
+          "Mon Jan 01 2024 00:03:00",
+          "Mon Jan 01 2024 00:04:00",
+          "Mon Jan 01 2024 00:05:00"
+        ]
+        assert.deepStrictEqual(result, expected)
+      }))
+
     it.effect("recur at time matching cron expression", () =>
       Effect.gen(function*($) {
         const ref = yield* $(Ref.make<ReadonlyArray<string>>([]))