Preserve unread events when NewEvents iteration stops early

[PratikDhanave]

Run.NewEvents advanced the read bookmark to the end of the event buffer at iterator start, before yielding any event:

current := run.lastBookmark
run.lastBookmark = len(run.eventSink) // advanced to end up front
for _, evt := range run.eventSink[current:] {
    if !yield(evt) {
        return
    }
}

A consumer that stops iterating early — an ordinary break in a for evt := range run.NewEvents() loop, e.g. stopping once it sees the output event — therefore permanently loses every event it had not yet seen. NewEventCount() drops to 0 and a later NewEvents() call yields nothing, even though those events were never delivered.

The method already guards early termination with if !yield(evt) { return }, so early break is an anticipated usage; only the bookmark accounting was wrong.

Fix

Advance the bookmark as each event is delivered, so the cursor tracks what the consumer has actually seen:

for run.lastBookmark < len(run.eventSink) {
    evt := run.eventSink[run.lastBookmark]
    run.lastBookmark++
    if !yield(evt) {
        return
    }
}

This yields exactly-once delivery: no drops on early break, no re-delivery on full drain. Every in-tree caller drains fully today, so their behavior is unchanged.

Test

TestRun_NewEventsEarlyBreakPreservesUnreadEvents runs a workflow (6 buffered events), consumes exactly one via an early break, then asserts the remaining events are still reported by NewEventCount() and re-iterable via NewEvents(). It fails on the old code (remaining reported as 0, want 5) and passes with the fix.

[Copilot]

Pull request overview

Fixes workflow/inproc.Run.NewEvents() so that an early stop (e.g., break from for evt := range run.NewEvents()) no longer advances the read bookmark past unread buffered events, preventing silent event loss. This aligns the bookmark with what has actually been yielded and adds a regression test to lock in the behavior.

Changes:

• Update Run.NewEvents() to advance lastBookmark incrementally per yielded event rather than jumping to the end up front.
• Add TestRun_NewEventsEarlyBreakPreservesUnreadEvents to verify unread events remain available after an early break.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
workflow/inproc/run.go	Adjusts NewEvents() bookmark advancement to prevent dropping unread events on early iteration termination.
workflow/inproc/events_test.go	Adds a regression test ensuring unread events remain countable and iterable after an early break from NewEvents().

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.