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Fix "unexpected null" handling overlapping file changes #1083

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Fix "unexpected null" handling overlapping file changes #1083

wants to merge 1 commit into from

Commits on Sep 14, 2023

  1. Fix "unexpected null" handling overlapping file changes 

    Summary:
Fixes a crash where a race in overlapping file change processing could occasionally throw "unexpected null".

Fixes facebook#1015

## Root cause
The problem logic is in some instrumentation code introduced in D40346848.

`metro-file-map` batches changes using `setInterval(emitChange, 30)`. The instrumentation attempted to record the time of the first event in a given batch, and then "reset the clock" when the batch is emitted to the consumer.

The problem occurred when an emit interval fell such that a non-empty `emitChange` occurred *during* the async `_processFile` step of a subsequent change. The first emit would reset the "start time" to `null`, and the second `emitChange` would fail at `nullthrows(eventStartTimestamp)`, because `eventStartTimestamp` is only set *before* `_processFile` starts (and only if already `null`).

The (over)writing of `eventStartTimestamp` was flawed, because we don't know at the start of change processing that this event is going to be the start of a batch - this only becomes clear when an event is subsequently enqueued for emit after file processing.

## This diff
This changes the recording of the start time such that we take a timestamp on every change, and record it as the batch start time on the creation of a new batch. We rearrange the various moving parts into a nullable object, so that "first event time" is tightly coupled to the batch it describes, and can never be `null` for a non-empty batch.

## Changelog
```
 * **[Fix]:** Fix "unexpected null" crash when handling a batch of file changes.
```

Differential Revision: D49272782
    @robhogan @facebook-github-bot
    robhogan authored and facebook-github-bot committed Sep 14, 2023
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