iamaleksey commented on code in PR #3495:
URL: https://github.com/apache/cassandra/pull/3495#discussion_r1743736074
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src/java/org/apache/cassandra/journal/Journal.java:
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@@ -542,8 +542,11 @@ private void advanceSegment(ActiveSegment<K, V> oldSegment)
// if a segment is ready, take it now, otherwise wait for the
allocator thread to construct it
if (availableSegment != null)
{
+ ActiveSegment<K, V> nextSegment = availableSegment;
+ availableSegment = null;
// success - change allocatingFrom and activeSegments
(which must be kept in order) before leaving the critical section
- addNewActiveSegment(currentSegment = availableSegment);
+ addNewActiveSegment(nextSegment);
Review Comment:
The updates of `currentSegment` and the `Segments` can't happen
simultaneously, unfortunately, so we have to make a decision regarding which
one happens first. Current order is intentional. The comment above explicitly
says so (though variable names need an update). Code that is potentially
affected by this needs to take this possibility into account.
`Journal#getActiveSegment()` is an example of this - there, we check
`currentSegment` first, and then `segments()` if need be. Similarly,
`Journal#isFlushed()` must do the same thing.
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