CQ: Fix shared store crashes (backport #11076) #11111
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Crashes could happen because compaction would wrongly write over valid messages, or truncate over valid messages, because when looking for messages into the files it would encounter leftover data that made it look like there was a message, which prompted compaction to not look for the real messages hidden within. To avoid this we ensure that there can't be leftover data as a result of compaction. We get this guarantee by blanking data in the holes in the file before we start copying messages closer to the start of the file. This requires us to do a few more writes but we know that the only data in the files at any point are valid messages. Note that it's possible that some of the messages in the files are no longer referenced; that's OK. We filter them out after scanning the file. This was also a good time to merge two almost identical scan functions, and be more explicit about what messages should be dropped after scanning the file (the messages no longer in the ets index and the fan-out messages that ended up re-written in a more recent file). (cherry picked from commit fcd011f)
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Crashes could happen because compaction would wrongly write over valid messages, or truncate over valid messages, because when looking for messages into the files it would encounter leftover data that made it look like there was a message, which prompted compaction to not look for the real messages hidden within.
To avoid this we ensure that there can't be leftover data as a result of compaction. We get this guarantee by blanking data in the holes in the file before we start copying messages closer to the start of the file. This requires us to do a few more writes but we know that the only data in the files at any point are valid messages.
Related discussion: #10902
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xin the boxes that applyThis is an automatic backport of pull request #11076 done by [Mergify](https://mergify.com).