core/rawdb: fix datarace in freezer #22728
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fjl
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atif-konasl
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Oct 15, 2021
The Append / truncate operations were racy. When a datafile reaches 2Gb, a new file is needed. For this operation, we require a writelock, which is not needed in the 99.99% of all cases where the data does fit in the current head-file. This transition from readlock to writelock was incorrect, and as the readlock was released, a truncate operation could slip in between, and truncate the data. This would have been fine, however, the Append operation continued writing as if no truncation had occurred, e.g writing item 5 where item 0 should reside. This PR changes the behaviour, so that if when we run into the situation that a new file is needed, it aborts, and retries, this time with a writelock. The outcome of the situation described above, running on this PR, would instead be that the Append operation exits with a failure.
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This PR fixes #22714 .
The Append / truncate operations were racy. When a datafile reaches
2Gb, a new file is needed. For this operation, we require a writelock, which is not needed in the 99.99% of all cases where the data does fit in the current head-file.This transition from readlock to writelock was incorrect, and as the readlock was released, a
truncateoperation could slip in between, and truncate the data. This would have been fine, however, theAppendoperation continued writing as if no truncation had occurred, e.g writing item5where item0should reside.This PR changes the behaviour, so that if when we run into the situation that a new file is needed, it aborts, and retries, this time with a writelock.
The outcome of the situation described above, running on this PR, would instead be that the
Appendoperation exits with a failure.The attached testcase, when enabled, finds the (original) bug pretty quickly.