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Do not truncate WAL if in read_only mode (#8313)
Summary: I noticed ```openat``` system call with ```O_WRONLY``` flag and ```sync_file_range``` and ```truncate``` on WAL file when using ```rocksdb::DB::OpenForReadOnly``` by way of ```db_bench --readonly=true --benchmarks=readseq --use_existing_db=1 --num=1 ...``` Noticed in ```strace``` after seeing the last modification time of the WAL file change after each run (with ```--readonly=true```). I think introduced by 7d7f144 from #8122 I added a test to catch the WAL file being truncated and the modification time on it changing. I am not sure if a mock filesystem with mock clock could be used to avoid having to sleep 1.1s. The test could also check the set of files is the same and that the sizes are also unchanged. Before: ``` [ RUN ] DBBasicTest.ReadOnlyReopenMtimeUnchanged db/db_basic_test.cc:182: Failure Expected equality of these values: file_mtime_after_readonly_reopen Which is: 1621611136 file_mtime_before_readonly_reopen Which is: 1621611135 file is: 000010.log [ FAILED ] DBBasicTest.ReadOnlyReopenMtimeUnchanged (1108 ms) ``` After: ``` [ RUN ] DBBasicTest.ReadOnlyReopenMtimeUnchanged [ OK ] DBBasicTest.ReadOnlyReopenMtimeUnchanged (1108 ms) ``` Pull Request resolved: #8313 Reviewed By: pdillinger Differential Revision: D28656925 Pulled By: jay-zhuang fbshipit-source-id: ea9e215cb53e7c830e76bc5fc75c45e21f12a1d6
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