promtail: write positions to new file first, move to target location afterwards #1304
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…rds. Signed-off-by: Peter Štibraný <peter.stibrany@grafana.com>
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If the rename fails, should the new file be deleted? WDYT? |
Nah, it will be overwritten on next try, or can be left there for inspection / manual rename by admin. |
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Should we use https://github.com/natefinch/atomic/ ? |
From what I can tell, it's doing exactly the same as Go stdlib: calls rename on Unix/Linux, and uses same MoveFileEx system call on Windows with MOVEFILE_REPLACE_EXISTING flag that Go stdlib does. Perhaps Go didn't do that in the past? This lib seems to be 5+ years old. |
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promtail uses positions file to write positions in watched log files. It's doing so in an unsafe way, overwriting existing file. If error happens while writing to the file, file ends up being inconsistent. And old version is gone.
This PR changes positions writing, so that it's first written to new file, and only if that succeeds, is it move to target position. New file is file in the same directory, but with "-new" appended.
os.Rename guarantees that it replaces existing file, or it fails, but it cannot leave file inconsistent.