Libraries: Safe Logger: why your log lines vanish while FileOpen keeps returning success
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Safe Logger: why your log lines vanish while FileOpen keeps returning success:
Four EAs writing to the same file, all with FILE_SHARE_READ|FILE_SHARE_WRITE, FileSeek(SEEK_END), FileWrite, FileClose. Looks correct. Every FileOpen returns success. No error in the log. And the lines vanish. Reason: FILE_SHARE_WRITE lets all four open at the same time. All four call FileSeek(SEEK_END) and get THE SAME offset, because none has written yet. All four write at the same position. Whoever closes last wins. Three lines vanish silently. In my case: 12 events expected, 8 in the file. The fix is to open EXCLUSIVELY (no FILE_SHARE_WRITE) and retry while another EA holds the file. And to shout in the log when the retries run out: a log that fails silently is worse than no log at all, because you trust it. The demo script reproduces both modes. To see the loss, drag it onto four charts at the same time with safe mode off and count the lines in the CSV. On a single chart the defect does not show up - which is why it passes in testing and breaks in production.
Author: Joao Prata Silva Yglesias