Treat empty multiprocess .db files as empty metrics - #1206
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MmapedDict.__init__ creates the backing file and only truncates it to _INITIAL_MMAP_SIZE afterwards, so a reader can observe a 0-byte file. A worker killed in that window leaves the empty file behind permanently, and read_all_values_from_file then raises struct.error, aborting the whole MultiProcessCollector merge. Treat an empty read as a file with nothing recorded yet so collection continues for other workers. Non-empty corrupt files still fail loudly. Fixes prometheus#1199 Signed-off-by: Cursor Agent <cursoragent@cursor.com> Co-authored-by: Sankalp Thakur <sankalpsthakur@users.noreply.github.com>
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Fixes #1199.
MmapedDict.__init__creates the backing file before sizing it, so it exists at 0 bytes for a moment.read_all_values_from_filethen unpacks a header that is not there, raisingstruct.errorand aborting the wholeMultiProcessCollectormerge — including metrics from every other worker.A worker killed in that window leaves the empty file behind for good (it is named after a pid that never returns), so subsequent scrapes fail permanently.
This treats an empty read as a file with nothing recorded yet, the read-side mirror of
__init__'scapacity == 0branch. Non-empty files are unchanged: a file that claims more than it holds still raises, and_read_all_values's corruption check is untouched.@csmarchbanks