Previously an existing output file was only detected at the final rename,
after the whole collection had run. Add an up-front check in the FileWriter
constructor that fails before acquisition arms.
Only the master file is checked, and only by the single writer that owns it
(write_master_file). In a multi-writer TCP/ZMQ setup the per-image data files
are staggered across writers by file number, and a writer does not even know
the total writer count, so it cannot enumerate its own subset; checking all
data files in every writer would make each writer stat files it never writes
and race the writers already creating them. Data-file conflicts stay caught by
their owning writer at the final rename.
Gate the check on whether the transport can report the failure to the broker:
the direct HDF5 pusher throws in-process and the TCP writer returns a
START-failure ACK, so both abort the collection cleanly (verified: an asymmetric
rejection cancels the already-started sibling). The ZeroMQ path is fire-and-forget
with no back-channel, so StreamWriter opts out via image_puller.SupportsAck() and
keeps writing .tmp files, failing only at the final rename (leaving the images on
disk). Documented in docs/JFJOCH_WRITER.md.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
leonarski_fandClaude Fable 5
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