The upload directory was named from the destination path alone, on the
reasoning that two files could then not collide. Two devices uploading
the same file could, and did: both wrote 00001…00009 into one directory,
and whichever MOVEd first assembled a mix of the two — a catalog of
exactly the right size whose pages came from two different databases.
SQLite called it malformed, every client declined to overwrite it, and
collections stopped syncing on all of them for a week. A transfer that
died on a phone's link left its chunks there for the next device to
assemble in, by the same mechanism.
The name now carries a nonce as well, so no two uploads share a
directory, and a failed transfer deletes its own directory on the way
out rather than leaving 5 MB chunks for the server to sweep eventually.
dtourolle
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