zlacker

Interesting; looks like DirectoryQueue uses directories, rather than file locks (man 2 flock), to lock the queue messages. This might actually work, since mkdir returns an error if you attempt to create a directory that already exists. The implementation seems to be handling most of the obvious failure cases, or at least tries to.

https://metacpan.org/release/Directory-Queue/source/lib/Dire...

So how does one lock a message in s3? Does s3 have a "createIfDoesNotExistOrError"? I'm still having difficulty understanding how the proposed system avoids race conditions.

>>archgo+(OP)
maildir is specified at https://cr.yp.to/proto/maildir.html and billions of message uses it each year. So that's pretty safe.

I can't vouch for the queueing code but I believe it's quite robust too.