- asynchronous sockets using an event loop
- connect & starttls have completion callback parameters
- callbacks for notification about filled input buffer and emptied
output buffer
- unsent imap command queue
- used when
- socket output buffer is non-empty
- number of commands in flight exceeds limit
- last sent command requires round-trip
- command has a dependency on completion of previous command
- trashnc is tri-state so only a single "scout" trash APPEND/COPY is
sent at first. a possibly resulting CREATE is injected in front of
the remaining trash commands, so they can succeed (or be cancel()d
if it fails).
- queue's presence necessitates imap_cancel implementation
this prepares the code for being called from a callback.
notably, this makes the imap list parser have a "soft stack", so the
recursion can be suspended at any time.
instead of returning a write()-like result, return only a binary status
code - write errors are handled internally anyway, so user code doesn't
have to check the write length.
