Writing to a connection goes through the flate library to compress. That writes
the compressed bytes to the underlying connection. But that underlying
connection is wrapped to raise a panic with an i/o error instead of returning a
normal error. Jumping out of flate leaves the internal state of the compressor
in undefined state. So far so good. But as part of cleaning up the connection,
we could try to flush output again. Specifically: If we were writing user data,
we had switched from tracing of protocol data to tracing of user data, and we
registered a defer that restored the tracing kind and flushed (to ensure data
was traced at the right level). That flush would cause a write into the
compressor again, which could panic with an out of bounds slice access due to
its inconsistent internal state.
This fix prevents that compressor panic in two ways:
1. We wrap the flate.Writer with a moxio.FlateWriter that keeps track of
whether a panic came out of an operation on it. If so, any further operation
raises the same panic. This prevents access to the inconsistent internal flate
state entirely.
2. Once we raise an i/o error, we mark the connection as broken and that makes
flushes a no-op.
REPLACE can be used to update draft messages as you are editing. Instead of
requiring an APPEND and STORE of \Deleted and EXPUNGE. REPLACE works
atomically.
It has a syntax similar to APPEND, just allows you to specify the message to
replace that's in the currently selected mailbox. The regular REPLACE-command
works on a message sequence number, the UID REPLACE commands on a uid. The
destination mailbox, of the updated message, can be different. For example to
move a draft message from the Drafts folder to the Sent folder.
We have to do quite some bookkeeping, e.g. for updating (message) counts for
the mailbox, checking quota, un/retraining the junk filter. During a
synchronizing literal, we check the parameters early and reject if the replace
would fail (eg over quota, bad destination mailbox).
