Do not hold write transaction in prune() loop

[link2xt]

Here write connection is created and transaction is started immediately:

mailadm/src/mailadm/commands.py

Line 63 in 09f6f2a

with db.write_transaction() as conn:

This command is called by prune loop every second.

Opening a transaction locks the database in such a way that new_email will not be able to get a transaction here and timeout:

mailadm/src/mailadm/web.py

Line 27 in 09f6f2a

with db.write_transaction() as conn:

Transaction should be short-lived. At the very least begin immediate should be replaced with begin here:

mailadm/src/mailadm/db.py

Line 44 in 09f6f2a

sqlconn.execute("begin immediate")

This will postpone locking the database until the first write statement: https://www.sqlite.org/lang_transaction.html

But I don't see the reason to explicitly open transactions in the prune loop at all, it's enough to open a write connection without a transaction.

[link2xt]

Note that #71 does not help at all, because both the prune loop and new_email request handler are writers, and WAL only helps to avoid readers locking writers.