A systematic way of handling Xapian lock errors?

[Matt Armstrong <marmstrong@google.com>]

Simple notmuch commands like "notmuch tag" can fail to grab the Xapian
lock.  When this occurs they bail with:

A Xapian exception occurred opening database: Unable to get write lock on /example/.notmuch/xapian: already locked

I've noticed a few issues with this:

1) The notmuch command line doesn't define semantics for exit codes.  I
notice that notmuch exits with 1 for "xapian write lock error".  I
suspect this code is not reserved for write lock errors?  That would be
needed for any sensible downgrade of the write lock error into a soft
failure with a retry loop in things like shell scripts.

2) The notmuch Emacs client just bails whatever it was trying when this
occurs.  I run background mail fetching, which I expect is typical, and
this makes for an unpleasant experience as errors are thrown at me
randomly.  Since my post-injection filter commands run quickly, I'd
rather Emacs simply spin a short while against the lock trying to
perform the command.  Even a one second spin attempt is likely to
succeed most of the time.  But I would happily configure an infinite
retry loop -- Emacs can show me a spinning ball, and I'd rather not be
subject to odd effects due to failures.

To address both, has something like this ever been considered:

  notmuch --lock_timeout <seconds> COMMAND ARG...

Then frontends like Emacs could lean on retry logic written in C.  If
this seems workable, it is something I can try implementing.