[mu bug] postlist.glass grew to half a terabyte due to "DatabaseError: leaf key < left dividing key in level above"

[bramadams]

Describe the bug

xapian-check: DatabaseError: Block 356 item 209: leaf key < left dividing key in level above

Having run almost out of disk space, I noticed that ~/.cache/mu/xapian/postlist.glass had grown to half a terabyte, for an email folder of 8.6GB. Running xapian-check yielded:

postlist:
blocksize=8K items=1146885 firstunused=29809 revision=2622 levels=2 root=26897
xapian-check: DatabaseError: Block 356 item 209: leaf key < left dividing key in level above

While waiting for a fix, I reran mu init and mu index to get rid of the large xapian cache files, inspired by https://github.com/djcb/mu/issues/2706#issuecomment-2427165843.

How to Reproduce

I could not observe a clear trigger. I have not monitored the file's size over time, but only recently my laptop became sluggish, so at least the problem must have happened suddenly.

Environment

MacOS Sonoma 14.7 on MacBook Air M2. Emacs 29.3 with mu 1.12.6 and xapian 1.4.26.

Checklist

• [X] you are running either the latest 1.10.x/1.12.x release or master (otherwise, please upgrade).