I'm beginning to understand the problem. Or at least one of them. :) Dovecot is currently flushing attribute cache for files it wants to stat/open. But that's not enough. It also has to flush attribute cache for the directory, or it might stat/open a file that has already been unlinked (but which still has links somewhere in the filesystem so that it doesn't fail with ESTALE).
dotlock_use_excl=yes seems to help somewhat with this problem for now. It also gives better performance.
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