For IMAP, our account creation process used to manually invoke syncFolderList and then once that completed we called ensureEssentialFolder to fix-up unhappy IMAP accounts. I moved syncFolderList out to be a separate job, losing the call to ensureEssentialFolders in the process. IMAP's syncFolderList should call ensureEssentialFolders after each run. It's reasonably harmless, mostly lightweight, and we'd be screwed without those folders.

It's notable that the one spot where we do call ensureEssentialFolders is in local_do_delete. The logic is sound but ideally would never be triggered since the deferral process breaks ordering and not something we want all.

This was likely fixed in bug 921050 by the introduction of a call to ensureEssentialOnlineFolders in _syncFolderComputeDeltas. There may be some awkward edge-cases relating to creating an account with a sufficiently flaky network such that we end up failing to create the folder in a sufficiently timely fashion.

Could you provide the detailed reproduce steps and video for me to verify this bug. Thanks!

This bug is superseded by bug 1096058 which provides back-end automated tests and is not yet landed on v2.1 but will be landed there soon. This bug cannot be verified itself on v2.1 because of the breakage from other v2.1 changes that are dealt with in that bug.