From an internal report by @mdehoog:

I'm running an L3 sequencer locally (using Base Sepolia as the "L1") and noticed that block building halts pretty consistently, after the following logs:

t=2024-10-01T23:05:36+0000 lvl=info msg="Started sequencing new block" parent=0xe3411af59e2177377734ab199d2a8698f07db2a4cb11fdf80dbf228c21ef6024:209 l1Origin=0x74f255a508a8219038aa95e56e39de5aaf7b3b5781b9033b404bcd53a7d1c266:16027824
t=2024-10-01T23:05:36+0000 lvl=warn msg="Engine temporary error" err="temp: failed to fetch L1 block info and receipts: querying block: not found"
t=2024-10-01T23:05:36+0000 lvl=debug msg="Engine reported temporary error, but sequencer is not using engine" err="temp: failed to fetch L1 block info and receipts: querying block: not found"

after this temporary error, there are no more "Sequencer action" logs. I'm a little nervous about this happening in prod.
Can anyone explain why we don't schedule another action in this conditional?

optimism/op-node/rollup/sequencing/sequencer.go

Lines 385 to 386 in 73038c8

	d.log.Debug("Engine reported temporary error, but sequencer is not using engine", "err", x.Err)
	return

My initial take: I think what happens is that when the sequencer enters startBuildingBlock, the building state in d.latest is cleared because of a previous d.onPayloadSuccess. It then hits a temp error and returns, but it never set any field in the d.latest BuildingState, which only happens at the end right before emitting a BuildStartEvent. This temp error then lands in onEngineTemporaryError where it checks if there's any non-zero BuildState at d.latest to make the decision whether the "sequencer is using the engine", and then returns early because it's still clear, so no future action is scheduled.