chainreg: send dummy epoch in no_chain_backend

[djkazic]

Change Description

Currently, getStartingBeat() does not work properly when the chain backend is nochainbackend.

This is because if the function never receives a bestBlock from blockEpochs.Epochs, the code drops into the case <-s.quit: branch (or simply times out), doesn’t set beat.

server.Start() sees no error from getStartingBeat(), so it calls s.txPublisher.Start(beat) with a nil beat.

Inside TxPublisher.Start(), we call t.currentHeight.Store(beat.Height()) which tries to do beat.Height().

Since beat is nil, we throw a panic: panic: runtime error: invalid memory address or nil pointer dereference.

That is in turn because the RegisterBlockEpochNtfn() method returns a channel that’s never written to.

This PR introduces a dummy payload to send back to unblock startup and avoid these nil pointer dereferences.

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1. Deploy LND 0.19.0-rc1 in remote signing mode.

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[yyforyongyu]

hmmm I thought it was fixed in #9642?

[djkazic]

Ah I think you're right. I had not checked master for a fix.