Having it set to nil caused https://github.com/lightningnetwork/lnd/issues/5115
The problem was several layers removed from the fix. The link decides to
clean up a `fwdPkg` only if it's completed, otherwise it renotifies the
HTLCs. A package is only set to complete if it's `addAck` and
`settleFail` filters are full. For forwarded HTLCs, the `addAck` was
never being set so it would never be considered complete under this
criteria.
`addAck` is set for an HTLC when signing the next commitment TX in the
`LightningChannel`. The path for this is:
* `LightningChannel#SettleHtlc` adds the HTLC to `localUpdates`
* `LightningChannel#SignNextCommitment` builds the `ackAddRef` for all
updates with `SourceRef != nil`.
* `LightningChannel#SignNextCommitment` then passes the list of
`ackAddRef` to `OpenChannel#AppendRemoteCommitChain` to persist the new
acks in the filter
Since `SourceRef` was nil for interceptor packages, `SignNextCommitment`
ignored it and the ack was never persisted.
In this commit we implement a wrapper arround the switch, called
InterceptableSwitch. This kind of wrapper behaves like a proxy which
intercepts forwarded packets and allows an external interceptor to
signal if it is interested to hold this forward and resolve it
manually later or let the switch execute its default behavior.
This infrastructure allows the RPC layer to expose interceptor
registration API to the user and by that enable the implementation
of custom routing behavior.
