OmnichainGovernanceExecutor.sol#_blockingLzReceive() - If the contract gets paused, it will block cross chain communication because of where the modifier is placed
OmnichainGovernanceExecutor implements Pausable, through BaseOmnichainControllerDest
The modifier _whenNotPaused is attached to _blockingLzReceive
_blockingLzReceive is hit and the code will execute in the following order.
whenNotPaused -> _blockingLzReceive -> _nonblockingLzReceive
This means that if the contract is paused, it will revert outside _noblockingLzReceive and the tx will not be stored inside failedMessages to be later retried with retryMessage, instead it will block the communicaton channel between the two chains.
If any subsequent messages are sent to the destination chain, while the cross chain communication is blocked, then the nonces will become out of sync, permanently blocking the channel between the two chains.
Another scenario that can occur, with technically the same root cause:
paused = false
Proposal1 fails inside _nonblockingLzReceive and is stored inside failedMessages
paused = true
Someone calls retryMessage and successfully retries the message and _nonblockingLzReceive is hit and the tx doesn't revert, because whenNotPaused isn't attached to the function.
We recommend adding the _whenNotPaused modifier to _nonblockingLzReceive instead of _blockingLzReceive. This way if the destination is paused, the messages will be stored inside failedMessages where they can later be executed, the cross chain communication will not be blocked and users won't have to create completely new proposals on the source chain.
This will also stop the second scenario from occurring when the destination gets paused.