server: configurable feature negotiation + downgrade to optional compat #1838
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In this commit, we thread down an error chan from the server's ConnectToPeer method to the point at which a peer is started. This enables the caller to be aware of issues in exchange init messages. This is additional info is useful for subsystems like autopilot, which will currently try to open a channel after knowing the dial succeeded, but unaware if the peers are actually compatible.
Since peer start errors are now bubbled up to the caller, this error can now be received when connecting non-permanent peers. If we encounter this when trying EnsureConnceted, we will assume the peers are trying to connect, and wait for the peers to appear in each other's list of peers.
This commit adds 3 basic levels of feature bit negotiation: off, optional, and required. Only optional and required will be used initially, as fully disabling a known feature would require a fair amount of extra work. However, these values are intended to correspond to command line flags that can be passed to lnd on startup.
This commit adds a conf struct for setting feature bits from the command line. The current supported features are related gossip queries and data loss protection.
This commit adds the required logic to downgrade a required feature bit to an optional one if they do not understand the required feature bit. On the first connection, we will detect the case in which we offered required and they gave optional. If the peers are disconnected for any reason, we note the peer's support for optional gq or dlp, and initiate a downgraded attempt on the next connection if we wish the feature to be required. This downgrade will be applied on all subsequent connections to remove uncertainty that not understanding the required feature bits was the reason for disconnecting the first time. Finally, if the are using a downgraded connection because the peer initially reported optional support, but later advertises that they don't support the feature at all, we fail the connection. This is to maintain the fact that we can only enter a downgraded connection if the user requested that it be required. Accepting the connection would break that contract with the user user's expectations.
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Alternative approach to #1801