Because we restrict custom SCID aliases to be in a specific range, we
export the range start and end values so a user of the RPCs we're going
to add in the next commits can adjust their values to fit within the
range.
With the new RPC calls that we are going to add in the next commits, it
will be possible for users to add (local only, non-gossipped) SCID
aliases for channels. Since those will be in the same range as the ones
given out by RequestAlias, we need to make sure that when we generate a
new one that it doesn't collide with an already existing one.
This introduces the a store for managing all things alias-related.
There are two maps:
* baseToSet:
This stores the "base" short channel id as the key. The value is
the set of all aliases issued for this channel. The "base" SCID is
whatever is stored in the OpenChannel's ShortChannelID member. For
everything other than zero-conf channels, this is the confirmed SCID.
For zero-conf channels, this is the very first alias assigned. This is
used mostly by the Switch to retrieve a set of aliases and determine
if it is safe to forward.
* aliasToBase:
This stores the reverse mapping of baseToSet. Each key is an alias
SCID and the value is the "base" SCID. This is exclusively used by
the gossiper to determine if an alias in a ChannelUpdate our peer
sends actually references a channel we know of.
The functions make use of the above two maps:
* AddLocalAlias:
This persists the {alias, base} pair in the database. The baseToSet
map is populated. The aliasToBase is optionally populated depending on
where this function is called from. Upgrade cases, where the
scid-alias feature bit is toggled and channels already exist, will
not persist to the gossip map. This is mainly to simplify the tangle
of logic that would otherwise occur.
* GetAliases:
This fetches the set of aliases by using the passed-in base SCID. This
is used in the Switch and other places where the alias set is needed.
* FindBaseSCID:
This fetches the base given an alias. This is used in the gossiper to
determine validity of a peer's ChannelUpdate that contains an alias.
* DeleteSixConfs:
This removes the aliasToBase map entry for the given "base". This is
used when the gossiper mappings are no longer needed, i.e. when the
channel has six confirmations and is public.
* PutPeerAlias:
Stores the peer's alias.
* GetPeerAlias:
Fetches the peer's alias.
* RequestAlias:
Generates an alias for us in the range 16000000:0:0 and
16250000:16777215:65535
