This commit modifies the LightningNode.ForEachChannel method to give
the caller the option of re-using an existing database transaction
instead of always creating a new db transaction with each invocation.
Internally boltdb will run into an error/dead-lock if a nested
transaction is attempted.
Such an action might be attempted if one were to use the traversal
functions in a path finding algorithm. Therefore in order to avoid
that after, we now allow the re-use of transactions to facilitate
nested calls to ForEachChannel.
This commit introduces a new capability to the database: storage of an
on-disk directed channel graph. The on-disk representation of the graph
within boltdb is essentially a modified adjacency list which separates
the storage of the edge’s existence and the storage of the edge
information itself.
The new objects provided within he ChannelGraph carry an API which
facilitates easy graph traversal via their ForEach* methods. As a
result, path finding algorithms will be able to be expressed in a
natural way using the range methods as a for-range language extension
within Go.
Additionally caching will likely be added either at this layer or the
layer above (the RoutingManager) in order keep queries and outgoing
payments speedy. In a future commit a new set of RPC’s to query the
state of a particular edge or node will also be added.
