In order to make it possible for higher layers to compare transaction quality
(ordering within the implicit total ordering on the mempool), expose a comparison
function and test it.
Before this commit, if a TxGraph::Ref object is destroyed, it becomes impossible
to refer to, but the actual corresponding transaction node in the TxGraph remains,
and remains indefinitely as there is no way to remove it.
Fix this by making the destruction of TxGraph::Ref trigger immediate removal of
the corresponding transaction in TxGraph, both in main and staging if it exists.
In order to make it easy to evaluate proposed changes to a TxGraph, introduce a
"staging" mode, where mutators (AddTransaction, AddDependency, RemoveTransaction)
do not modify the actual graph, but just a staging version of it. That staging
graph can then be commited (replacing the main one with it), or aborted (discarding
the staging).
Move a number of related modifications to TxGraphImpl into a separate
function for removal of transactions. This is preparation for a later
commit where this will be useful in more than one place.
This is a preparation for a next commit where a TxGraph will start representing
potentially two distinct graphs (a main one, and a staging one with proposed
changes).
When transactions are removed from the tail of a cluster, we know the existing
linearization remains acceptable (if it already was), but may just need splitting
and postlinearization, so special case these into separate quality levels.
Chunk-based information (primarily, chunk feerates) are never accessed without
first bringing the relevant Clusters to an "acceptable" quality level. Thus,
while operations are ongoing and Clusters are not acceptable, we can omit
computing the chunkings and chunk feerates for Clusters.
Since m_deps_to_add has been sorted by child Cluster* already, all dependencies
with the same child will be processed consecutively. Take advantage of this by
remember the last partition merged with, and reusing that if applicable.
The m_deps_to_add vector is sorted by child Cluster*, which matches the
order of an_clusters. This means we can walk through m_deps_to_add while
doing the representative lookups for an_clusters, and reuse them.
Instead of leaving the responsibility on higher layers to guarantee that
no connected component within TxGraph (a barely exposed concept, except through
GetCluster()) exceeds the cluster count limit, move this responsibility to
TxGraph itself:
* TxGraph retains a cluster count limit, but it becomes configurable at construction
time (this primarily helps with testing that it is properly enforced).
* It is always allowed to perform mutators on TxGraph, even if they would cause the
cluster count limit to be exceeded. Instead, TxGraph exposes an IsOversized()
function, which queries whether it is in a special "oversize" state.
* During oversize state, many inspectors are unavailable, but mutators remain valid,
so the higher layer can "fix" the oversize state before continuing.
Instead construct a single vector with the list of all clusters in all groups,
and then store per-group offset/range in that list.
For dependencies, reuse m_deps_to_add, and store offset/range into that.
To make testing more powerful, expose a function to perform an internal sanity
check on the state of a TxGraph. This is especially important as TxGraphImpl
contains many redundantly represented pieces of information:
* graph contains clusters, which refer to entries, but the entries refer back
* graph maintains pointers to Ref objects, which point back to the graph.
This lets us make sure they are always in sync.
This adds an initial version of the txgraph module, with the TxGraph class.
It encapsulates knowledge about the fees, sizes, and dependencies between all
mempool transactions, but nothing else.
In particular, it lacks knowledge about txids, inputs, outputs, CTransactions,
... and so forth. Instead, it exposes a generic TxGraph::Ref type to reference
nodes in the TxGraph, which can be passed around and stored by layers on top.
