Merge bitcoin/bitcoin#27501: mempool / rpc: add getprioritisedtransactions, delete a mapDeltas entry when delta==0

67b7fecacd0489809690982c89ba2d0acdca938c [mempool] clear mapDeltas entry if prioritisetransaction sets delta to 0 (glozow)
c1061acb9d502cdf8c6996c818d9a8a281cbe40c [functional test] prioritisation is not removed during replacement and expiry (glozow)
0e5874f0b06114d9b077e0ff582915e4f83059e6 [functional test] getprioritisedtransactions RPC (glozow)
99f8046829f699ff2eace266aa8cea1d9f7cb65a [rpc] add getprioritisedtransactions (glozow)
9e9ca36c80013749faaf2aa777d52bd07d9d24ec [mempool] add GetPrioritisedTransactions (glozow)

Pull request description:

  Add an RPC to get prioritised transactions (also tells you whether the tx is in mempool or not), helping users clean up `mapDeltas` manually. When `CTxMemPool::PrioritiseTransaction` sets a delta to 0, remove the entry from `mapDeltas`.

  Motivation / Background
  - `mapDeltas` entries are never removed from mapDeltas except when the tx is mined in a block or conflicted.
  - Mostly it is a feature to allow `prioritisetransaction` for a tx that isn't in the mempool {yet, anymore}. A user can may resbumit a tx and it retains its priority, or mark a tx as "definitely accept" before it is seen.
  - Since #8448, `mapDeltas` is persisted to mempool.dat and loaded on restart. This is also good, otherwise we lose prioritisation on restart.
  - Note the removal due to block/conflict is only done when `removeForBlock` is called, i.e. when the block is received. If you load a mempool.dat containing `mapDeltas` with transactions that were mined already (e.g. the file was saved prior to the last few blocks), you don't delete them.
  - Related: #4818 and #6464.
  - There is no way to query the node for not-in-mempool `mapDeltas`. If you add a priority and forget what the value was, the only way to get that information is to inspect mempool.dat.
  - Calling `prioritisetransaction` with an inverse value does not remove it from `mapDeltas`, it just sets the value to 0. It disappears on a restart (`LoadMempool` checks if delta is 0), but that might not happen for a while.

  Added together, if a user calls `prioritisetransaction` very regularly and not all those transactions get mined/conflicted, `mapDeltas` might keep lots of entries of delta=0 around. A user should clean up the not-in-mempool prioritisations, but that's currently difficult without keeping track of what those txids/amounts are.

ACKs for top commit:
  achow101:
    ACK 67b7fecacd0489809690982c89ba2d0acdca938c
  theStack:
    Code-review ACK 67b7fecacd0489809690982c89ba2d0acdca938c
  instagibbs:
    code review ACK 67b7fecacd0489809690982c89ba2d0acdca938c
  ajtowns:
    ACK 67b7fecacd0489809690982c89ba2d0acdca938c code review only, some nits

Tree-SHA512: 9df48b622ef27f33db1a2748f682bb3f16abe8172fcb7ac3c1a3e1654121ffb9b31aeaad5570c4162261f7e2ff5b5912ddc61a1b8beac0e9f346a86f5952260a

1 files changed, 13 insertions, 0 deletions

diff --git a/src/txmempool.h b/src/txmempool.h
index 769b7f69ea..000033086b 100644
--- a/src/txmempool.h
+++ b/src/txmempool.h
@@ -516,6 +516,19 @@ public:
     void ApplyDelta(const uint256& hash, CAmount &nFeeDelta) const EXCLUSIVE_LOCKS_REQUIRED(cs);
     void ClearPrioritisation(const uint256& hash) EXCLUSIVE_LOCKS_REQUIRED(cs);
 
+    struct delta_info {
+        /** Whether this transaction is in the mempool. */
+        const bool in_mempool;
+        /** The fee delta added using PrioritiseTransaction(). */
+        const CAmount delta;
+        /** The modified fee (base fee + delta) of this entry. Only present if in_mempool=true. */
+        std::optional<CAmount> modified_fee;
+        /** The prioritised transaction's txid. */
+        const uint256 txid;
+    };
+    /** Return a vector of all entries in mapDeltas with their corresponding delta_info. */
+    std::vector<delta_info> GetPrioritisedTransactions() const EXCLUSIVE_LOCKS_REQUIRED(!cs);
+
     /** Get the transaction in the pool that spends the same prevout */
     const CTransaction* GetConflictTx(const COutPoint& prevout) const EXCLUSIVE_LOCKS_REQUIRED(cs);