log: mempool: log removal reason in validation interface

Currently the exact reason a transaction is removed from the mempool isn't
logged. It is sometimes detectable from context, but adding the `reason` to
the validation interface logs (where it is already passed) seems like an easy
way to disambiguate.

For example, in the case of mempool expiry, the logs look like this:

```
[validationinterface.cpp:220] [TransactionRemovedFromMempool] [validation] Enqueuing TransactionRemovedFromMempool: txid=<txid> wtxid=<wtxid>
[txmempool.cpp:1050] [RemoveUnbroadcastTx] [mempool] Removed <txid> from set of unbroadcast txns before confirmation that txn was sent out
[validationinterface.cpp:220] [operator()] [validation] TransactionRemovedFromMempool: txid=<txid> wtxid=<wtxid>
[validation.cpp:267] [LimitMempoolSize] [mempool] Expired 1 transactions from the memory pool
```

There is no context-free way to know $txid was evicted on the basis of expiry.
This change will make that case (and probably others) clear.

1 files changed, 14 insertions, 0 deletions

diff --git a/src/txmempool.cpp b/src/txmempool.cpp
index 84ed2e9ef5..6a4cd842fb 100644
--- a/src/txmempool.cpp
+++ b/src/txmempool.cpp
@@ -1182,3 +1182,17 @@ void CTxMemPool::SetLoadTried(bool load_tried)
     LOCK(cs);
     m_load_tried = load_tried;
 }
+
+
+const std::string RemovalReasonToString(const MemPoolRemovalReason& r) noexcept
+{
+    switch (r) {
+        case MemPoolRemovalReason::EXPIRY: return "expiry";
+        case MemPoolRemovalReason::SIZELIMIT: return "sizelimit";
+        case MemPoolRemovalReason::REORG: return "reorg";
+        case MemPoolRemovalReason::BLOCK: return "block";
+        case MemPoolRemovalReason::CONFLICT: return "conflict";
+        case MemPoolRemovalReason::REPLACED: return "replaced";
+    }
+    assert(false);
+}