mempool: add a comment and missing changelog entry (#2996)

Refs #2994

CHANGELOG_PENDING.md

@@ -23,4 +23,5 @@ Special thanks to external contributors on this release:
 
 ### BUG FIXES:
 - [kv indexer] \#2912 don't ignore key when executing CONTAINS
+- [mempool] \#2994 Don't allow txs with negative gas wanted
 - [p2p] \#2715 fix a bug where seeds don't disconnect from a peer after 3h

mempool/mempool.go

@@ -490,11 +490,15 @@ func (mem *Mempool) ReapMaxBytesMaxGas(maxBytes, maxGas int64) types.Txs {
 			return txs
 		}
 		totalBytes += int64(len(memTx.tx)) + aminoOverhead
-		// Check total gas requirement
-		if maxGas > -1 && totalGas+memTx.gasWanted > maxGas {
+		// Check total gas requirement.
+		// If maxGas is negative, skip this check.
+		// Since newTotalGas < masGas, which
+		// must be non-negative, it follows that this won't overflow.
+		newTotalGas := totalGas + memTx.gasWanted
+		if maxGas > -1 && newTotalGas > maxGas {
 			return txs
 		}
-		totalGas += memTx.gasWanted
+		totalGas = newTotalGas
 		txs = append(txs, memTx.tx)
 	}
 	return txs