mempool: Introduce (still-unused) MemPoolLimits

They live as a CTxMemPool member.

[META] These limits will be used in subsequent commits to replace calls
       to gArgs.

src/kernel/mempool_limits.h

@@ -0,0 +1,26 @@
+// Copyright (c) 2022 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+#ifndef BITCOIN_KERNEL_MEMPOOL_LIMITS_H
+#define BITCOIN_KERNEL_MEMPOOL_LIMITS_H
+
+#include <policy/policy.h>
+
+#include <cstdint>
+
+namespace kernel {
+/**
+ * Options struct containing limit options for a CTxMemPool. Default constructor
+ * populates the struct with sane default values which can be modified.
+ *
+ * Most of the time, this struct should be referenced as CTxMemPool::Limits.
+ */
+struct MemPoolLimits {
+    int64_t ancestor_count{DEFAULT_ANCESTOR_LIMIT};
+    int64_t ancestor_size_vbytes{DEFAULT_ANCESTOR_SIZE_LIMIT_KVB * 1'000};
+    int64_t descendant_count{DEFAULT_DESCENDANT_LIMIT};
+    int64_t descendant_size_vbytes{DEFAULT_DESCENDANT_SIZE_LIMIT_KVB * 1'000};
+};
+} // namespace kernel
+
+#endif // BITCOIN_KERNEL_MEMPOOL_LIMITS_H

src/kernel/mempool_options.h

@@ -4,6 +4,8 @@
 #ifndef BITCOIN_KERNEL_MEMPOOL_OPTIONS_H
 #define BITCOIN_KERNEL_MEMPOOL_OPTIONS_H
 
+#include <kernel/mempool_limits.h>
+
 #include <chrono>
 #include <cstdint>
 
@@ -29,6 +31,7 @@ struct MemPoolOptions {
     int check_ratio{0};
     int64_t max_size_bytes{DEFAULT_MAX_MEMPOOL_SIZE_MB * 1'000'000};
     std::chrono::seconds expiry{std::chrono::hours{DEFAULT_MEMPOOL_EXPIRY_HOURS}};
+    MemPoolLimits limits{};
 };
 } // namespace kernel