init: cap -maxmempool to 500 MB on 32-bit systems

32-bit architecture is limited to 4GiB, so it doesn't make sense to set a too high value. 500 MB is
chosen as an arbitrary maximum value that seems reasonable.

Author: darosior

src/node/mempool_args.cpp

@@ -25,6 +25,9 @@ using common::AmountErrMsg;
 using kernel::MemPoolLimits;
 using kernel::MemPoolOptions;
 
+//! Maximum mempool size on 32-bit systems.
+static constexpr int MAX_32BIT_MEMPOOL_MB{500};
+
 namespace {
 void ApplyArgsManOptions(const ArgsManager& argsman, MemPoolLimits& mempool_limits)
 {
@@ -42,7 +45,13 @@ util::Result<void> ApplyArgsManOptions(const ArgsManager& argsman, const CChainP
 {
     mempool_opts.check_ratio = argsman.GetIntArg("-checkmempool", mempool_opts.check_ratio);
 
-    if (auto mb = argsman.GetIntArg("-maxmempool")) mempool_opts.max_size_bytes = *mb * 1'000'000;
+    if (auto mb = argsman.GetIntArg("-maxmempool")) {
+        constexpr bool is_32bit{sizeof(void*) == 4};
+        if (is_32bit && *mb > MAX_32BIT_MEMPOOL_MB) {
+            return util::Error{Untranslated(strprintf("-maxmempool is set to %i but can't be over %i MB on 32-bit systems", *mb, MAX_32BIT_MEMPOOL_MB))};
+        }
+        mempool_opts.max_size_bytes = *mb * 1'000'000;
+    }
 
     if (auto hours = argsman.GetIntArg("-mempoolexpiry")) mempool_opts.expiry = std::chrono::hours{*hours};