mempool: Don't share mempool with dbcache in blocksonly

When -blockonly is set, reduce mempool size to 5MB unless -maxmempool
is also set.

See #9569

Author: willcl-ark

src/init.cpp

@@ -719,10 +719,13 @@ void InitParameterInteraction(ArgsManager& args)
             LogPrintf("%s: parameter interaction: -externalip set -> setting -discover=0\n", __func__);
     }
 
-    // disable whitelistrelay in blocksonly mode
     if (args.GetBoolArg("-blocksonly", DEFAULT_BLOCKSONLY)) {
+        // disable whitelistrelay in blocksonly mode
         if (args.SoftSetBoolArg("-whitelistrelay", false))
             LogPrintf("%s: parameter interaction: -blocksonly=1 -> setting -whitelistrelay=0\n", __func__);
+        // Reduce default mempool size in blocksonly mode to avoid unexpected resource usage
+        if (args.SoftSetArg("-maxmempool", ToString(DEFAULT_BLOCKSONLY_MAX_MEMPOOL_SIZE_MB)))
+            LogPrintf("%s: parameter interaction: -blocksonly=1 -> setting -maxmempool=%d\n", __func__, DEFAULT_BLOCKSONLY_MAX_MEMPOOL_SIZE_MB);
     }
 
     // Forcing relay from whitelisted hosts implies we will accept relays from them in the first place.

src/kernel/mempool_options.h

@@ -18,6 +18,8 @@ class CBlockPolicyEstimator;
 
 /** Default for -maxmempool, maximum megabytes of mempool memory usage */
 static constexpr unsigned int DEFAULT_MAX_MEMPOOL_SIZE_MB{300};
+//** Default for -maxmempool when blocksonly is set */
+static constexpr unsigned int DEFAULT_BLOCKSONLY_MAX_MEMPOOL_SIZE_MB{5};
 /** Default for -mempoolexpiry, expiration time for mempool transactions in hours */
 static constexpr unsigned int DEFAULT_MEMPOOL_EXPIRY_HOURS{336};
 /** Default for -mempoolfullrbf, if the transaction replaceability signaling is ignored */