Check -dbcache maximum at startup

Previously the number was silently rounded down.

doc/release-notes-25358.md

@@ -2,3 +2,4 @@ Node
 ------
 
 - The maximum value for `-dbcache` has been increased to 32,000 MB due to recent UTXO set growth. Be sure to check that you have enough RAM before increasing this.
+- Setting `-dbcache` above the maximum causes the node to abort startup. Previously it would round the number down.

src/init.cpp

@@ -925,6 +925,10 @@ bool AppInitParameterInteraction(const ArgsManager& args)
         return InitError(Untranslated("Cannot set -listen=0 together with -listenonion=1"));
     }
 
+    if (args.GetIntArg("-dbcache", nDefaultDbCache) >  nMaxDbCache) {
+        return InitError(Untranslated(strprintf("-dbcache must be at most %d MB", nMaxDbCache)));
+    }
+
     // Make sure enough file descriptors are available
     int nBind = std::max(nUserBind, size_t(1));
     nUserMaxConnections = args.GetIntArg("-maxconnections", DEFAULT_MAX_PEER_CONNECTIONS);

src/node/caches.cpp

@@ -13,7 +13,7 @@ CacheSizes CalculateCacheSizes(const ArgsManager& args, size_t n_indexes)
 {
     int64_t nTotalCache = (args.GetIntArg("-dbcache", nDefaultDbCache) << 20);
     nTotalCache = std::max(nTotalCache, nMinDbCache << 20); // total cache cannot be less than nMinDbCache
-    nTotalCache = std::min(nTotalCache, nMaxDbCache << 20); // total cache cannot be greater than nMaxDbcache
+    assert(nTotalCache <= nMaxDbCache);
     CacheSizes sizes;
     sizes.block_tree_db = std::min(nTotalCache / 8, nMaxBlockDBCache << 20);
     nTotalCache -= sizes.block_tree_db;