Merge #18637: coins: allow cache resize after init

f19fdd47a6 test: add test for CChainState::ResizeCoinsCaches() (James O'Beirne)
8ac3ef4699 add ChainstateManager::MaybeRebalanceCaches() (James O'Beirne)
f36aaa6392 Add CChainState::ResizeCoinsCaches (James O'Beirne)
b223111da2 txdb: add CCoinsViewDB::ChangeCacheSize (James O'Beirne)

Pull request description:

  This is part of the [assumeutxo project](https://github.com/bitcoin/bitcoin/projects/11):

  Parent PR: #15606
  Issue: #15605
  Specification: https://github.com/jamesob/assumeutxo-docs/tree/master/proposal

  ---

  In the assumeutxo implementation draft (#15056), once a UTXO snapshot is loaded, a new chainstate object is created after initialization. This means that we have to reclaim some of the cache that we've allocated to the original chainstate (per `dbcache=`) to repurpose for the snapshot chainstate.

  Furthermore, it makes sense to have different cache allocations depending on which chainstate is more active. While the snapshot chainstate is working to get to the network tip (and the background validation chainstate is idle), it makes sense that the snapshot chainstate should have the majority of cache allocation. And contrariwise once the snapshot has reached network tip, most of the cache should be given to the background validation chainstate.

  This set of changes (detailed in the commit messages) allows us to dynamically resize the various coins caches. None of the functionality introduced here is used at the moment, but will be in the next AU PR (which introduces `ActivateSnapshot`).

  `ChainstateManager::MaybeRebalanceCaches()` defines the (somewhat normative) cache allocations between the snapshot and background validation chainstates. I'd be interested in feedback if anyone has thoughts on the proportions I've set there.

ACKs for top commit:
  ajtowns:
    weak utACK f19fdd47a6 -- didn't find any major problems, but not super confident that I didn't miss anything
  fjahr:
    Code review ACK f19fdd4
  ryanofsky:
    Code review ACK f19fdd47a6. Only change since last review is constructor cleanup (no change in behavior). I think the suggestions here from ajtowns and others are good, but shouldn't delay merging the PR (and hold up assumeutxo)

Tree-SHA512: fffb7847fb6993dd4a1a41cf11179b211b0b20b7eb5f7cf6266442136bfe9d43b830bbefcafd475bfd4af273f5573500594aa41fff03e0ed5c2a1e8562ff9269

src/init.cpp

@@ -1534,7 +1534,7 @@ bool AppInitMain(const util::Ref& context, NodeContext& node)
     int64_t nCoinDBCache = std::min(nTotalCache / 2, (nTotalCache / 4) + (1 << 23)); // use 25%-50% of the remainder for disk cache
     nCoinDBCache = std::min(nCoinDBCache, nMaxCoinsDBCache << 20); // cap total coins db cache
     nTotalCache -= nCoinDBCache;
-    nCoinCacheUsage = nTotalCache; // the rest goes to in-memory cache
+    int64_t nCoinCacheUsage = nTotalCache; // the rest goes to in-memory cache
     int64_t nMempoolSizeMax = gArgs.GetArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000;
     LogPrintf("Cache configuration:\n");
     LogPrintf("* Using %.1f MiB for block index database\n", nBlockTreeDBCache * (1.0 / 1024 / 1024));
@@ -1563,6 +1563,9 @@ bool AppInitMain(const util::Ref& context, NodeContext& node)
             try {
                 LOCK(cs_main);
                 chainman.InitializeChainstate();
+                chainman.m_total_coinstip_cache = nCoinCacheUsage;
+                chainman.m_total_coinsdb_cache = nCoinDBCache;
+
                 UnloadBlockIndex();
 
                 // new CBlockTreeDB tries to delete the existing file, which
@@ -1646,7 +1649,7 @@ bool AppInitMain(const util::Ref& context, NodeContext& node)
                     }
 
                     // The on-disk coinsdb is now in a good state, create the cache
-                    chainstate->InitCoinsCache();
+                    chainstate->InitCoinsCache(nCoinCacheUsage);
                     assert(chainstate->CanFlushToDisk());
 
                     if (!is_coinsview_empty(chainstate)) {