Merge bitcoin/bitcoin#31439: validation: In case of a continued reindex, only activate chain in the end

c9136ca validation: fix issue with an interrupted -reindex (Martin Zumsande)
a267589 validation: Don't loop over all chainstates in LoadExternalBlock (Martin Zumsande)

Pull request description:

  If a user interrupts a reindex while it is iterating over the block files, it will continue to reindex with the next node start (if the `-reindex` arg is dropped, otherwise it will start reindexing from scratch).
  However, due to an early call to `ActivateBestChainState()` that only exists to connect the genesis block during
  the original `-reindex`, it wil start connecting blocks immediately before having iterated through all block files.
  Because later headers above the minchainwork threshold won't be loaded in this case, `-assumevalid` will not
  be applied and the process is much slower due to script validation being done.

  Fix this by only calling `ActivateBestChainState()` here if Genesis is not connected yet (equivalent to `ActiveHeight() == -1`).
  Also simplify this spot by only doing this for the active chainstate instead of looping over all chainstates (first commit).

  This issue was discussed in the thread below bitcoin/bitcoin#31346 (comment), the impact on assumevalid was found by l0rinc.

  The fix can be tested by manually aborting a `-reindex` e.g. on signet and observing in the debug log the order in which blockfiles are indexed / blocks are connected with this branch vs master.

ACKs for top commit:
  achow101:
    ACK c9136ca
  ryanofsky:
    Code review ACK c9136ca. Only comments changed since last review. Appreciate the new comments, I think they make a little clearer what things code is trying to do and what things are just side-effects.
  TheCharlatan:
    Re-ACK c9136ca

Tree-SHA512: 6f34abc317ad7e605ccc0c2f4615e4ea6978223d207f80f768f39cc135a9ac0adf31681fadfa2aed45324a5d27a4f68c5e118ee7eec18ca5c40ef177caa9cc47

Author: achow101

src/validation.cpp

@@ -5169,16 +5169,15 @@ void ChainstateManager::LoadExternalBlockFile(
                 }
 
                 // Activate the genesis block so normal node progress can continue
-                if (hash == params.GetConsensus().hashGenesisBlock) {
-                    bool genesis_activation_failure = false;
-                    for (auto c : GetAll()) {
-                        BlockValidationState state;
-                        if (!c->ActivateBestChain(state, nullptr)) {
-                            genesis_activation_failure = true;
-                            break;
-                        }
-                    }
-                    if (genesis_activation_failure) {
+                // During first -reindex, this will only connect Genesis since
+                // ActivateBestChain only connects blocks which are in the block tree db,
+                // which only contains blocks whose parents are in it.
+                // But do this only if genesis isn't activated yet, to avoid connecting many blocks
+                // without assumevalid in the case of a continuation of a reindex that
+                // was interrupted by the user.
+                if (hash == params.GetConsensus().hashGenesisBlock && WITH_LOCK(::cs_main, return ActiveHeight()) == -1) {
+                    BlockValidationState state;
+                    if (!ActiveChainstate().ActivateBestChain(state, nullptr)) {
                         break;
                     }
                 }