wallet: avoid rescans if under the snapshot

Refuse to load a wallet if it requires a rescan lower than the height of
an unvalidated snapshot we're running -- in more general terms, if we
don't have data for the blocks.

Author: jamesob

src/interfaces/chain.h

@@ -286,6 +286,9 @@ class Chain
     //! to be prepared to handle this by ignoring notifications about unknown
     //! removed transactions and already added new transactions.
     virtual void requestMempoolTransactions(Notifications& notifications) = 0;
+
+    //! Return true if an assumed-valid chain is in use.
+    virtual bool hasAssumedValidChain() = 0;
 };
 
 //! Interface to let node manage chain clients (wallets, or maybe tools for

src/node/interfaces.cpp

@@ -723,6 +723,11 @@ class ChainImpl : public Chain
             notifications.transactionAddedToMempool(entry.GetSharedTx(), 0 /* mempool_sequence */);
         }
     }
+    bool hasAssumedValidChain() override
+    {
+        return Assert(m_node.chainman)->IsSnapshotActive();
+    }
+
     NodeContext& m_node;
 };
 } // namespace

src/wallet/wallet.cpp

@@ -2903,20 +2903,31 @@ bool CWallet::AttachChain(const std::shared_ptr<CWallet>& walletInstance, interf
 
     if (tip_height && *tip_height != rescan_height)
     {
-        if (chain.havePruned()) {
+        // Technically we could execute the code below in any case, but performing the
+        // `while` loop below can make startup very slow, so only check blocks on disk
+        // if necessary.
+        if (chain.havePruned() || chain.hasAssumedValidChain()) {
             int block_height = *tip_height;
             while (block_height > 0 && chain.haveBlockOnDisk(block_height - 1) && rescan_height != block_height) {
                 --block_height;
             }
 
             if (rescan_height != block_height) {
-                // We can't rescan beyond non-pruned blocks, stop and throw an error.
+                // We can't rescan beyond blocks we don't have data for, stop and throw an error.
                 // This might happen if a user uses an old wallet within a pruned node
                 // or if they ran -disablewallet for a longer time, then decided to re-enable
                 // Exit early and print an error.
+                // It also may happen if an assumed-valid chain is in use and therefore not
+                // all block data is available.
                 // If a block is pruned after this check, we will load the wallet,
                 // but fail the rescan with a generic error.
-                error = _("Prune: last wallet synchronisation goes beyond pruned data. You need to -reindex (download the whole blockchain again in case of pruned node)");
+
+                error = chain.hasAssumedValidChain() ?
+                     _(
+                        "Assumed-valid: last wallet synchronisation goes beyond "
+                        "available block data. You need to wait for the background "
+                        "validation chain to download more blocks.") :
+                     _("Prune: last wallet synchronisation goes beyond pruned data. You need to -reindex (download the whole blockchain again in case of pruned node)");
                 return false;
             }
         }