[FOLD] Fallback to acquiring ledgers:

When first starting up, a node may still be working to acquire the
ledgers for validations it has received. When calculating the preferred
ledger in this case, we should still rely on these validations over peer
counts until we acquire our first validated ledger. This should fix the
speed at which a newly started node syncs with the network.

src/ripple/consensus/Validations.h

@@ -656,7 +656,27 @@ class Validations
 
         // No trusted validations to determine branch
         if (preferred.seq == Seq{0})
+        {
+            // fall back to majority over acquiring ledgers
+            auto it = std::max_element(
+                acquiring_.begin(),
+                acquiring_.end(),
+                [](auto const& a, auto const& b) {
+                    std::pair<Seq, ID> const& aKey = a.first;
+                    typename hash_set<NodeKey>::size_type const& aSize =
+                        a.second.size();
+                    std::pair<Seq, ID> const& bKey = b.first;
+                    typename hash_set<NodeKey>::size_type const& bSize =
+                        b.second.size();
+                    // order by number of trusted peers validating that ledger
+                    // break ties with ledger ID
+                    return std::tie(aSize, aKey.second) <
+                        std::tie(bSize, bKey.second);
+                });
+            if(it != acquiring_.end())
+                return it->first;
             return std::make_pair(preferred.seq, preferred.id);
+        }
 
         // If we are the parent of the preferred ledger, stick with our
         // current ledger since we might be about to generate it

src/test/consensus/Validations_test.cpp

@@ -970,6 +970,20 @@ class Validations_test : public beast::unit_test::suite
         BEAST_EXPECT(harness.vals().numTrustedForLedger(ID{2}) == 1);
         // but ledger based data is not
         BEAST_EXPECT(harness.vals().getNodesAfter(genesisLedger, ID{0}) == 0);
+        // Initial preferred branch falls back to the ledger we are trying to
+        // acquire
+        BEAST_EXPECT(
+            harness.vals().getPreferred(genesisLedger) ==
+            std::make_pair(Seq{2}, ID{2}));
+
+        // After adding another unavailable validation, the preferred ledger
+        // breaks ties via higher ID
+        BEAST_EXPECT(
+            ValStatus::current ==
+            harness.add(b.validate(ID{3}, Seq{2}, 0s, 0s, true)));
+        BEAST_EXPECT(
+            harness.vals().getPreferred(genesisLedger) ==
+            std::make_pair(Seq{2}, ID{3}));
 
         // Create the ledger
         Ledger ledgerAB = h["ab"];