Acquire logic fixes:

In InboundLedgers::acquire, properly return a ledger if we are
able to create one immediately.

In LedgerMaster::walkHashBySeq, don't walk the whole ledger if
we already know we have it.

src/ripple/app/ledger/Ledger.h

@@ -545,6 +545,25 @@ boost::optional<uint256>
 hashOfSeq (Ledger& ledger, LedgerIndex seq,
     SLECache& cache, beast::Journal journal);
 
+/** Find a ledger index from which we could easily get the requested ledger
+
+    The index that we return should meet two requirements:
+        1) It must be the index of a ledger that has the hash of the ledger
+            we are looking for. This means that its sequence must be equal to
+            greater than the sequence that we want but not more than 256 greater
+            since each ledger contains the hashes of the 256 previous ledgers.
+
+        2) Its hash must be easy for us to find. This means it must be 0 mod 256
+            because every such ledger is permanently enshrined in a LedgerHashes
+            page which we can easily retrieve via the skip list.
+*/
+inline
+LedgerIndex
+getCandidateLedger (LedgerIndex requested)
+{
+    return (requested + 255) & (~255);
+}
+
 } // ripple
 
 #endif

src/ripple/app/ledger/impl/InboundLedgers.cpp

@@ -57,8 +57,7 @@ public:
     Ledger::pointer acquire (uint256 const& hash, std::uint32_t seq, InboundLedger::fcReason reason)
     {
         assert (hash.isNonZero ());
-        Ledger::pointer ret;
-
+        InboundLedger::pointer inbound;
         {
             ScopedLockType sl (mLock);
 
@@ -67,26 +66,26 @@ public:
                 auto it = mLedgers.find (hash);
                 if (it != mLedgers.end ())
                 {
-                    // Don't touch failed acquires so they can expire
-                    if (! it->second->isFailed ())
-                    {
-                        it->second->update (seq);
-                        if (it->second->isComplete ())
-                            ret = it->second->getLedger ();
-                    }
+                    inbound = it->second;
 
+                    // If the acquisition failed, don't mark the item as
+                    // recently accessed so that it can expire.
+                    if (! inbound->isFailed ())
+                        inbound->update (seq);
                 }
                 else
                 {
-                    auto il = std::make_shared <InboundLedger> (hash, seq, reason, std::ref (m_clock));
-                    mLedgers.insert (std::make_pair (hash, il));
-                    il->init (sl);
+                    inbound = std::make_shared <InboundLedger> (
+                        hash, seq, reason, std::ref (m_clock));
+                    mLedgers.emplace (hash, inbound);
+                    inbound->init (sl);
                     ++mCounter;
                 }
             }
         }
-
-        return ret;
+        if (inbound && inbound->isComplete ())
+            return inbound->getLedger();
+        return {};
     }
 
     InboundLedger::pointer find (uint256 const& hash)

src/ripple/app/ledger/impl/LedgerCleaner.cpp

@@ -352,7 +352,7 @@ public:
             {
                 // No, Try to get another ledger that might have the hash we need
                 // Compute the index and hash of a ledger that will have the hash we need
-                LedgerIndex refIndex = (ledgerIndex + 255) & (~255);
+                LedgerIndex refIndex = getCandidateLedger (ledgerIndex);
                 LedgerHash refHash = getLedgerHash (referenceLedger, refIndex);
 
                 bool const nonzero (refHash.isNonZero ());

src/ripple/app/ledger/impl/LedgerMaster.cpp

@@ -1450,17 +1450,34 @@ public:
         if (ledgerHash)
             return *ledgerHash;
 
-        // No, Try to get another ledger that might have the hash we need
-        // Compute the index and hash of a ledger that will have the hash we need
-        LedgerIndex refIndex = (index + 255) & (~255);
+        // The hash is not in the reference ledger. Get another ledger which can
+        // be located easily and should contain the hash.
+        LedgerIndex refIndex = getCandidateLedger(index);
         auto const refHash = hashOfSeq(*referenceLedger, refIndex,
             getApp().getSLECache(), m_journal);
         assert(refHash);
         if (refHash)
         {
-            // We found the hash and sequence of a better reference ledger
-            auto const ledger =
-                getApp().getInboundLedgers().acquire (
+            // Try the hash and sequence of a better reference ledger just found
+            auto ledger = mLedgerHistory.getLedgerByHash (*refHash);
+
+            if (ledger)
+            {
+                try
+                {
+                    ledgerHash = hashOfSeq(*ledger, index,
+                        getApp().getSLECache(), m_journal);
+                }
+                catch(SHAMapMissingNode&)
+                {
+                    ledger.reset();
+                }
+            }
+
+            // Try to acquire the complete ledger
+            if (!ledger)
+            {
+                auto const ledger = getApp().getInboundLedgers().acquire (
                     *refHash, refIndex, InboundLedger::fcGENERIC);
                 if (ledger)
                 {
@@ -1469,6 +1486,7 @@ public:
                     assert (ledgerHash);
                 }
             }
+        }
         return ledgerHash ? *ledgerHash : zero; // kludge
     }
 

src/ripple/rpc/handlers/LedgerRequest.cpp

@@ -71,7 +71,7 @@ Json::Value doLedgerRequest (RPC::Context& context)
         if (! ledgerHash)
         {
             // Find a ledger more likely to have the hash of the desired ledger
-            auto refIndex = (ledgerIndex + 255) & (~255);
+            auto const refIndex = getCandidateLedger(ledgerIndex);
             auto refHash = hashOfSeq(*ledger, refIndex,
                 getApp().getSLECache(), j);
             assert(refHash);