Negative cache support for node store

Adds support to TaggedCache to support smart replacement
(Needed to avoid race conditions with negative caching.)

Create a "hotDUMMY" object that represents the absence
of an object.

Allow DatabaseNodeImp::asyncFetch to complete immediately
if object is in cache (positive or negative).

Fix a bug in asyncFetch where the object returned may not
be the correct canonical version because we stash the
object in the results array before we canonicalize it.

src/ripple/basics/TaggedCache.h

@@ -300,19 +300,16 @@ public:
 
         @param key The key corresponding to the object
         @param data A shared pointer to the data corresponding to the object.
-        @param replace `true` if `data` is the up to date version of the object.
+        @param replace Function that decides if cache should be replaced
 
         @return `true` If the key already existed.
     */
-private:
-    template <bool replace>
+public:
     bool
     canonicalize(
         const key_type& key,
-        std::conditional_t<
-            replace,
-            std::shared_ptr<T> const,
-            std::shared_ptr<T>>& data)
+        std::shared_ptr<T>& data,
+        std::function<bool(std::shared_ptr<T> const&)>&& replace)
     {
         // Return canonical value, store if needed, refresh in cache
         // Return values: true=we had the data already
@@ -335,7 +332,7 @@ private:
 
         if (entry.isCached())
         {
-            if constexpr (replace)
+            if (replace(entry.ptr))
             {
                 entry.ptr = data;
                 entry.weak_ptr = data;
@@ -352,7 +349,7 @@ private:
 
         if (cachedData)
         {
-            if constexpr (replace)
+            if (replace(entry.ptr))
             {
                 entry.ptr = data;
                 entry.weak_ptr = data;
@@ -374,19 +371,22 @@ private:
         return false;
     }
 
-public:
     bool
     canonicalize_replace_cache(
         const key_type& key,
         std::shared_ptr<T> const& data)
     {
-        return canonicalize<true>(key, data);
+        return canonicalize(
+            key,
+            const_cast<std::shared_ptr<T>&>(data),
+            [](std::shared_ptr<T> const&) { return true; });
     }
 
     bool
     canonicalize_replace_client(const key_type& key, std::shared_ptr<T>& data)
     {
-        return canonicalize<false>(key, data);
+        return canonicalize(
+            key, data, [](std::shared_ptr<T> const&) { return false; });
     }
 
     std::shared_ptr<T>

src/ripple/nodestore/Database.h

@@ -156,7 +156,7 @@ public:
                 object is stored, used by the shard store.
         @param callback Callback function when read completes
     */
-    void
+    virtual void
     asyncFetch(
         uint256 const& hash,
         std::uint32_t ledgerSeq,

src/ripple/nodestore/NodeObject.h

@@ -33,7 +33,8 @@ enum NodeObjectType : std::uint32_t {
     hotUNKNOWN = 0,
     hotLEDGER = 1,
     hotACCOUNT_NODE = 3,
-    hotTRANSACTION_NODE = 4
+    hotTRANSACTION_NODE = 4,
+    hotDUMMY = 512  // an invalid or missing object
 };
 
 /** A simple object that the Ledger uses to store entries.

src/ripple/nodestore/impl/DatabaseNodeImp.cpp

@@ -33,7 +33,34 @@ DatabaseNodeImp::store(
 {
     storeStats(1, data.size());
 
-    backend_->store(NodeObject::createObject(type, std::move(data), hash));
+    auto obj = NodeObject::createObject(type, std::move(data), hash);
+    backend_->store(obj);
+    if (cache_)
+    {
+        // After the store, replace a negative cache entry if there is one
+        cache_->canonicalize(
+            hash, obj, [](std::shared_ptr<NodeObject> const& n) {
+                return n->getType() == hotDUMMY;
+            });
+    }
+}
+
+void
+DatabaseNodeImp::asyncFetch(
+    uint256 const& hash,
+    std::uint32_t ledgerSeq,
+    std::function<void(std::shared_ptr<NodeObject> const&)>&& callback)
+{
+    if (cache_)
+    {
+        std::shared_ptr<NodeObject> obj = cache_->fetch(hash);
+        if (obj)
+        {
+            callback(obj->getType() == hotDUMMY ? nullptr : obj);
+            return;
+        }
+    }
+    Database::asyncFetch(hash, ledgerSeq, std::move(callback));
 }
 
 void
@@ -75,8 +102,19 @@ DatabaseNodeImp::fetchNodeObject(
         switch (status)
         {
             case ok:
-                if (nodeObject && cache_)
-                    cache_->canonicalize_replace_client(hash, nodeObject);
+                if (cache_)
+                {
+                    if (nodeObject)
+                        cache_->canonicalize_replace_client(hash, nodeObject);
+                    else
+                    {
+                        auto notFound =
+                            NodeObject::createObject(hotDUMMY, {}, hash);
+                        cache_->canonicalize_replace_client(hash, notFound);
+                        if (notFound->getType() != hotDUMMY)
+                            nodeObject = notFound;
+                    }
+                }
                 break;
             case notFound:
                 break;
@@ -95,6 +133,8 @@ DatabaseNodeImp::fetchNodeObject(
     {
         JLOG(j_.trace()) << "fetchNodeObject " << hash
                          << ": record found in cache";
+        if (nodeObject->getType() == hotDUMMY)
+            nodeObject.reset();
     }
 
     if (nodeObject)
@@ -127,7 +167,7 @@ DatabaseNodeImp::fetchBatch(std::vector<uint256> const& hashes)
         }
         else
         {
-            results[i] = nObj;
+            results[i] = nObj->getType() == hotDUMMY ? nullptr : nObj;
             // It was in the cache.
             ++hits;
         }
@@ -140,9 +180,8 @@ DatabaseNodeImp::fetchBatch(std::vector<uint256> const& hashes)
 
     for (size_t i = 0; i < dbResults.size(); ++i)
     {
-        auto nObj = dbResults[i];
+        auto nObj = std::move(dbResults[i]);
         size_t index = indexMap[cacheMisses[i]];
-        results[index] = nObj;
         auto const& hash = hashes[index];
 
         if (nObj)
@@ -156,7 +195,15 @@ DatabaseNodeImp::fetchBatch(std::vector<uint256> const& hashes)
             JLOG(j_.error())
                 << "fetchBatch - "
                 << "record not found in db or cache. hash = " << strHex(hash);
+            if (cache_)
+            {
+                auto notFound = NodeObject::createObject(hotDUMMY, {}, hash);
+                cache_->canonicalize_replace_client(hash, notFound);
+                if (notFound->getType() != hotDUMMY)
+                    nObj = std::move(notFound);
+            }
         }
+        results[index] = std::move(nObj);
     }
 
     auto fetchDurationUs =

src/ripple/nodestore/impl/DatabaseNodeImp.h

@@ -111,6 +111,7 @@ public:
         // only one database
         return true;
     }
+
     void
     sync() override
     {
@@ -120,6 +121,13 @@ public:
     std::vector<std::shared_ptr<NodeObject>>
     fetchBatch(std::vector<uint256> const& hashes);
 
+    void
+    asyncFetch(
+        uint256 const& hash,
+        std::uint32_t ledgerSeq,
+        std::function<void(std::shared_ptr<NodeObject> const&)>&& callback)
+        override;
+
     bool
     storeLedger(std::shared_ptr<Ledger const> const& srcLedger) override
     {