Refactor tally initialization for snapshot rebuild

This updates the tally initialization procedure to better handle accrual
snapshot state issues. The snapshot system detects problems with accrual
snapshots on disk and rebuilds those as needed. Since the tally needs to
rescan the chain to prepare the research accounts state for the snapshot
rebuild process, this refactors the code to fit that process more easily
and improve performance. It fixes an issue that failed an assertion when
a rebuild attempted to record duplicate blocks.

Author: cyrossignol

src/neuralnet/accrual/snapshot.h

@@ -750,6 +750,10 @@ class AccrualSnapshotRegistry
     {
         LogPrintf("Initializing accrual snapshot registry...");
 
+        if (!Close()) {
+            return false;
+        }
+
         try {
             CAutoFile registry_file(
                 fsbridge::fopen(RegistryPath(), "rb"),
@@ -994,7 +998,7 @@ class AccrualSnapshotRegistry
         }
     }; // DiskEntry
 
-    FILE* m_file;                 //!< Handle of the registry file.
+    FILE* m_file = nullptr;       //!< Handle of the registry file.
     std::vector<Entry> m_entries; //!< Ordered heights of each snapshot.
 
     //!
@@ -1185,14 +1189,29 @@ class AccrualSnapshotRepository
     }
 
     //!
-    //! \brief Check the integrity of each snapshot file on disk by comparing
-    //! the file hashes to those in the registry. Clean up any stray snapshot
-    //! files.
+    //! \brief Assert that the registry contains an entry for a snapshot at the
+    //! specified height and that the snapshot file hash matches.
     //!
-    //! \throws SnapshotHashMismatchError If the hash of a disk snapshot does
-    //! not match the hash recorded in the registry.
+    //! \param height Height of the snapshot to check the hash for.
+    //!
+    //! \throws SnapshotHashMismatchError If the supplied hash does not match
+    //! the hash recorded in the registry.
+    //! \throws SnapshotMissingError If the registry contains no entry for the
+    //! supplied height.
+    //!
+    void AssertMatch(const uint64_t height) const
+    {
+        if (const auto* entry = m_registry.TryHeight(height)) {
+            entry->AssertHash(AccrualSnapshotReader::Hash(SnapshotPath(height)));
+        } else {
+            throw SnapshotMissingError(height);
+        }
+    }
+
+    //!
+    //! \brief Clean up extraneous accrual snapshot files.
     //!
-    void AuditSnapshotIntegrity(const CBlockIndex* snapshot_baseline_pindex) const
+    void PruneSnapshotFiles() const
     {
         for (const auto& file : fs::directory_iterator(SnapshotDirectory())) {
             const fs::path& file_path = file.path();
@@ -1202,11 +1221,7 @@ class AccrualSnapshotRepository
             }
 
             if (const uint64_t height = AccrualSnapshotFile::ParseHeight(file_path)) {
-                // If the snapshot height does not exist in the registry, fall-
-                // through to the end of the loop and remove the file:
-                //
-                if (const auto* entry = m_registry.TryHeight(height)) {
-                    entry->AssertHash(AccrualSnapshotReader::Hash(file_path));
+                if (m_registry.TryHeight(height)) {
                     continue;
                 }
             }
@@ -1218,37 +1233,6 @@ class AccrualSnapshotRepository
 
             AccrualSnapshotFile::Remove(file_path);
         }
-
-        // Iterate through the superblocks from the baseline_superblock
-        // forward. At this point, because the hashes of the
-        // registry entries have been checked against the files,
-        // and orphan files eliminated, the only thing left to check
-        // is whether the registry entries actually correspond to the
-        // superblocks on chain.
-
-        // First rewind to the first SB encountered lower than the snapshot
-        // baseline pindex, which is the baseline SB. Then seek forward
-        // through all of the SB's.
-        const CBlockIndex* pindex = snapshot_baseline_pindex;
-
-        for (; pindex; pindex = pindex->pprev) {
-
-            if (pindex->nIsSuperBlock == 1) {
-                break;
-            }
-
-        }
-
-        for (; pindex; pindex = pindex->pnext) {
-
-            if (pindex->nIsSuperBlock != 1) {
-                continue;
-            }
-
-            if (m_registry.TryHeight(pindex->nHeight) == nullptr) {
-                throw SnapshotMissingError(pindex->nHeight);
-            }
-        }
     }
 
     //!