log: LogError with FlatFilePos in UndoReadFromDisk

These errors should never happen in normal operation. If they do,
knowing the FlatFilePos may be useful to determine if data corruption
happened. Also, handle the error pos.IsNull() as part of OpenUndoFile,
because it may as well have happened due to data corruption.

This mirrors the LogError behavior from ReadBlockFromDisk.

Author: MarcoFalke

src/node/blockstorage.cpp

@@ -703,15 +703,10 @@ bool BlockManager::UndoReadFromDisk(CBlockUndo& blockundo, const CBlockIndex& in
 {
     const FlatFilePos pos{WITH_LOCK(::cs_main, return index.GetUndoPos())};
 
-    if (pos.IsNull()) {
-        LogError("%s: no undo data available\n", __func__);
-        return false;
-    }
-
     // Open history file to read
     AutoFile filein{OpenUndoFile(pos, true)};
     if (filein.IsNull()) {
-        LogError("%s: OpenUndoFile failed\n", __func__);
+        LogError("%s: OpenUndoFile failed for %s\n", __func__, pos.ToString());
         return false;
     }
 
@@ -723,13 +718,13 @@ bool BlockManager::UndoReadFromDisk(CBlockUndo& blockundo, const CBlockIndex& in
         verifier >> blockundo;
         filein >> hashChecksum;
     } catch (const std::exception& e) {
-        LogError("%s: Deserialize or I/O error - %s\n", __func__, e.what());
+        LogError("%s: Deserialize or I/O error - %s at %s\n", __func__, e.what(), pos.ToString());
         return false;
     }
 
     // Verify checksum
     if (hashChecksum != verifier.GetHash()) {
-        LogError("%s: Checksum mismatch\n", __func__);
+        LogError("%s: Checksum mismatch at %s\n", __func__, pos.ToString());
         return false;
     }