blockstorage: Return on fatal block file flush error

By returning an error code if `FlushBlockFile` fails, the caller now has
to explicitly handle block file flushing errors. Before this change
such errors were non-explicitly ignored without a clear rationale.

Prior to this patch `FlushBlockFile` may have failed silently in
`Chainstate::FlushStateToDisk`. Improve this with a log line. Also add a
TODO comment to flesh out whether returning early in the case of an
error is appropriate or not. Returning early might be appropriate to
prohibit `WriteBlockIndexDB` from writing a block index entry that does
not refer to a fully flushed block.

Besides `Chainstate::FlushStateToDisk`, `FlushBlockFile` is also called
by `FindBlockPos`. Don't change the abort behavior there, since we don't
want to fail the function if the flushing of already written blocks
fails. Instead, just document it.

src/node/blockstorage.cpp

@@ -546,8 +546,9 @@ void BlockManager::FlushUndoFile(int block_file, bool finalize)
     }
 }
 
-void BlockManager::FlushBlockFile(bool fFinalize, bool finalize_undo)
+bool BlockManager::FlushBlockFile(bool fFinalize, bool finalize_undo)
 {
+    bool success = true;
     LOCK(cs_LastBlockFile);
 
     if (m_blockfile_info.size() < 1) {
@@ -555,17 +556,19 @@ void BlockManager::FlushBlockFile(bool fFinalize, bool finalize_undo)
         // chainstate init, when we call ChainstateManager::MaybeRebalanceCaches() (which
         // then calls FlushStateToDisk()), resulting in a call to this function before we
         // have populated `m_blockfile_info` via LoadBlockIndexDB().
-        return;
+        return true;
     }
     assert(static_cast<int>(m_blockfile_info.size()) > m_last_blockfile);
 
     FlatFilePos block_pos_old(m_last_blockfile, m_blockfile_info[m_last_blockfile].nSize);
     if (!BlockFileSeq().Flush(block_pos_old, fFinalize)) {
         m_opts.notifications.flushError("Flushing block file to disk failed. This is likely the result of an I/O error.");
+        success = false;
     }
     // we do not always flush the undo file, as the chain tip may be lagging behind the incoming blocks,
     // e.g. during IBD or a sync after a node going offline
     if (!fFinalize || finalize_undo) FlushUndoFile(m_last_blockfile, finalize_undo);
+    return success;
 }
 
 uint64_t BlockManager::CalculateCurrentUsage()
@@ -658,7 +661,19 @@ bool BlockManager::FindBlockPos(FlatFilePos& pos, unsigned int nAddSize, unsigne
         if (!fKnown) {
             LogPrint(BCLog::BLOCKSTORAGE, "Leaving block file %i: %s\n", m_last_blockfile, m_blockfile_info[m_last_blockfile].ToString());
         }
-        FlushBlockFile(!fKnown, finalize_undo);
+
+        // Do not propagate the return code. The flush concerns a previous block
+        // and undo file that has already been written to. If a flush fails
+        // here, and we crash, there is no expected additional block data
+        // inconsistency arising from the flush failure here. However, the undo
+        // data may be inconsistent after a crash if the flush is called during
+        // a reindex. A flush error might also leave some of the data files
+        // untrimmed.
+        if (!FlushBlockFile(!fKnown, finalize_undo)) {
+            LogPrintLevel(BCLog::BLOCKSTORAGE, BCLog::Level::Warning,
+                          "Failed to flush previous block file %05i (finalize=%i, finalize_undo=%i) before opening new block file %05i\n",
+                          m_last_blockfile, !fKnown, finalize_undo, nFile);
+        }
         m_last_blockfile = nFile;
         m_undo_height_in_last_blockfile = 0; // No undo data yet in the new file, so reset our undo-height tracking.
     }

src/node/blockstorage.h

@@ -91,7 +91,10 @@ private:
      */
     bool LoadBlockIndex()
         EXCLUSIVE_LOCKS_REQUIRED(cs_main);
-    void FlushBlockFile(bool fFinalize = false, bool finalize_undo = false);
+
+    /** Return false if block file flushing fails. */
+    [[nodiscard]] bool FlushBlockFile(bool fFinalize = false, bool finalize_undo = false);
+
     void FlushUndoFile(int block_file, bool finalize = false);
     [[nodiscard]] bool FindBlockPos(FlatFilePos& pos, unsigned int nAddSize, unsigned int nHeight, uint64_t nTime, bool fKnown);
     bool FindUndoPos(BlockValidationState& state, int nFile, FlatFilePos& pos, unsigned int nAddSize);

src/validation.cpp

@@ -2511,7 +2511,11 @@ bool Chainstate::FlushStateToDisk(
                 LOG_TIME_MILLIS_WITH_CATEGORY("write block and undo data to disk", BCLog::BENCH);
 
                 // First make sure all block and undo data is flushed to disk.
-                m_blockman.FlushBlockFile();
+                // TODO: Handle return error, or add detailed comment why it is
+                // safe to not return an error upon failure.
+                if (!m_blockman.FlushBlockFile()) {
+                    LogPrintLevel(BCLog::VALIDATION, BCLog::Level::Warning, "%s: Failed to flush block file.\n", __func__);
+                }
             }
 
             // Then update all block file information (which may refer to block and undo files).