blockstorage: Return on fatal undo file flush error

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

Besides the call to `FlushUndoFile` in `FlushBlockFile`, ignore its
return code at its call site in `WriteUndoDataForBlock`. There, a failed
flush of the undo data should not be indicative of a failed write.

Add [[nodiscard]] annotations to `FlushUndoFile` such that its return
value is not just ignored in the future.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/node/blockstorage.h b/src/node/blockstorage.h
index 3e0962386f..9950d9c477 100644
--- a/src/node/blockstorage.h
+++ b/src/node/blockstorage.h
@@ -92,10 +92,12 @@ private:
     bool LoadBlockIndex()
         EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 
-    /** Return false if block file flushing fails. */
+    /** Return false if block file or undo file flushing fails. */
     [[nodiscard]] bool FlushBlockFile(bool fFinalize = false, bool finalize_undo = false);
 
-    void FlushUndoFile(int block_file, bool finalize = false);
+    /** Return false if undo file flushing fails. */
+    [[nodiscard]] bool 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);