blockstorage: Rename FindBlockPos and have it return a FlatFilePos

The new name reflects that it is no longer called with existing blocks
for which the position is already known.

Returning a FlatFilePos directly simplifies the interface.

src/node/blockstorage.cpp

@@ -848,7 +848,7 @@ fs::path BlockManager::GetBlockPosFilename(const FlatFilePos& pos) const
     return BlockFileSeq().FileName(pos);
 }
 
-bool BlockManager::FindBlockPos(FlatFilePos& pos, unsigned int nAddSize, unsigned int nHeight, uint64_t nTime)
+FlatFilePos BlockManager::FindNextBlockPos(unsigned int nAddSize, unsigned int nHeight, uint64_t nTime)
 {
     LOCK(cs_LastBlockFile);
 
@@ -897,6 +897,7 @@ bool BlockManager::FindBlockPos(FlatFilePos& pos, unsigned int nAddSize, unsigne
             m_blockfile_info.resize(nFile + 1);
         }
     }
+    FlatFilePos pos;
     pos.nFile = nFile;
     pos.nPos = m_blockfile_info[nFile].nSize;
 
@@ -927,14 +928,14 @@ bool BlockManager::FindBlockPos(FlatFilePos& pos, unsigned int nAddSize, unsigne
     size_t bytes_allocated = BlockFileSeq().Allocate(pos, nAddSize, out_of_space);
     if (out_of_space) {
         m_opts.notifications.fatalError(_("Disk space is too low!"));
-        return false;
+        return {};
     }
     if (bytes_allocated != 0 && IsPruneMode()) {
         m_check_for_pruning = true;
     }
 
     m_dirty_fileinfo.insert(nFile);
-    return true;
+    return pos;
 }
 
 
@@ -1032,7 +1033,7 @@ bool BlockManager::WriteUndoDataForBlock(const CBlockUndo& blockundo, BlockValid
         // we want to flush the rev (undo) file once we've written the last block, which is indicated by the last height
         // in the block file info as below; note that this does not catch the case where the undo writes are keeping up
         // with the block writes (usually when a synced up node is getting newly mined blocks) -- this case is caught in
-        // the FindBlockPos function
+        // the FindNextBlockPos function
         if (_pos.nFile < cursor.file_num && static_cast<uint32_t>(block.nHeight) == m_blockfile_info[_pos.nFile].nHeightLast) {
             // Do not propagate the return code, a failed flush here should not
             // be an indication for a failed write. If it were propagated here,
@@ -1151,12 +1152,12 @@ bool BlockManager::ReadRawBlockFromDisk(std::vector<uint8_t>& block, const FlatF
 FlatFilePos BlockManager::SaveBlockToDisk(const CBlock& block, int nHeight)
 {
     unsigned int nBlockSize = ::GetSerializeSize(TX_WITH_WITNESS(block));
-    FlatFilePos blockPos;
     // Must also account for the serialization header
     // (the 4 magic message start bytes + the 4 length bytes = 8 bytes = BLOCK_SERIALIZATION_HEADER_SIZE)
     nBlockSize += static_cast<unsigned int>(BLOCK_SERIALIZATION_HEADER_SIZE);
-    if (!FindBlockPos(blockPos, nBlockSize, nHeight, block.GetBlockTime())) {
-        LogError("%s: FindBlockPos failed\n", __func__);
+    FlatFilePos blockPos{FindNextBlockPos(nBlockSize, nHeight, block.GetBlockTime())};
+    if (blockPos.IsNull()) {
+        LogError("%s: FindNextBlockPos failed\n", __func__);
         return FlatFilePos();
     }
     if (!WriteBlockToDisk(block, blockPos)) {

src/node/blockstorage.h

@@ -156,10 +156,15 @@ class BlockManager
     [[nodiscard]] bool FlushUndoFile(int block_file, bool finalize = false);
 
     /**
+     * Helper function performing various preparations before a block can be saved to disk:
+     * Returns the correct position for the block to be saved, which may be in the current or a new
+     * block file depending on nAddSize. May flush the previous blockfile to disk if full, updates
+     * blockfile info, and checks if there is enough disk space to save the block.
+     *
      * The nAddSize argument passed to this function should include not just the size of the serialized CBlock, but the also size of
      * separator fields which are written before it by WriteBlockToDisk (BLOCK_SERIALIZATION_HEADER_SIZE).
      */
-    [[nodiscard]] bool FindBlockPos(FlatFilePos& pos, unsigned int nAddSize, unsigned int nHeight, uint64_t nTime);
+    [[nodiscard]] FlatFilePos FindNextBlockPos(unsigned int nAddSize, unsigned int nHeight, uint64_t nTime);
     [[nodiscard]] bool FlushChainstateBlockFile(int tip_height);
     bool FindUndoPos(BlockValidationState& state, int nFile, FlatFilePos& pos, unsigned int nAddSize);
 
@@ -216,7 +221,7 @@ class BlockManager
     //! effectively.
     //!
     //! This data structure maintains separate blockfile number cursors for each
-    //! BlockfileType. The ASSUMED state is initialized, when necessary, in FindBlockPos().
+    //! BlockfileType. The ASSUMED state is initialized, when necessary, in FindNextBlockPos().
     //!
     //! The first element is the NORMAL cursor, second is ASSUMED.
     std::array<std::optional<BlockfileCursor>, BlockfileType::NUM_TYPES>