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LibSQL: Find free blocks when opening a database file
The free block list now gets populated on opening a database file. Ideally we persist this list inside the heap itself, but for now this prevents excessive heap growth.
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2 changed files with 53 additions and 3 deletions
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@ -127,3 +127,43 @@ TEST_CASE(heap_reuse_freed_blocks_after_storage_trim)
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auto heap_size_after_second_storage = MUST(heap->file_size_in_bytes());
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EXPECT(heap_size_after_second_storage <= original_heap_size);
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}
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TEST_CASE(heap_reuse_freed_blocks_after_reopening_file)
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{
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ScopeGuard guard([]() { MUST(Core::System::unlink(db_path)); });
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size_t original_heap_size = 0;
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StringBuilder builder;
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MUST(builder.try_append_repeated('x', SQL::Block::DATA_SIZE * 4));
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auto long_string = builder.string_view();
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{
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auto heap = create_heap();
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// First, write storage spanning 4 blocks
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auto first_index = heap->request_new_block_index();
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TRY_OR_FAIL(heap->write_storage(first_index, long_string.bytes()));
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MUST(heap->flush());
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original_heap_size = MUST(heap->file_size_in_bytes());
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// Then, overwrite the first storage and reduce it to 2 blocks
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builder.clear();
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MUST(builder.try_append_repeated('x', SQL::Block::DATA_SIZE * 2));
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long_string = builder.string_view();
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TRY_OR_FAIL(heap->write_storage(first_index, long_string.bytes()));
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MUST(heap->flush());
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auto heap_size_after_reduction = MUST(heap->file_size_in_bytes());
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EXPECT(heap_size_after_reduction <= original_heap_size);
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}
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// Reopen the database file; we expect the heap to support reading back free blocks somehow.
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// Add the second storage spanning 2 blocks - heap should not have grown compared to the original storage.
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{
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auto heap = create_heap();
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auto second_index = heap->request_new_block_index();
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TRY_OR_FAIL(heap->write_storage(second_index, long_string.bytes()));
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MUST(heap->flush());
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auto heap_size_after_second_storage = MUST(heap->file_size_in_bytes());
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EXPECT(heap_size_after_second_storage <= original_heap_size);
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}
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}
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@ -68,7 +68,16 @@ ErrorOr<void> Heap::open()
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return open();
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}
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dbgln_if(SQL_DEBUG, "Heap file {} opened; number of blocks = {}", name(), m_highest_block_written);
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// Perform a heap scan to find all free blocks
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// FIXME: this is very inefficient; store free blocks in a persistent heap structure
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for (Block::Index index = 1; index <= m_highest_block_written; ++index) {
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auto block_data = TRY(read_raw_block(index));
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auto size_in_bytes = *reinterpret_cast<u32*>(block_data.data());
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if (size_in_bytes == 0)
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TRY(m_free_block_indices.try_append(index));
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}
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dbgln_if(SQL_DEBUG, "Heap file {} opened; number of blocks = {}; free blocks = {}", name(), m_highest_block_written, m_free_block_indices.size());
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return {};
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}
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@ -159,8 +168,8 @@ ErrorOr<ByteBuffer> Heap::read_raw_block(Block::Index index)
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VERIFY(m_file);
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VERIFY(index < m_next_block);
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if (auto data = m_write_ahead_log.get(index); data.has_value())
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return data.value();
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if (auto wal_entry = m_write_ahead_log.get(index); wal_entry.has_value())
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return wal_entry.value();
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TRY(m_file->seek(index * Block::SIZE, SeekMode::SetPosition));
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auto buffer = TRY(ByteBuffer::create_uninitialized(Block::SIZE));
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@ -211,6 +220,7 @@ ErrorOr<void> Heap::write_block(Block const& block)
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{
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VERIFY(block.index() < m_next_block);
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VERIFY(block.next_block() < m_next_block);
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VERIFY(block.size_in_bytes() > 0);
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VERIFY(block.data().size() <= Block::DATA_SIZE);
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auto size_in_bytes = block.size_in_bytes();
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