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.

Tests/LibSQL/TestSqlHeap.cpp

@@ -127,3 +127,43 @@ TEST_CASE(heap_reuse_freed_blocks_after_storage_trim)
     auto heap_size_after_second_storage = MUST(heap->file_size_in_bytes());
     EXPECT(heap_size_after_second_storage <= original_heap_size);
 }
+
+TEST_CASE(heap_reuse_freed_blocks_after_reopening_file)
+{
+    ScopeGuard guard([]() { MUST(Core::System::unlink(db_path)); });
+
+    size_t original_heap_size = 0;
+    StringBuilder builder;
+    MUST(builder.try_append_repeated('x', SQL::Block::DATA_SIZE * 4));
+    auto long_string = builder.string_view();
+
+    {
+        auto heap = create_heap();
+
+        // First, write storage spanning 4 blocks
+        auto first_index = heap->request_new_block_index();
+        TRY_OR_FAIL(heap->write_storage(first_index, long_string.bytes()));
+        MUST(heap->flush());
+        original_heap_size = MUST(heap->file_size_in_bytes());
+
+        // Then, overwrite the first storage and reduce it to 2 blocks
+        builder.clear();
+        MUST(builder.try_append_repeated('x', SQL::Block::DATA_SIZE * 2));
+        long_string = builder.string_view();
+        TRY_OR_FAIL(heap->write_storage(first_index, long_string.bytes()));
+        MUST(heap->flush());
+        auto heap_size_after_reduction = MUST(heap->file_size_in_bytes());
+        EXPECT(heap_size_after_reduction <= original_heap_size);
+    }
+
+    // Reopen the database file; we expect the heap to support reading back free blocks somehow.
+    // Add the second storage spanning 2 blocks - heap should not have grown compared to the original storage.
+    {
+        auto heap = create_heap();
+        auto second_index = heap->request_new_block_index();
+        TRY_OR_FAIL(heap->write_storage(second_index, long_string.bytes()));
+        MUST(heap->flush());
+        auto heap_size_after_second_storage = MUST(heap->file_size_in_bytes());
+        EXPECT(heap_size_after_second_storage <= original_heap_size);
+    }
+}

Userland/Libraries/LibSQL/Heap.cpp

@@ -68,7 +68,16 @@ ErrorOr<void> Heap::open()
         return open();
     }
 
-    dbgln_if(SQL_DEBUG, "Heap file {} opened; number of blocks = {}", name(), m_highest_block_written);
+    // Perform a heap scan to find all free blocks
+    // FIXME: this is very inefficient; store free blocks in a persistent heap structure
+    for (Block::Index index = 1; index <= m_highest_block_written; ++index) {
+        auto block_data = TRY(read_raw_block(index));
+        auto size_in_bytes = *reinterpret_cast<u32*>(block_data.data());
+        if (size_in_bytes == 0)
+            TRY(m_free_block_indices.try_append(index));
+    }
+
+    dbgln_if(SQL_DEBUG, "Heap file {} opened; number of blocks = {}; free blocks = {}", name(), m_highest_block_written, m_free_block_indices.size());
     return {};
 }
 
@@ -159,8 +168,8 @@ ErrorOr<ByteBuffer> Heap::read_raw_block(Block::Index index)
     VERIFY(m_file);
     VERIFY(index < m_next_block);
 
-    if (auto data = m_write_ahead_log.get(index); data.has_value())
-        return data.value();
+    if (auto wal_entry = m_write_ahead_log.get(index); wal_entry.has_value())
+        return wal_entry.value();
 
     TRY(m_file->seek(index * Block::SIZE, SeekMode::SetPosition));
     auto buffer = TRY(ByteBuffer::create_uninitialized(Block::SIZE));
@@ -211,6 +220,7 @@ ErrorOr<void> Heap::write_block(Block const& block)
 {
     VERIFY(block.index() < m_next_block);
     VERIFY(block.next_block() < m_next_block);
+    VERIFY(block.size_in_bytes() > 0);
     VERIFY(block.data().size() <= Block::DATA_SIZE);
 
     auto size_in_bytes = block.size_in_bytes();