LibSQL: Implement freeing heap storage

This allows us to free entire chains of blocks in one go.

Tests/LibSQL/TestSqlHeap.cpp

@@ -167,3 +167,27 @@ TEST_CASE(heap_reuse_freed_blocks_after_reopening_file)
         EXPECT(heap_size_after_second_storage <= original_heap_size);
     }
 }
+
+TEST_CASE(heap_free_storage)
+{
+    ScopeGuard guard([]() { MUST(Core::System::unlink(db_path)); });
+    auto heap = create_heap();
+    auto storage_block_id = heap->request_new_block_index();
+
+    // Write large storage spanning multiple blocks
+    StringBuilder builder;
+    MUST(builder.try_append_repeated('x', SQL::Block::DATA_SIZE * 4));
+    auto long_string = builder.string_view();
+    TRY_OR_FAIL(heap->write_storage(storage_block_id, long_string.bytes()));
+    MUST(heap->flush());
+    auto heap_size = MUST(heap->file_size_in_bytes());
+
+    // Free the storage
+    TRY_OR_FAIL(heap->free_storage(storage_block_id));
+
+    // Again, write some large storage spanning multiple blocks
+    TRY_OR_FAIL(heap->write_storage(storage_block_id, long_string.bytes()));
+    MUST(heap->flush());
+    auto new_heap_size = MUST(heap->file_size_in_bytes());
+    EXPECT(new_heap_size <= heap_size);
+}

Userland/Libraries/LibSQL/Heap.cpp

@@ -154,11 +154,8 @@ ErrorOr<void> Heap::write_storage(Block::Index index, ReadonlyBytes data)
     }
 
     // Free remaining blocks in existing chain, if any
-    while (existing_next_block_index > 0) {
+    if (existing_next_block_index > 0)
-        auto existing_block = TRY(read_block(existing_next_block_index));
+        TRY(free_storage(existing_next_block_index));
-        existing_next_block_index = existing_block.next_block();
-        TRY(free_block(existing_block));
-    }
 
     return {};
 }
@@ -235,6 +232,19 @@ ErrorOr<void> Heap::write_block(Block const& block)
     return write_raw_block_to_wal(block.index(), move(heap_data));
 }
 
+ErrorOr<void> Heap::free_storage(Block::Index index)
+{
+    dbgln_if(SQL_DEBUG, "{}({})", __FUNCTION__, index);
+    VERIFY(index > 0);
+
+    while (index > 0) {
+        auto block = TRY(read_block(index));
+        TRY(free_block(block));
+        index = block.next_block();
+    }
+    return {};
+}
+
 ErrorOr<void> Heap::free_block(Block const& block)
 {
     auto index = block.index();

Userland/Libraries/LibSQL/Heap.h

@@ -116,6 +116,7 @@ public:
 
     ErrorOr<ByteBuffer> read_storage(Block::Index);
     ErrorOr<void> write_storage(Block::Index, ReadonlyBytes);
+    ErrorOr<void> free_storage(Block::Index);
 
     ErrorOr<void> flush();