The entry.tag is incorrect after full gc

[JaySon-Huang]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

TEST_F(PageStorageTest, EntryTagAfterFullGC)
try
{
    {
        PageStorage::Config config;
        config.blob_heavy_gc_valid_rate = 1.0; /// always run full gc
        page_storage = reopenWithConfig(config);
    }

    const size_t buf_sz = 1024;
    char c_buff[buf_sz];

    for (size_t i = 0; i < buf_sz; ++i)
    {
        c_buff[i] = i % 0xff;
    }

    PageId page_id = 120;
    UInt64 tag = 12345;
    {
        WriteBatch batch;
        batch.putPage(page_id, tag, std::make_shared<ReadBufferFromMemory>(c_buff, buf_sz), buf_sz, {});
        page_storage->write(std::move(batch));
    }

    {
        auto entry = page_storage->getEntry(page_id);
        ASSERT_EQ(entry.tag, tag);
        auto page = page_storage->read(page_id);
        for (size_t i = 0; i < buf_sz; ++i)
        {
            EXPECT_EQ(*(page.data.begin() + i), static_cast<char>(i % 0xff));
        }
    }

    auto done_full_gc = page_storage->gc();
    EXPECT_TRUE(done_full_gc);

    {
        auto entry = page_storage->getEntry(page_id);
        ASSERT_EQ(entry.tag, tag);
        auto page = page_storage->read(page_id);
        for (size_t i = 0; i < buf_sz; ++i)
        {
            EXPECT_EQ(*(page.data.begin() + i), static_cast<char>(i % 0xff));
        }
    }
}
CATCH

2. What did you expect to see? (Required)

the entry.tag remain unchanged after full gc

3. What did you see instead (Required)

new_entry.tag is not set after full gc

tiflash/dbms/src/Storages/Page/V3/BlobStore.cpp

Lines 1120 to 1135 in 5b61ae7

	PageEntryV3 new_entry;
	read(file_id, entry.offset, data_pos, entry.size, read_limiter, /*background*/ true);
	// No need do crc again, crc won't be changed.
	new_entry.checksum = entry.checksum;
	// Need copy the field_offsets
	new_entry.field_offsets = entry.field_offsets;
	// Entry size won't be changed.
	new_entry.size = entry.size;
	new_entry.file_id = blobfile_id;
	new_entry.offset = file_offset_beg + offset_in_data;

this will lead to more IO caused by RegionPersister::doPersist

tiflash/dbms/src/Storages/Transaction/RegionPersister.cpp

Lines 111 to 115 in 5b61ae7

	if (page_reader)
	{
	auto entry = page_reader->getPageEntry(region_id);
	if (entry.isValid() && entry.tag > applied_index)
	return;

4. What is your TiFlash version? (Required)

master