Repository: bitcoin/bitcoin
File: test/functional/feature_reindex.py (Python)

Code before:
    def set_test_params(self):
        self.setup_clean_chain = True
        self.num_nodes = 1

    def reindex(self, justchainstate=False):
        self.generatetoaddress(self.nodes[0], 3, self.nodes[0].get_deterministic_priv_key().address)
        blockcount = self.nodes[0].getblockcount()
        self.stop_nodes()
        extra_args = [["-reindex-chainstate" if justchainstate else "-reindex"]]
        self.start_nodes(extra_args)
        assert_equal(self.nodes[0].getblockcount(), blockcount)  # start_node is blocking on reindex
        self.log.info("Success")

    # Check that blocks can be processed out of order
    def out_of_order(self):
        # The previous test created 12 blocks
        assert_equal(self.nodes[0].getblockcount(), 12)
        self.stop_nodes()

        # In this test environment, blocks will always be in order (since
        # we're generating them rather than getting them from peers), so to
        # test out-of-order handling, swap blocks 1 and 2 on disk.
        blk0 = self.nodes[0].blocks_path / "blk00000.dat"
        with open(blk0, 'r+b') as bf:
            # Read at least the first few blocks (including genesis)
            b = bf.read(2000)

            # Find the offsets of blocks 2, 3, and 4 (the first 3 blocks beyond genesis)
            # by searching for the regtest marker bytes (see pchMessageStart).
            def find_block(b, start):
                return b.find(MAGIC_BYTES["regtest"], start)+4

            genesis_start = find_block(b, 0)
            assert_equal(genesis_start, 4)
            b2_start = find_block(b, genesis_start)
            b3_start = find_block(b, b2_start)
            b4_start = find_block(b, b3_start)

            # Blocks 2 and 3 should be the same size.
            assert_equal(b3_start-b2_start, b4_start-b3_start)

            # Swap the second and third blocks (don't disturb the genesis block).
            bf.seek(b2_start)
            bf.write(b[b3_start:b4_start])
            bf.write(b[b2_start:b3_start])

        # The reindexing code should detect and accommodate out of order blocks.
        with self.nodes[0].assert_debug_log([
            'LoadExternalBlockFile: Out of order block',
            'LoadExternalBlockFile: Processing out of order child',
        ]):

Review comment (suggestion):
I guess it may be simpler to serialize and deserialize the xor key as a raw byte span (without the size indicator). This should make it even easier to read in external scripts, for example in vanilla python.

Edit: And readers can assume it to be constant size. For example:

```
NUM_XOR_BYTES{8}

Code after:
    def set_test_params(self):
        self.setup_clean_chain = True
        self.num_nodes = 1

    def reindex(self, justchainstate=False):
        self.generatetoaddress(self.nodes[0], 3, self.nodes[0].get_deterministic_priv_key().address)
        blockcount = self.nodes[0].getblockcount()
        self.stop_nodes()
        extra_args = [["-reindex-chainstate" if justchainstate else "-reindex"]]
        self.start_nodes(extra_args)
        assert_equal(self.nodes[0].getblockcount(), blockcount)  # start_node is blocking on reindex
        self.log.info("Success")

    # Check that blocks can be processed out of order
    def out_of_order(self):
        # The previous test created 12 blocks
        assert_equal(self.nodes[0].getblockcount(), 12)
        self.stop_nodes()

        # In this test environment, blocks will always be in order (since
        # we're generating them rather than getting them from peers), so to
        # test out-of-order handling, swap blocks 1 and 2 on disk.
        blk0 = self.nodes[0].blocks_path / "blk00000.dat"
        with open(self.nodes[0].blocks_path / "xor.dat", "rb") as xor_f:
            NUM_XOR_BYTES = 8 # From InitBlocksdirXorKey::xor_key.size()
            xor_dat = xor_f.read(NUM_XOR_BYTES)

        def util_xor(data, key, *, offset):
            data = bytearray(data)
            for i in range(len(data)):
                data[i] ^= key[(i + offset) % len(key)]
            return bytes(data)

        with open(blk0, 'r+b') as bf:
            # Read at least the first few blocks (including genesis)
            b = util_xor(bf.read(2000), xor_dat, offset=0)

            # Find the offsets of blocks 2, 3, and 4 (the first 3 blocks beyond genesis)
            # by searching for the regtest marker bytes (see pchMessageStart).
            def find_block(b, start):
                return b.find(MAGIC_BYTES["regtest"], start)+4

            genesis_start = find_block(b, 0)
            assert_equal(genesis_start, 4)
            b2_start = find_block(b, genesis_start)
            b3_start = find_block(b, b2_start)
            b4_start = find_block(b, b3_start)

            # Blocks 2 and 3 should be the same size.
            assert_equal(b3_start - b2_start, b4_start - b3_start)