Merge bitcoin/bitcoin#33657: rest: allow reading partial block data from storage

07135290c1 rest: allow reading partial block data from storage (Roman Zeyde)
4e2af1c065 blockstorage: allow reading partial block data from storage (Roman Zeyde)
f2fd1aa21c blockstorage: return an error code from `ReadRawBlock()` (Roman Zeyde)

Pull request description:

  It allows fetching specific transactions using an external index, following https://github.com/bitcoin/bitcoin/pull/32541#issuecomment-3267485313.

  Currently, electrs and other indexers map between an address/scripthash to the list of the relevant transactions.

  However, in order to fetch those transactions from bitcoind, electrs relies on reading the whole block and post-filtering for a specific transaction[^1]. Other indexers use a `txindex` to fetch a transaction using its txid [^2][^3][^4].

  The above approach has significant storage and CPU overhead, since the `txid` is a pseudo-random 32-byte value. Also, mainnet `txindex` takes ~60GB today.

  This PR is adding support for using the transaction's position within its block to be able to fetch it directly using [REST API](https://github.com/bitcoin/bitcoin/blob/master/doc/REST-interface.md), using the following HTTP request:

  ```
  GET /rest/blockpart/BLOCKHASH.bin?offset=OFFSET&size=SIZE
  ```

  - The offsets' index can be encoded much more efficiently ([~1.3GB today](https://github.com/romanz/bindex-rs/pull/66#issuecomment-3508476436)).

  - Address history query performance can be tested on mainnet using [1BitcoinEaterAddressDontSendf59kuE](https://mempool.space/address/1BitcoinEaterAddressDontSendf59kuE) - assuming warm OS block cache, [it takes <1s to fetch 5200 txs, i.e. <0.2ms per tx](https://github.com/romanz/bindex-rs/pull/66#issuecomment-3508476436) with [bindex](https://github.com/romanz/bindex-rs).

  - Only binary and hex response formats are supported.

  [^1]: https://github.com/romanz/electrs/blob/master/doc/schema.md
  [^2]: https://github.com/Blockstream/electrs/blob/new-index/doc/schema.md#txstore
  [^3]: https://github.com/spesmilo/electrumx/blob/master/docs/HOWTO.rst#prerequisites
  [^4]: https://github.com/cculianu/Fulcrum/blob/master/README.md#requirements

ACKs for top commit:
  maflcko:
    review ACK 07135290c1 🏪
  l0rinc:
    ACK 07135290c1
  hodlinator:
    re-ACK 07135290c1

Tree-SHA512: bcce7bf4b9a3e5e920ab5a83e656f50d5d7840cdde6b7147d329cf578f8a2db555fc1aa5334e8ee64d5630d25839ece77a2cf421c6c3ac1fa379bb453163bd4f

src/test/blockmanager_tests.cpp

@@ -138,6 +138,68 @@ BOOST_FIXTURE_TEST_CASE(blockmanager_block_data_availability, TestChain100Setup)
     BOOST_CHECK(!blockman.CheckBlockDataAvailability(tip, *last_pruned_block));
 }
 
+BOOST_FIXTURE_TEST_CASE(blockmanager_block_data_part, TestChain100Setup)
+{
+    LOCK(::cs_main);
+    auto& chainman{m_node.chainman};
+    auto& blockman{chainman->m_blockman};
+    const CBlockIndex& tip{*chainman->ActiveTip()};
+    const FlatFilePos tip_block_pos{tip.GetBlockPos()};
+
+    auto block{blockman.ReadRawBlock(tip_block_pos)};
+    BOOST_REQUIRE(block);
+    BOOST_REQUIRE_GE(block->size(), 200);
+
+    const auto expect_part{[&](size_t offset, size_t size) {
+        auto res{blockman.ReadRawBlock(tip_block_pos, std::pair{offset, size})};
+        BOOST_CHECK(res);
+        const auto& part{res.value()};
+        BOOST_CHECK_EQUAL_COLLECTIONS(part.begin(), part.end(), block->begin() + offset, block->begin() + offset + size);
+    }};
+
+    expect_part(0, 20);
+    expect_part(0, block->size() - 1);
+    expect_part(0, block->size() - 10);
+    expect_part(0, block->size());
+    expect_part(1, block->size() - 1);
+    expect_part(10, 20);
+    expect_part(block->size() - 1, 1);
+}
+
+BOOST_FIXTURE_TEST_CASE(blockmanager_block_data_part_error, TestChain100Setup)
+{
+    LOCK(::cs_main);
+    auto& chainman{m_node.chainman};
+    auto& blockman{chainman->m_blockman};
+    const CBlockIndex& tip{*chainman->ActiveTip()};
+    const FlatFilePos tip_block_pos{tip.GetBlockPos()};
+
+    auto block{blockman.ReadRawBlock(tip_block_pos)};
+    BOOST_REQUIRE(block);
+    BOOST_REQUIRE_GE(block->size(), 200);
+
+    const auto expect_part_error{[&](size_t offset, size_t size) {
+        auto res{blockman.ReadRawBlock(tip_block_pos, std::pair{offset, size})};
+        BOOST_CHECK(!res);
+        BOOST_CHECK_EQUAL(res.error(), node::ReadRawError::BadPartRange);
+    }};
+
+    expect_part_error(0, 0);
+    expect_part_error(0, block->size() + 1);
+    expect_part_error(0, std::numeric_limits<size_t>::max());
+    expect_part_error(1, block->size());
+    expect_part_error(2, block->size() - 1);
+    expect_part_error(block->size() - 1, 2);
+    expect_part_error(block->size() - 2, 3);
+    expect_part_error(block->size() + 1, 0);
+    expect_part_error(block->size() + 1, 1);
+    expect_part_error(block->size() + 2, 2);
+    expect_part_error(block->size(), 0);
+    expect_part_error(block->size(), 1);
+    expect_part_error(std::numeric_limits<size_t>::max(), 1);
+    expect_part_error(std::numeric_limits<size_t>::max(), std::numeric_limits<size_t>::max());
+}
+
 BOOST_FIXTURE_TEST_CASE(blockmanager_readblock_hash_mismatch, TestingSetup)
 {
     CBlockIndex index;