Merge bitcoin/bitcoin#29668: prune, rpc: Check undo data when finding pruneheight

8789dc8f31 doc: Add note to getblockfrompeer on missing undo data (Fabian Jahr)
4a1975008b rpc: Make pruneheight also reflect undo data presence (Fabian Jahr)
96b4facc91 refactor, blockstorage: Generalize GetFirstStoredBlock (Fabian Jahr)

Pull request description:

  The function `GetFirstStoredBlock()` helps us find the first block for which we have data. So far this function only looked for a block with `BLOCK_HAVE_DATA`. However, this doesn't mean that we also have the undo data of that block, and undo data might be required for what a user would like to do with those blocks. One example of how this might happen is if some blocks were fetched using the `getblockfrompeer` RPC. Blocks fetched from a peer will have data but no undo data.

  The first commit here allows `GetFirstStoredBlock()` to check for undo data as well by passing a parameter. This alone is useful for #29553 and I would use it there.

  In the second commit I am applying the undo check to the RPCs that report `pruneheight` to the user. I find this much more intuitive because I think the user expects to be able to do all operations on blocks up until the `pruneheight` but that is not the case if undo data is missing. I personally ran into this once before and now again when testing for assumeutxo when I had used `getblockfrompeer`. The following commit adds test coverage for this change of behavior.

  The last commit adds a note in the docs of `getblockfrompeer` that undo data will not be available.

ACKs for top commit:
  achow101:
    ACK 8789dc8f31
  furszy:
    Code review ACK 8789dc8f31.
  stickies-v:
    ACK 8789dc8f31

Tree-SHA512: 90ae8bdd07a496ade579aa25240609c61c9ed173ad38d30533f6c631fe674e5a41727478ade69ca4b71a571ad94c9da4b33ebba6b5d8821109313c2de3bdfb3d

src/test/blockchain_tests.cpp

@@ -5,7 +5,9 @@
 #include <boost/test/unit_test.hpp>
 
 #include <chain.h>
+#include <node/blockstorage.h>
 #include <rpc/blockchain.h>
+#include <sync.h>
 #include <test/util/setup_common.h>
 #include <util/string.h>
 
@@ -76,4 +78,36 @@ BOOST_AUTO_TEST_CASE(get_difficulty_for_very_high_target)
     TestDifficulty(0x12345678, 5913134931067755359633408.0);
 }
 
+//! Prune chain from height down to genesis block and check that
+//! GetPruneHeight returns the correct value
+static void CheckGetPruneHeight(node::BlockManager& blockman, CChain& chain, int height) EXCLUSIVE_LOCKS_REQUIRED(::cs_main)
+{
+    AssertLockHeld(::cs_main);
+
+    // Emulate pruning all blocks from `height` down to the genesis block
+    // by unsetting the `BLOCK_HAVE_DATA` flag from `nStatus`
+    for (CBlockIndex* it{chain[height]}; it != nullptr && it->nHeight > 0; it = it->pprev) {
+        it->nStatus &= ~BLOCK_HAVE_DATA;
+    }
+
+    const auto prune_height{GetPruneHeight(blockman, chain)};
+    BOOST_REQUIRE(prune_height.has_value());
+    BOOST_CHECK_EQUAL(*prune_height, height);
+}
+
+BOOST_FIXTURE_TEST_CASE(get_prune_height, TestChain100Setup)
+{
+    LOCK(::cs_main);
+    auto& chain = m_node.chainman->ActiveChain();
+    auto& blockman = m_node.chainman->m_blockman;
+
+    // Fresh chain of 100 blocks without any pruned blocks, so std::nullopt should be returned
+    BOOST_CHECK(!GetPruneHeight(blockman, chain).has_value());
+
+    // Start pruning
+    CheckGetPruneHeight(blockman, chain, 1);
+    CheckGetPruneHeight(blockman, chain, 99);
+    CheckGetPruneHeight(blockman, chain, 100);
+}
+
 BOOST_AUTO_TEST_SUITE_END()