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

test/functional/feature_pruning.py

@@ -25,6 +25,7 @@ from test_framework.util import (
     assert_equal,
     assert_greater_than,
     assert_raises_rpc_error,
+    try_rpc,
 )
 
 # Rescans start at the earliest block up to 2 hours before a key timestamp, so
@@ -479,8 +480,12 @@ class PruneTest(BitcoinTestFramework):
         self.log.info("Test invalid pruning command line options")
         self.test_invalid_command_line_options()
 
+        self.log.info("Test scanblocks can not return pruned data")
         self.test_scanblocks_pruned()
 
+        self.log.info("Test pruneheight reflects the presence of block and undo data")
+        self.test_pruneheight_undo_presence()
+
         self.log.info("Done")
 
     def test_scanblocks_pruned(self):
@@ -494,5 +499,18 @@ class PruneTest(BitcoinTestFramework):
         assert_raises_rpc_error(-1, "Block not available (pruned data)", node.scanblocks,
             "start", [{"desc": f"raw({false_positive_spk.hex()})"}], 0, 0, "basic", {"filter_false_positives": True})
 
+    def test_pruneheight_undo_presence(self):
+        node = self.nodes[2]
+        pruneheight = node.getblockchaininfo()["pruneheight"]
+        fetch_block = node.getblockhash(pruneheight - 1)
+
+        self.connect_nodes(1, 2)
+        peers = node.getpeerinfo()
+        node.getblockfrompeer(fetch_block, peers[0]["id"])
+        self.wait_until(lambda: not try_rpc(-1, "Block not available (pruned data)", node.getblock, fetch_block), timeout=5)
+
+        new_pruneheight = node.getblockchaininfo()["pruneheight"]
+        assert_equal(pruneheight, new_pruneheight)
+
 if __name__ == '__main__':
     PruneTest().main()