Merge bitcoin/bitcoin#20295: rpc: getblockfrompeer

dce8c4c381 rpc: getblockfrompeer (Sjors Provoost)
b884ababc2 rpc: move Ensure* helpers to server_util.h (Sjors Provoost)

Pull request description:

  This adds an RPC method to fetch a block directly from a peer. This can used to fetch stale blocks with lower proof of work that are normally ignored by the node (`headers-only` in `getchaintips`).

  Usage:
  ```
  bitcoin-cli getblockfrompeer HASH peer_n
  ```

  Closes #20155

  Limitations:
  * you have to specify which peer to fetch the block from
  * the node must already have the header

ACKs for top commit:
  jnewbery:
    ACK dce8c4c381
  fjahr:
     re-ACK dce8c4c381

Tree-SHA512: 843ba2b7a308f640770d624d0aa3265fdc5c6ea48e8db32269b96a082b7420f7953d1d8d1ef2e6529392c7172dded9d15639fbc9c24e7bfa5cfb79e13a5498c8

test/functional/rpc_getblockfrompeer.py

@@ -0,0 +1,76 @@
+#!/usr/bin/env python3
+# Copyright (c) 2020 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+"""Test the getblockfrompeer RPC."""
+
+from test_framework.authproxy import JSONRPCException
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.util import (
+    assert_equal,
+    assert_raises_rpc_error,
+)
+
+class GetBlockFromPeerTest(BitcoinTestFramework):
+    def set_test_params(self):
+        self.num_nodes = 2
+
+    def setup_network(self):
+        self.setup_nodes()
+
+    def check_for_block(self, hash):
+        try:
+            self.nodes[0].getblock(hash)
+            return True
+        except JSONRPCException:
+            return False
+
+    def run_test(self):
+        self.log.info("Mine 4 blocks on Node 0")
+        self.generate(self.nodes[0], 4, sync_fun=self.no_op)
+        assert_equal(self.nodes[0].getblockcount(), 204)
+
+        self.log.info("Mine competing 3 blocks on Node 1")
+        self.generate(self.nodes[1], 3, sync_fun=self.no_op)
+        assert_equal(self.nodes[1].getblockcount(), 203)
+        short_tip = self.nodes[1].getbestblockhash()
+
+        self.log.info("Connect nodes to sync headers")
+        self.connect_nodes(0, 1)
+        self.sync_blocks()
+
+        self.log.info("Node 0 should only have the header for node 1's block 3")
+        for x in self.nodes[0].getchaintips():
+            if x['hash'] == short_tip:
+                assert_equal(x['status'], "headers-only")
+                break
+        else:
+            raise AssertionError("short tip not synced")
+        assert_raises_rpc_error(-1, "Block not found on disk", self.nodes[0].getblock, short_tip)
+
+        self.log.info("Fetch block from node 1")
+        peers = self.nodes[0].getpeerinfo()
+        assert_equal(len(peers), 1)
+        peer_0_peer_1_id = peers[0]["id"]
+
+        self.log.info("Arguments must be sensible")
+        assert_raises_rpc_error(-8, "hash must be of length 64 (not 4, for '1234')", self.nodes[0].getblockfrompeer, "1234", 0)
+
+        self.log.info("We must already have the header")
+        assert_raises_rpc_error(-1, "Block header missing", self.nodes[0].getblockfrompeer, "00" * 32, 0)
+
+        self.log.info("Non-existent peer generates error")
+        assert_raises_rpc_error(-1, f"Peer nodeid {peer_0_peer_1_id + 1} does not exist", self.nodes[0].getblockfrompeer, short_tip, peer_0_peer_1_id + 1)
+
+        self.log.info("Successful fetch")
+        result = self.nodes[0].getblockfrompeer(short_tip, peer_0_peer_1_id)
+        self.wait_until(lambda: self.check_for_block(short_tip), timeout=1)
+        assert(not "warnings" in result)
+
+        self.log.info("Don't fetch blocks we already have")
+        result = self.nodes[0].getblockfrompeer(short_tip, peer_0_peer_1_id)
+        assert("warnings" in result)
+        assert_equal(result["warnings"], "Block already downloaded")
+
+if __name__ == '__main__':
+    GetBlockFromPeerTest().main()

test/functional/test_runner.py

@@ -217,6 +217,7 @@ BASE_SCRIPTS = [
     'wallet_txn_clone.py --mineblock',
     'feature_notifications.py',
     'rpc_getblockfilter.py',
+    'rpc_getblockfrompeer.py',
     'rpc_invalidateblock.py',
     'feature_utxo_set_hash.py',
     'feature_rbf.py --legacy-wallet',