Merge bitcoin/bitcoin#26355: p2p: Handle IsContinuationOfLowWorkHeadersSync return value correctly when new headers sync is started

7ad15d1100 [net processing] Handle IsContinuationOfLowWorkHeadersSync return value correctly when new headers sync is started (dergoegge)

Pull request description:

  This PR fixes a bug in the headers sync logic that enables submitting headers to a nodes block index that don't lead to a chain that surpasses our DoS limit.

  The issue is that we ignore the return value on [the first `IsContinuationOfLowWorkHeadersSync` call after a new headers sync is started](https://github.com/bitcoin/bitcoin/blob/fabc0310480b49e159a15d494525c5aa15072cba/src/net_processing.cpp#L2553-L2568), which leads to us passing headers to [`ProcessNewBlockHeaders`](https://github.com/bitcoin/bitcoin/blob/fabc0310480b49e159a15d494525c5aa15072cba/src/net_processing.cpp#L2856) when that initial `IsContinuationOfLowWorkHeadersSync` call returns `false`. One easy way (maybe the only?) to trigger this is by sending 2000 headers where the last header has a different `nBits` value than the prior headers (which fails the pre-sync logic [here](https://github.com/bitcoin/bitcoin/blob/fabc0310480b49e159a15d494525c5aa15072cba/src/headerssync.cpp#L189)). Those 2000 headers will be passed to `ProcessNewBlockHeaders`.

  I haven't included a test here so far because we can't test this without changing the default value for `CRegTestParams::consensus.fPowAllowMinDifficultyBlocks` or doing some more involved refactoring.

ACKs for top commit:
  sipa:
    ACK 7ad15d1100
  glozow:
    ACK 7ad15d1100

Tree-SHA512: 9aabb8bf3700401e79863d0accda0befd2a83c4d469a53f97d827e51139e2f826aee08cdfbc8866b311b153f61fdac9b7aa515fcfa2a21c5e2812c2bf3c03664

src/net_processing.cpp

@@ -2565,14 +2565,22 @@ bool PeerManagerImpl::TryLowWorkHeadersSync(Peer& peer, CNode& pfrom, const CBlo
 
             // Now a HeadersSyncState object for tracking this synchronization is created,
             // process the headers using it as normal.
-            return IsContinuationOfLowWorkHeadersSync(peer, pfrom, headers);
+            if (!IsContinuationOfLowWorkHeadersSync(peer, pfrom, headers)) {
+                // Something went wrong, reset the headers sync.
+                peer.m_headers_sync.reset(nullptr);
+                LOCK(m_headers_presync_mutex);
+                m_headers_presync_stats.erase(peer.m_id);
+            }
         } else {
             LogPrint(BCLog::NET, "Ignoring low-work chain (height=%u) from peer=%d\n", chain_start_header->nHeight + headers.size(), pfrom.GetId());
-            // Since this is a low-work headers chain, no further processing is required.
-            headers = {};
-            return true;
         }
+
+        // The peer has not yet given us a chain that meets our work threshold,
+        // so we want to prevent further processing of the headers in any case.
+        headers = {};
+        return true;
     }
+
     return false;
 }