[net processing] Only accept sendcmpct with version=2

Subsequent commits will remove support for other versions of compact blocks.

Add a test that a received `sendcmpct` message with version = 1 is
ignored.

src/net_processing.cpp

@@ -2883,7 +2883,10 @@ void PeerManagerImpl::ProcessMessage(CNode& pfrom, const std::string& msg_type,
         bool fAnnounceUsingCMPCTBLOCK = false;
         uint64_t nCMPCTBLOCKVersion = 0;
         vRecv >> fAnnounceUsingCMPCTBLOCK >> nCMPCTBLOCKVersion;
-        if (nCMPCTBLOCKVersion == 1 || nCMPCTBLOCKVersion == 2) {
+
+        // Only support compact block relay with witnesses
+        if (nCMPCTBLOCKVersion != CMPCTBLOCKS_VERSION) return;
+
         LOCK(cs_main);
         // fProvidesHeaderAndIDs is used to "lock in" version of compact blocks we send (fWantsCmpctWitness)
         if (!State(pfrom.GetId())->fProvidesHeaderAndIDs) {
@@ -2899,7 +2902,6 @@ void PeerManagerImpl::ProcessMessage(CNode& pfrom, const std::string& msg_type,
         if (!State(pfrom.GetId())->fSupportsDesiredCmpctVersion) {
             State(pfrom.GetId())->fSupportsDesiredCmpctVersion = (nCMPCTBLOCKVersion == 2);
         }
-        }
         return;
     }
 

test/functional/p2p_compactblocks.py

@@ -179,6 +179,7 @@ class CompactBlocksTest(BitcoinTestFramework):
 
     # Test "sendcmpct" (between peers preferring the same version):
     # - No compact block announcements unless sendcmpct is sent.
+    # - If sendcmpct is sent with version = 1, the message is ignored.
     # - If sendcmpct is sent with version > 2, the message is ignored.
     # - If sendcmpct is sent with boolean 0, then block announcements are not
     #   made with compact blocks.
@@ -221,6 +222,13 @@ class CompactBlocksTest(BitcoinTestFramework):
         # Before each test, sync the headers chain.
         test_node.request_headers_and_sync(locator=[tip])
 
+        # Now try a SENDCMPCT message with too-low version
+        test_node.send_and_ping(msg_sendcmpct(announce=True, version=1))
+        check_announcement_of_new_block(node, test_node, lambda p: "cmpctblock" not in p.last_message)
+
+        # Headers sync before next test.
+        test_node.request_headers_and_sync(locator=[tip])
+
         # Now try a SENDCMPCT message with too-high version
         test_node.send_and_ping(msg_sendcmpct(announce=True, version=3))
         check_announcement_of_new_block(node, test_node, lambda p: "cmpctblock" not in p.last_message)