Remove protocol downgrade restriction (#161)

* Remove protocol downgrade restriction

* Conditionally downgrade

* Fix log message to clarify client protocol version downgrade

* Restrict client protocol version downgrade to minor versions only

* Remove redundant assignment of helloBackMajor in protocol downgrade handling

Author: nbolton

src/lib/client/Client.cpp

@@ -651,26 +651,6 @@ void Client::handleClipboardGrabbed(const Event &event, void *)
   }
 }
 
-bool Client::isCompatible(int major, int minor) const
-{
-  const std::map<int, std::set<int>> compatibleTable{
-      {6, {7, 8}}, // 1.6 is compatible with 1.7 and 1.8
-      {7, {8}}     // 1.7 is compatible with 1.8
-  };
-
-  bool isCompatible = false;
-
-  if (major == kProtocolMajorVersion) {
-    auto versions = compatibleTable.find(minor);
-    if (versions != compatibleTable.end()) {
-      auto compatibleVersions = versions->second;
-      isCompatible = compatibleVersions.find(kProtocolMinorVersion) != compatibleVersions.end();
-    }
-  }
-
-  return isCompatible;
-}
-
 void Client::handleHello(const Event &, void *)
 {
   SInt16 major, minor;
@@ -682,24 +662,26 @@ void Client::handleHello(const Event &, void *)
   }
 
   // check versions
-  LOG((CLOG_DEBUG1 "got hello version %d.%d", major, minor));
+  LOG((CLOG_DEBUG "got hello version %d.%d", major, minor));
   SInt16 helloBackMajor = kProtocolMajorVersion;
   SInt16 helloBackMinor = kProtocolMinorVersion;
 
-  if (isCompatible(major, minor)) {
-    // because 1.6 is comptable with 1.7 and 1.8 - downgrading protocol for
-    // server
-    LOG((CLOG_NOTE "downgrading protocol version for server"));
+  // only allow client minor version to downgrade, as major versions will likely not be compatible.
+  if (major == kProtocolMajorVersion && minor < kProtocolMinorVersion) {
     helloBackMinor = minor;
-  } else if (major < kProtocolMajorVersion || (major == kProtocolMajorVersion && minor < kProtocolMinorVersion)) {
+    LOG_NOTE(
+        "downgrading client protocol version from %d.%d to %d.%d", //
+        kProtocolMajorVersion, kProtocolMinorVersion, helloBackMajor, helloBackMinor
+    );
+  } else if (major != kProtocolMajorVersion) {
     sendConnectionFailedEvent(XIncompatibleClient(major, minor).what());
     cleanupTimer();
     cleanupConnection();
     return;
   }
 
   // say hello back
-  LOG((CLOG_DEBUG1 "say hello version %d.%d", helloBackMajor, helloBackMinor));
+  LOG((CLOG_DEBUG "say hello version %d.%d", helloBackMajor, helloBackMinor));
   ProtocolUtil::writef(m_stream, kMsgHelloBack, helloBackMajor, helloBackMinor, &m_name);
 
   // now connected but waiting to complete handshake

src/lib/client/Client.h

@@ -217,7 +217,6 @@ class Client : public IClient, public INode
   void handleDisconnected(const Event &, void *);
   void handleShapeChanged(const Event &, void *);
   void handleClipboardGrabbed(const Event &, void *);
-  bool isCompatible(int major, int minor) const;
   void handleHello(const Event &, void *);
   void handleSuspend(const Event &event, void *);
   void handleResume(const Event &event, void *);