Improve error messaging on process connect errors

[jasonmolenda]

Summary

process connect (aka gdb-remote) isn't the clearest in its error messaging. We have everything we need to distinguish between three common failure modes: Nothing is listening to the port (or we could not reach the port), the remote side did not respond to the initial handshake packet quickly enough, or the remote connection accepts the connection and then drops it immediately (e.g. a debugserver might reject the connection because it is only accepting connections from certain hosts).

With this patch, we get

(lldb) gdb-remote 7000
error: Failed to connect port

(lldb) gdb-remote 4000
error: failed to get reply to handshake packet within timeout of 6.0 seconds

(lldb) gdb-remote work-imacpro:4000
error: Connection shut down by remote side while waiting for reply to initial handshake packet

and I think it's a little easier to understand the nature of the problem. Just fixing a little thing that's bugged me for a while.

lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunicationClient.cpp

if (IsConnected())
     Disconnect();
 }
 
 bool GDBRemoteCommunicationClient::HandshakeWithServer(Status *error_ptr) {
   ResetDiscoverableSettings(false);
 
   // Start the read thread after we send the handshake ack since if we fail to
   // send the handshake ack, there is no reason to continue...
+  std::chrono::steady_clock::time_point start_of_handshake =
+      std::chrono::steady_clock::now();
   if (SendAck()) {
     // Wait for any responses that might have been queued up in the remote
     // GDB server and flush them all
     StringExtractorGDBRemote response;
     PacketResult packet_result = PacketResult::Success;
     while (packet_result == PacketResult::Success)
       packet_result = ReadPacket(response, milliseconds(10), false);
 
     // The return value from QueryNoAckModeSupported() is true if the packet
     // was sent and _any_ response (including UNIMPLEMENTED) was received), or
     // false if no response was received. This quickly tells us if we have a
     // live connection to a remote GDB server...
     if (QueryNoAckModeSupported()) {
       return true;
     } else {
-      if (error_ptr)
+      std::chrono::steady_clock::time_point end_of_handshake =
+          std::chrono::steady_clock::now();
+      auto handshake_timeout =
+          std::chrono::duration<double>(end_of_handshake - start_of_handshake)
+              .count();
+      if (error_ptr) {
+        if (packet_result == PacketResult::ErrorDisconnected)
+          error_ptr->SetErrorString("Connection shut down by remote side "
+                                    "while waiting for reply to initial "
+                                    "handshake packet");
+        else if (packet_result == PacketResult::ErrorReplyTimeout)
+          error_ptr->SetErrorStringWithFormat(
+              "failed to get reply to handshake packet within timeout of "
+              "%.1f seconds",
+              handshake_timeout);
+        else
           error_ptr->SetErrorString("failed to get reply to handshake packet");
       }
+    }
   } else {
     if (error_ptr)
       error_ptr->SetErrorString("failed to send the handshake ack");
   }
   return false;
 }
 
 bool GDBRemoteCommunicationClient::GetEchoSupported() {