Make more robust in face of random other stdout

Fuzzlyn.ExecutionServer/Program.cs

@@ -64,6 +64,9 @@ public static void Main(string[] args)
  return;
  }
 
+ // This identifier must be in sync with
+ // RunningExecutionServer.RequestAndReceive.
+ Console.Write("@!EXEC_SERVER_RESPONSE!@");
  Console.WriteLine(JsonSerializer.Serialize(resp));
  if (req.Kind == RequestKind.Shutdown)
  return;

Fuzzlyn/RunningExecutionServer.cs

@@ -27,7 +27,31 @@ private ReceiveResult RequestAndReceive(Request req, TimeSpan timeout)
  {
  using var cts = new CancellationTokenSource(timeout);
  using var reg = cts.Token.Register(() => { killed = true; Kill(); });
- line = _process.StandardOutput.ReadLine();
+ bool first = true;
+ while (true)
+ {
+ line = _process.StandardOutput.ReadLine();
+
+ if (line == null)
+ {
+ break;
+ }
+
+ if (line.StartsWith("@!EXEC_SERVER_RESPONSE!@"))
+ {
+ line = line["@!EXEC_SERVER_RESPONSE!@".Length..];
+ break;
+ }
+
+ if (first)
+ {
+ Console.WriteLine("Received unexpected output from execution server:");
+ }
+
+ Console.WriteLine(line);
+
+ first = false;
+ }
  }
 
  LastUseTimer.Restart();
@@ -43,20 +67,19 @@ private ReceiveResult RequestAndReceive(Request req, TimeSpan timeout)
  return new ReceiveResult { Ended = true, Stderr = stderr };
  }
 
- Response resp;
  try
  {
- resp = JsonSerializer.Deserialize<Response>(line);
+ Response resp = JsonSerializer.Deserialize<Response>(line);
+ return new ReceiveResult { Response = resp };
  }
  catch (JsonException ex)
  {
- Console.WriteLine("Received malformed JSON response");
+ Console.WriteLine("Could not parse JSON response");
  Console.WriteLine(ex.ToString());
  Console.WriteLine(line);
  return new ReceiveResult { Ended = true, Stderr = "Malformed result" };
  }
 
- return new ReceiveResult { Response = JsonSerializer.Deserialize<Response>(line) };
  }
 
  public RunSeparatelyResults RunPair(ProgramPair pair, TimeSpan timeout)