Add a test for a second client connecting during a mirror

tests/fakes/source/connect_during_hello.rb

@@ -0,0 +1,36 @@
+#!/usr/bin/env ruby
+
+# Connect to the destination, then hang.  Connect a second time to the
+# destinatioin.  This will trigger the destination's thread clearer.
+
+require 'socket'
+require 'timeout'
+
+addr, port = *ARGV
+
+# client_sock1 is a connection the destination is expecting.
+client_sock1 = nil
+begin
+  Timeout.timeout(2) do
+    client_sock1 = TCPSocket.open( addr, port )
+  end
+rescue Timeout::Error
+  $stderr.puts "Timed out connecting."
+  exit 1
+end
+
+#client_sock2 is the interloper.
+client_sock2 = nil
+begin
+  Timeout.timeout(2) do
+    client_sock2 = TCPSocket.open( addr, port )
+  end
+rescue Timeout::Error
+  $stderr.puts "Timed out connecting a second time."
+  exit 1
+end
+
+# This is the expected source crashing after connect
+client_sock1.close
+# And this is just a tidy-up.
+client_sock2.close