Handle a failed disconnect correctly

If the sender disconnects its socket before sending the disconnect
message, the destination should restart the migration process.  This
patch makes sure that happens.

tests/acceptance/fakes/source/close_after_entrust_reply.rb

@@ -0,0 +1,32 @@
+#!/usr/bin/env ruby
+
+# Connect, send a migration, entrust then *immediately* disconnect.
+# This simulates a client which fails while the client is blocked.
+#
+# We attempt to reconnect immediately afterwards to prove that we can
+# retry the mirroring.
+
+require 'flexnbd/fake_source'
+include FlexNBD
+
+addr, port, srv_pid = *ARGV
+
+client = FakeSource.new( addr, port, "Timed out connecting" )
+client.read_hello
+client.write_write_request( 0, 8 )
+client.write_data( "12345678" )
+
+client.write_entrust_request
+client.read_response
+client.close
+
+sleep(0.25)
+
+client2 = FakeSource.new( addr, port, "Timed out reconnecting to mirror" )
+client2.send_mirror
+
+sleep(0.25)
+client3 = FakeSource.new( addr, port, "Timed out reconnecting to read" )
+client3.close
+
+exit(0)