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/dest/close_after_entrust_reply.rb

@@ -0,0 +1,36 @@
+#!/usr/bin/env ruby
+# encoding: utf-8
+
+# Receive a mirror, and disconnect after sending the entrust reply but
+# before it can send the disconnect signal.
+#
+# This test is currently unused: the sender can't detect that the
+# write failed.
+
+require 'flexnbd/fake_dest'
+include FlexNBD
+
+addr, port, src_pid = *ARGV
+server = FakeDest.new( addr, port )
+client = server.accept
+
+client.write_hello
+while (req = client.read_request; req[:type] == 1)
+  client.read_data( req[:len] )
+  client.write_reply( req[:handle] )
+end
+
+system "kill -STOP #{src_pid}"
+client.write_reply( req[:handle] )
+client.close
+system "kill -CONT #{src_pid}"
+
+sleep( 0.25 )
+client2 = server.accept( "Timed out waiting for a reconnection" )
+
+client2.close
+server.close
+
+$stderr.puts "done"
+exit(0)
+