add regression test which reproduces problem

Author: wahern

regress/157-interposed-methods-lost.lua

@@ -0,0 +1,34 @@
+#!/bin/sh
+_=[[
+	. "${0%%/*}/regress.sh"
+	exec runlua "$0" "$@"
+]]
+-- load dns.config first so that subsequent loads of DNS modules overwrite
+-- the config metatable
+local config = require"cqueues.dns.config"
+local resolver = require"cqueues.dns.resolver" -- ensure config mt reloaded
+require"regress".export".*"
+
+-- config.new would fail because the :set method it depended upon is an
+-- interposed Lua function added to the userdata metatable. When subsequent
+-- modules which depended on the metatable definition were loaded, they
+-- called dnsL_loadall. dnsL_loadall defined all the internal metatables,
+-- which since commit 4d66661 had the effect of replacing any pre-existing
+-- __index field with a new table containing only the original C-defined
+-- methods. Previously cqs_newmetatable short-circuited when a metatable
+-- existed. The purpose of 4d66661 was to permit forced reloading of all Lua
+-- modules by clearing package.loaded; the original behavior resulted in
+-- modules interposing the same functions multiple times.
+info"creating resolver config object"
+local cfg = config.new{
+	nameserver = { "1.2.3.4" },
+	lookup = { "file", "bind", "cache" },
+	options = { edns0 = true },
+}
+
+info("resolv.conf:")
+for ln in tostring(cfg):gmatch("[^\n]+") do
+	info("  %s", ln)
+end
+
+say"OK"