fix: the traffic-split plugin is invalid to bind upstream via upstream_id

[tzssangglass]

What this PR does / why we need it:

fix: #3740

Pre-submission checklist:

the cause of the bug reference: #3740 (comment)
this change temporarily stores the route's upstream_id in the matched_route table. If the current request match fails and the temporarily stored upstream_id exists in the matched_route table, the route's upstream_id is restored to the temporarily stored upstream_id.
when the route is configured with an upstream_id, the upstream_id is passed into the server_list.

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[spacewander]

After apply this change:

diff --git apisix/plugins/traffic-split.lua apisix/plugins/traffic-split.lua
index eb7a2028..3d5e7d8f 100644
--- apisix/plugins/traffic-split.lua
+++ apisix/plugins/traffic-split.lua
@@ -315,8 +315,7 @@ function _M.access(conf, ctx)
     end

     local route_upstream_id = ctx.matched_route.value.upstream_id
-    local rr_up, err = lrucache(weighted_upstreams, nil, new_rr_obj, weighted_upstreams,
-                                route_upstream_id)
+    local rr_up, err = new_rr_obj(weighted_upstreams, route_upstream_id)
     if not rr_up then
         core.log.error("lrucache roundrobin failed: ", err)
         return 500

many test cases are failed. Look like new_rr_obj corrupts the route's configuration. This may happen once the cache is evicted.

[tzssangglass]

Look like new_rr_obj corrupts the route's configuration. This may happen once the cache is evicted.

yes, according to my understanding, I think the design idea of traffic-split itself is to achieve the goal by corrupts the route's configuration.

take a look at

apisix/apisix/plugins/traffic-split.lua

Lines 322 to 323 in 6cbaf55

	elseif upstream and upstream ~= "plugin#upstream#is#empty" then
	ctx.matched_route.value.upstream_id = upstream

[spacewander]

There should be a way to do the same thing without corrupting the data. Otherwise once the cache is evicted we will be in trouble.

[tzssangglass]

There should be a way to do the same thing without corrupting the data. Otherwise once the cache is evicted we will be in trouble.

This PR does not do this work. If needed, we should open another issue to discuss it, as it would involve refactoring the plugin's implementation logic.