Re: [I] bug: ai-proxy-multi health checker creation always fails in timer context due to missing _dns_value [apisix]

["Baoyuantop (via GitHub)" <gi...@apache.org>]

Baoyuantop commented on issue #13101:
URL: https://github.com/apache/apisix/issues/13101#issuecomment-4241444801

   ### Reproduction Verified on APISIX 3.16.0
   
   Bug confirmed on **APISIX 3.16.0-debian** (latest release).
   
   #### Environment
   - Image: `apache/apisix:3.16.0-debian`
   - etcd: `bitnamilegacy/etcd:3.5`
   - Docker Compose with custom backends
   
   #### Reproduction Steps
   1. Create a route with `ai-proxy-multi` + `checks` (active health check enabled)
   2. Send a request to trigger health checker creation → checkers created successfully
   3. Update the route via Admin API (`PATCH` with any change like adding `desc`)
   4. Observe error logs: `timer_working_pool_check` crashes every second
   
   #### Error Output
   ```
   [error] healthcheck_manager.lua:288: failed to run timer_working_pool_check:
     /usr/local/apisix/apisix/healthcheck_manager.lua:238:
     attempt to index local 'upstream' (a nil value), context: ngx.timer
   ```
   
   The crash repeats every second indefinitely (5 crashes in 5 seconds observed).
   
   #### Root Cause
   When the route config is updated via Admin API, etcd watch triggers a config cache refresh with a new in-memory Lua table object. This new object does NOT have the `_dns_value` runtime field (which was set by `resolve_endpoint` only during request processing on the previous object). `construct_upstream` returns `nil`, and the subsequent `upstream.resource_key` indexing crashes.
   
   #### Key Observation
   The initial health checker creation succeeds because the request thread sets `_dns_value` on the shared config cache object before the timer fires. The bug only manifests after a config update refreshes the cache with a clean object.


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