fix(pluginservers): restart all instances of a plugin

When the plugin -- which acts as a plugin server -- dies for some
reason, all entities associated with it must be invalidated, not only
the current.

If two instances A and B of an external plugin P exist (e.g., if applied
to different route objects), they receive a unique ID [1] defined by the
the plugin server when the instance is started. If A and B are started
sequentially and are the first instances, they get IDs 0 and 1, respectively.
This ID is cached on the Kong side by the plugin server manager [2]. If
plugin P dies, Kong plugin server manager will detect this condition and
restart it.

As it was, the code would remove whichever instance first
detected the condition and leave the other in `running_instances`; once
the plugin server was up, the removed instance would be restarted, in
some cases with the same Id already in use by the stale instance that
should have been removed from `running_instances`. Subsequent requests
may then run the wrong instance of the external plugin. This change
fixes the issue by ensuring that both instances A and B are removed
from the `running_instances` table,

[1] https://github.com/Kong/go-pdk/blob/4deec45a5bfe421a0ceebcbf7931d395250949b0/server/instance.go#L47
[2] `kong/runloop/plugin_servers/process.lua`

CHANGELOG.md

@@ -103,6 +103,11 @@
 - Make `kong vault get` CLI command work in dbless mode by injecting the necessary directives into the kong cli nginx.conf.
   [#11127](https://github.com/Kong/kong/pull/11127)
   [#11291](https://github.com/Kong/kong/pull/11291)
+- Fix an issue where a crashing Go plugin server process would cause subsequent
+  requests proxied through Kong to execute Go plugins with inconsistent configurations.
+  The issue only affects scenarios where the same Go plugin is applied to different Route
+  or Service entities.
+  [#11306](https://github.com/Kong/kong/pull/11306)
 
 #### Admin API
 

kong/runloop/plugin_servers/init.lua

@@ -270,7 +270,20 @@ end
 --- reset_instance: removes an instance from the table.
 function reset_instance(plugin_name, conf)
   local key = type(conf) == "table" and kong.plugin.get_id() or plugin_name
-  running_instances[key] = nil
+  local current_instance = running_instances[key]
+
+  --
+  -- the same plugin (which acts as a plugin server) is shared among
+  -- instances of the plugin; for example, the same plugin can be applied
+  -- to many routes
+  -- `reset_instance` is called when (but not only) the plugin server died;
+  -- in such case, all associated instances must be removed, not only the current
+  --
+  for k, instance in pairs(running_instances) do
+    if instance.rpc == current_instance.rpc then
+      running_instances[k] = nil
+    end
+  end
 end