MM-57018: support reattaching plugins (#26421)

* ProfileImageBytes for EnsureBotOptions

* leverage plugintest.NewAPI

* fix linting

* add UpdateUserRoles to plugin api

* MM-57018: support reattaching plugins

Expose a local-only API for reattaching plugins: instead of the server starting and managing the process itself, allow the plugin to be launched externally (eg within a unit test) and reattach to an existing server instance to provide the unit test with a fully functional RPC API, sidestepping the need for mocking the plugin API in most cases.

In the future, this may become the basis for running plugins in a sidecar container.

Fixes: https://mattermost.atlassian.net/browse/MM-57018

* drop unused supervisor.pid

* factor out checkMinServerVersion

* factor out startPluginServer

* restore missing setPluginState on successful reattach

* avoid passing around a stale registeredPlugin

* inline initializePluginImplementation

* have IsValid return an error

* explicitly close rpcClient

In the case of reattached plugins, the Unix socket won't necessarily disappear leaving the muxBrokers blocked indefinitely. And `Kill()` doesn't do anything if there's no process being managed.

* explicitly detachPlugin

* emphasize gRPC not being supported

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/api4/plugin_local.go

@@ -3,6 +3,13 @@
 
 package api4
 
+import (
+	"encoding/json"
+	"net/http"
+
+	"github.com/mattermost/mattermost/server/public/model"
+)
+
 func (api *API) InitPluginLocal() {
 	api.BaseRoutes.Plugins.Handle("", api.APILocal(uploadPlugin, handlerParamFileAPI)).Methods("POST")
 	api.BaseRoutes.Plugins.Handle("", api.APILocal(getPlugins)).Methods("GET")
@@ -12,4 +19,44 @@ func (api *API) InitPluginLocal() {
 	api.BaseRoutes.Plugin.Handle("/disable", api.APILocal(disablePlugin)).Methods("POST")
 	api.BaseRoutes.Plugins.Handle("/marketplace", api.APILocal(installMarketplacePlugin)).Methods("POST")
 	api.BaseRoutes.Plugins.Handle("/marketplace", api.APILocal(getMarketplacePlugins)).Methods("GET")
+	api.BaseRoutes.Plugins.Handle("/reattach", api.APILocal(reattachPlugin)).Methods("POST")
+	api.BaseRoutes.Plugin.Handle("/detach", api.APILocal(detachPlugin)).Methods("POST")
+}
+
+// reattachPlugin allows the server to bind to an existing plugin instance launched elsewhere.
+//
+// This API is only exposed over a local socket.
+func reattachPlugin(c *Context, w http.ResponseWriter, r *http.Request) {
+	var pluginReattachRequest model.PluginReattachRequest
+	if err := json.NewDecoder(r.Body).Decode(&pluginReattachRequest); err != nil {
+		c.Err = model.NewAppError("reattachPlugin", "api4.plugin.reattachPlugin.invalid_request", nil, err.Error(), http.StatusBadRequest)
+		return
+	}
+
+	if err := pluginReattachRequest.IsValid(); err != nil {
+		c.Err = err
+		return
+	}
+
+	err := c.App.ReattachPlugin(pluginReattachRequest.Manifest, pluginReattachRequest.PluginReattachConfig)
+	if err != nil {
+		c.Err = err
+		return
+	}
+}
+
+// detachPlugin detaches a previously reattached plugin.
+//
+// This API is only exposed over a local socket.
+func detachPlugin(c *Context, w http.ResponseWriter, r *http.Request) {
+	c.RequirePluginId()
+	if c.Err != nil {
+		return
+	}
+
+	err := c.App.DetachPlugin(c.Params.PluginId)
+	if err != nil {
+		c.Err = err
+		return
+	}
 }