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/app/plugin_reattach.go

@@ -0,0 +1,50 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+package app
+
+import (
+	"net/http"
+
+	"github.com/mattermost/mattermost/server/public/model"
+)
+
+// ReattachPlugin allows the server to bind to an existing plugin instance launched elsewhere.
+func (a *App) ReattachPlugin(manifest *model.Manifest, pluginReattachConfig *model.PluginReattachConfig) *model.AppError {
+	return a.ch.ReattachPlugin(manifest, pluginReattachConfig)
+}
+
+// ReattachPlugin allows the server to bind to an existing plugin instance launched elsewhere.
+func (ch *Channels) ReattachPlugin(manifest *model.Manifest, pluginReattachConfig *model.PluginReattachConfig) *model.AppError {
+	pluginsEnvironment := ch.GetPluginsEnvironment()
+	if pluginsEnvironment == nil {
+		return model.NewAppError("ReattachPlugin", "app.plugin.disabled.app_error", nil, "", http.StatusNotImplemented)
+	}
+
+	ch.DetachPlugin(manifest.Id)
+
+	// Reattach to the plugin
+	if err := pluginsEnvironment.Reattach(manifest, pluginReattachConfig); err != nil {
+		return model.NewAppError("ReattachPlugin", "app.plugin.reattach.app_error", nil, "", http.StatusInternalServerError).Wrap(err)
+	}
+
+	return nil
+}
+
+// DetachPlugin allows the server to bind to an existing plugin instance launched elsewhere.
+func (a *App) DetachPlugin(pluginId string) *model.AppError {
+	return a.ch.DetachPlugin(pluginId)
+}
+
+// DetachPlugin allows the server to bind to an existing plugin instance launched elsewhere.
+func (ch *Channels) DetachPlugin(pluginID string) *model.AppError {
+	pluginsEnvironment := ch.GetPluginsEnvironment()
+	if pluginsEnvironment == nil {
+		return model.NewAppError("DetachPlugin", "app.plugin.disabled.app_error", nil, "", http.StatusNotImplemented)
+	}
+
+	// Deactivate and remove any existing plugin, if present.
+	pluginsEnvironment.Deactivate(pluginID)
+	pluginsEnvironment.RemovePlugin(pluginID)
+
+	return nil
+}