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/public/plugin/plugintest/example_reattach_test.go

@@ -0,0 +1,85 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package plugintest_test
+
+import (
+	"context"
+	"os"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+
+	goPlugin "github.com/hashicorp/go-plugin"
+	"github.com/mattermost/mattermost/server/public/model"
+	"github.com/mattermost/mattermost/server/public/plugin"
+)
+
+type UnitTestedPlugin struct {
+	plugin.MattermostPlugin
+}
+
+// This example demonstrates a plugin that's launched during a unit test and reattached to an
+// existing server instance to obtain a real PluginAPI.
+func Example_unitTestingPlugins() {
+	t := &testing.T{}
+
+	// The manifest is usually generated dynamically.
+	manifest := &model.Manifest{
+		Id: "reattach-plugin-test",
+	}
+
+	// ctx, and specifically cancel, gives us control over the plugin lifecycle
+	ctx, cancel := context.WithCancel(context.Background())
+
+	// reattachConfigCh is the means by which we get the Unix socket information to relay back
+	// to the server and finish the reattachment.
+	reattachConfigCh := make(chan *goPlugin.ReattachConfig)
+
+	// closeCh tells us when the plugin exits and allows for cleanup.
+	closeCh := make(chan struct{})
+
+	// plugin.ClientMain with options allows for reattachment.
+	go plugin.ClientMain(
+		&UnitTestedPlugin{},
+		plugin.WithTestContext(ctx),
+		plugin.WithTestReattachConfigCh(reattachConfigCh),
+		plugin.WithTestCloseCh(closeCh),
+	)
+
+	// Make sure the plugin shuts down normally with the test
+	t.Cleanup(func() {
+		cancel()
+
+		select {
+		case <-closeCh:
+		case <-time.After(5 * time.Second):
+			panic("plugin failed to close after 5 seconds")
+		}
+	})
+
+	// Wait for the plugin to start and then reattach to the server.
+	var reattachConfig *goPlugin.ReattachConfig
+	select {
+	case reattachConfig = <-reattachConfigCh:
+	case <-time.After(5 * time.Second):
+		t.Fatal("failed to get reattach config")
+	}
+
+	// Reattaching requires a local mode client.
+	socketPath := os.Getenv("MM_LOCALSOCKETPATH")
+	if socketPath == "" {
+		socketPath = model.LocalModeSocketPath
+	}
+
+	clientLocal := model.NewAPIv4SocketClient(socketPath)
+	_, err := clientLocal.ReattachPlugin(ctx, &model.PluginReattachRequest{
+		Manifest:             manifest,
+		PluginReattachConfig: model.NewPluginReattachConfig(reattachConfig),
+	})
+	require.NoError(t, err)
+
+	// At this point, the plugin is ready for unit testing and will be cleaned up automatically
+	// with the testing.T instance.
+}