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/client.go

@@ -4,6 +4,8 @@
 package plugin
 
 import (
+	"context"
+
 	"github.com/hashicorp/go-plugin"
 )
 
@@ -12,10 +14,51 @@ const (
 	BotUserKey        = InternalKeyPrefix + "botid"
 )
 
-// Starts the serving of a Mattermost plugin over net/rpc. gRPC is not yet supported.
+// WithTestContext provides a context typically used to terminate a plugin from a unit test.
+func WithTestContext(ctx context.Context) func(*plugin.ServeConfig) error {
+	return func(config *plugin.ServeConfig) error {
+		if config.Test == nil {
+			config.Test = &plugin.ServeTestConfig{}
+		}
+
+		config.Test.Context = ctx
+
+		return nil
+	}
+}
+
+// WithTestReattachConfigCh configures the channel to receive the ReattachConfig used to reattach
+// an externally launched plugin instance with the Mattermost server.
+func WithTestReattachConfigCh(reattachConfigCh chan<- *plugin.ReattachConfig) func(*plugin.ServeConfig) error {
+	return func(config *plugin.ServeConfig) error {
+		if config.Test == nil {
+			config.Test = &plugin.ServeTestConfig{}
+		}
+
+		config.Test.ReattachConfigCh = reattachConfigCh
+
+		return nil
+	}
+}
+
+// WithTestCloseCh provides a channel that signals when the plugin exits.
+func WithTestCloseCh(closeCh chan<- struct{}) func(*plugin.ServeConfig) error {
+	return func(config *plugin.ServeConfig) error {
+		if config.Test == nil {
+			config.Test = &plugin.ServeTestConfig{}
+		}
+
+		config.Test.CloseCh = closeCh
+
+		return nil
+	}
+}
+
+// Starts the serving of a Mattermost plugin over net/rpc. gRPC is not supported.
 //
-// Call this when your plugin is ready to start.
-func ClientMain(pluginImplementation any) {
+// Call this when your plugin is ready to start. Options allow configuring plugins for testing
+// scenarios.
+func ClientMain(pluginImplementation any, opts ...func(config *plugin.ServeConfig) error) {
 	impl, ok := pluginImplementation.(interface {
 		SetAPI(api API)
 		SetDriver(driver Driver)
@@ -30,10 +73,19 @@ func ClientMain(pluginImplementation any) {
 		"hooks": &hooksPlugin{hooks: pluginImplementation},
 	}
 
-	plugin.Serve(&plugin.ServeConfig{
+	serveConfig := &plugin.ServeConfig{
 		HandshakeConfig: handshake,
 		Plugins:         pluginMap,
-	})
+	}
+
+	for _, opt := range opts {
+		err := opt(serveConfig)
+		if err != nil {
+			panic("failed to start serving plugin: " + err.Error())
+		}
+	}
+
+	plugin.Serve(serveConfig)
 }
 
 type MattermostPlugin struct {