MM-56402: Introduce a pluginID to track RPC DB connections (#26424)

Previously, we relied on the plugin to close the DB connections
on shutdown. While this keeps the code simple, there is no guarantee
that the plugin author will remember to close the DB.

In that case, it's better to track the connections from the server side
and close them in case they weren't closed already. This complicates
the API slightly, but it's a price we need to pay.

https://mattermost.atlassian.net/browse/MM-56402

```release-note
We close any remaining unclosed DB RPC connections
after a plugin shuts down.
```


Co-authored-by: Jesse Hallam <jesse.hallam@gmail.com>
Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/plugin/driver.go

@@ -62,3 +62,14 @@ type Driver interface {
 	// ResetSession(ctx context.Context) error
 	// IsValid() bool
 }
+
+// AppDriver is an extension of the Driver interface to capture non-RPC APIs.
+type AppDriver interface {
+	Driver
+
+	// ConnWithPluginID is only used by the server, and isn't exposed via the RPC API.
+	ConnWithPluginID(isMaster bool, pluginID string) (string, error)
+	// This is an extra method needed to shutdown connections
+	// after a plugin shuts down.
+	ShutdownConns(pluginID string)
+}

server/public/plugin/environment.go

@@ -54,7 +54,7 @@ type Environment struct {
 	logger                           *mlog.Logger
 	metrics                          metricsInterface
 	newAPIImpl                       apiImplCreatorFunc
-	dbDriver                         Driver
+	dbDriver                         AppDriver
 	pluginDir                        string
 	webappPluginDir                  string
 	prepackagedPlugins               []*PrepackagedPlugin
@@ -64,7 +64,7 @@ type Environment struct {
 
 func NewEnvironment(
 	newAPIImpl apiImplCreatorFunc,
-	dbDriver Driver,
+	dbDriver AppDriver,
 	pluginDir string,
 	webappPluginDir string,
 	logger *mlog.Logger,

server/public/plugin/supervisor.go

@@ -24,6 +24,8 @@ import (
 
 type supervisor struct {
 	lock         sync.RWMutex
+	pluginID     string
+	appDriver    AppDriver
 	client       *plugin.Client
 	hooks        Hooks
 	implemented  [TotalHooksID]bool
@@ -31,6 +33,15 @@ type supervisor struct {
 	isReattached bool
 }
 
+type driverForPlugin struct {
+	AppDriver
+	pluginID string
+}
+
+func (d *driverForPlugin) Conn(isMaster bool) (string, error) {
+	return d.AppDriver.ConnWithPluginID(isMaster, d.pluginID)
+}
+
 func WithExecutableFromManifest(pluginInfo *model.BundleInfo) func(*supervisor, *plugin.ClientConfig) error {
 	return func(_ *supervisor, clientConfig *plugin.ClientConfig) error {
 		executable := pluginInfo.Manifest.GetExecutableForRuntime(runtime.GOOS, runtime.GOARCH)
@@ -74,8 +85,14 @@ func WithReattachConfig(pluginReattachConfig *model.PluginReattachConfig) func(*
 	}
 }
 
-func newSupervisor(pluginInfo *model.BundleInfo, apiImpl API, driver Driver, parentLogger *mlog.Logger, metrics metricsInterface, opts ...func(*supervisor, *plugin.ClientConfig) error) (retSupervisor *supervisor, retErr error) {
-	sup := supervisor{}
+func newSupervisor(pluginInfo *model.BundleInfo, apiImpl API, driver AppDriver, parentLogger *mlog.Logger, metrics metricsInterface, opts ...func(*supervisor, *plugin.ClientConfig) error) (retSupervisor *supervisor, retErr error) {
+	sup := supervisor{
+		pluginID: pluginInfo.Manifest.Id,
+	}
+	if driver != nil {
+		sup.appDriver = &driverForPlugin{AppDriver: driver, pluginID: pluginInfo.Manifest.Id}
+	}
+
 	defer func() {
 		if retErr != nil {
 			sup.Shutdown()
@@ -92,7 +109,7 @@ func newSupervisor(pluginInfo *model.BundleInfo, apiImpl API, driver Driver, par
 	pluginMap := map[string]plugin.Plugin{
 		"hooks": &hooksPlugin{
 			log:        wrappedLogger,
-			driverImpl: driver,
+			driverImpl: sup.appDriver,
 			apiImpl:    &apiTimerLayer{pluginInfo.Manifest.Id, apiImpl, metrics},
 		},
 	}
@@ -166,8 +183,12 @@ func (sup *supervisor) Shutdown() {
 	}
 
 	// Wait for API RPC server and DB RPC server to exit.
+	// And then shutdown conns.
 	if sup.hooksClient != nil {
 		sup.hooksClient.doneWg.Wait()
+		if sup.appDriver != nil {
+			sup.appDriver.ShutdownConns(sup.pluginID)
+		}
 	}
 }