oauthutil: add Shutdown method

Before this change, calling the `oauthutil.NewRenew` func may
cause goroutine leaks.

This change adds a `Shutdown` method to allow the caller to exit
the goroutine to avoid leaks.

Signed-off-by: rkonfj <rkonfj@gmail.com>

lib/oauthutil/renew.go

@@ -1,6 +1,7 @@
 package oauthutil
 
 import (
+	"sync"
 	"sync/atomic"
 
 	"github.com/rclone/rclone/fs"
@@ -8,10 +9,12 @@ import (
 
 // Renew allows tokens to be renewed on expiry if uploads are in progress.
 type Renew struct {
-	name    string       // name to use in logs
-	ts      *TokenSource // token source that needs renewing
-	uploads atomic.Int32 // number of uploads in progress
-	run     func() error // a transaction to run to renew the token on
+	name     string       // name to use in logs
+	ts       *TokenSource // token source that needs renewing
+	uploads  atomic.Int32 // number of uploads in progress
+	run      func() error // a transaction to run to renew the token on
+	done     chan any     // channel to end the go routine
+	shutdown sync.Once
 }
 
 // NewRenew creates a new Renew struct and starts a background process
@@ -24,6 +27,7 @@ func NewRenew(name string, ts *TokenSource, run func() error) *Renew {
 		name: name,
 		ts:   ts,
 		run:  run,
+		done: make(chan any),
 	}
 	go r.renewOnExpiry()
 	return r
@@ -36,7 +40,11 @@ func NewRenew(name string, ts *TokenSource, run func() error) *Renew {
 func (r *Renew) renewOnExpiry() {
 	expiry := r.ts.OnExpiry()
 	for {
-		<-expiry
+		select {
+		case <-expiry:
+		case <-r.done:
+			return
+		}
 		uploads := r.uploads.Load()
 		if uploads != 0 {
 			fs.Debugf(r.name, "Token expired - %d uploads in progress - refreshing", uploads)
@@ -72,3 +80,15 @@ func (r *Renew) Invalidate() {
 func (r *Renew) Expire() error {
 	return r.ts.Expire()
 }
+
+// Shutdown stops the timer and no more renewal will take place.
+func (r *Renew) Shutdown() {
+	if r == nil {
+		return
+	}
+	// closing a channel can only be done once
+	r.shutdown.Do(func() {
+		r.ts.expiryTimer.Stop()
+		close(r.done)
+	})
+}