memory: fix deadlock in operations.Purge

Before this change, the Memory backend had the potential to deadlock under
certain conditions, if the ListR callback required locking the b.mu mutex. This
was the case with operations.Purge, because Memory has no Purge method, and the
fallback option does:

	err = DeleteFiles(ctx, listToChan(ctx, f, dir))

which potentially starts removing objects before the listing has completed.

This change fixes the issue by batching all the entries before calling the
callback on them.

backend/memory/memory.go

@@ -38,8 +38,7 @@ func init() {
 }
 
 // Options defines the configuration for this backend
-type Options struct {
-}
+type Options struct{}
 
 // Fs represents a remote memory server
 type Fs struct {
@@ -385,10 +384,22 @@ func (f *Fs) List(ctx context.Context, dir string) (entries fs.DirEntries, err e
 func (f *Fs) ListR(ctx context.Context, dir string, callback fs.ListRCallback) (err error) {
 	bucket, directory := f.split(dir)
 	list := walk.NewListRHelper(callback)
+	entries := fs.DirEntries{}
 	listR := func(bucket, directory, prefix string, addBucket bool) error {
-		return f.list(ctx, bucket, directory, prefix, addBucket, true, func(remote string, entry fs.DirEntry, isDirectory bool) error {
-			return list.Add(entry)
+		err = f.list(ctx, bucket, directory, prefix, addBucket, true, func(remote string, entry fs.DirEntry, isDirectory bool) error {
+			entries = append(entries, entry) // can't list.Add here -- could deadlock
+			return nil
 		})
+		if err != nil {
+			return err
+		}
+		for _, entry := range entries {
+			err = list.Add(entry)
+			if err != nil {
+				return err
+			}
+		}
+		return nil
 	}
 	if bucket == "" {
 		entries, err := f.listBuckets(ctx)

backend/memory/memory_internal_test.go

@@ -0,0 +1,40 @@
+package memory
+
+import (
+	"context"
+	"fmt"
+	"testing"
+
+	_ "github.com/rclone/rclone/backend/local"
+	"github.com/rclone/rclone/fs/operations"
+	"github.com/rclone/rclone/fstest"
+	"github.com/rclone/rclone/fstest/fstests"
+	"github.com/stretchr/testify/require"
+)
+
+var t1 = fstest.Time("2001-02-03T04:05:06.499999999Z")
+
+// InternalTest dispatches all internal tests
+func (f *Fs) InternalTest(t *testing.T) {
+	t.Run("PurgeListDeadlock", func(t *testing.T) {
+		testPurgeListDeadlock(t)
+	})
+}
+
+// test that Purge fallback does not result in deadlock from concurrently listing and removing
+func testPurgeListDeadlock(t *testing.T) {
+	ctx := context.Background()
+	r := fstest.NewRunIndividual(t)
+	r.Mkdir(ctx, r.Fremote)
+	r.Fremote.Features().Disable("Purge") // force fallback-purge
+
+	// make a lot of files to prevent it from finishing too quickly
+	for i := 0; i < 100; i++ {
+		dst := "file" + fmt.Sprint(i) + ".txt"
+		r.WriteObject(ctx, dst, "hello", t1)
+	}
+
+	require.NoError(t, operations.Purge(ctx, r.Fremote, ""))
+}
+
+var _ fstests.InternalTester = (*Fs)(nil)