fix(rclone/operations): fix CheckPermissions for NFS

4f1adf6 introduced explicit cleanup of test dirs in CheckPermissions.
The problem was that the Purge happened while the file descriptor from
the "cat" verification was still held. On a regular filesystem this is
not an issue, as Purge would still just remove the dir with its contents
and make the file descriptor broken. The situation is different with
NFS, as when another process tries to remove file with open descriptor,
NFS will perform a "silly rename" and create an ephemeral .nfsXXX file
so that the descriptor is not broken. So when rclone runs Purge, it first
removes all its files, this results in creation of .nfsXXX files and when
it proceeds with dir removal, it fails with "directory not empty".

This commit fixes the bug related to holding the file descriptor for too
long. It also explicitly checks the permission to remove a file and
performs the cleanup without failing the CheckPermissions if just
the cleanup failed, as it is not critical.

Author: Michal-Leszczynski

pkg/rclone/operations/operations.go

@@ -4,6 +4,7 @@ package operations
 
 import (
 	"context"
+	stderr "errors"
 	"io"
 	"os"
 	"path/filepath"
@@ -110,7 +111,7 @@ func CheckPermissions(ctx context.Context, l fs.Fs) error {
 		}
 	}
 
-	// Cat remote file.
+	// Create, cat and remove remote file.
 	{
 		o, err := l.NewObject(ctx, filepath.Join(testDirName, testFileName))
 		if err != nil {
@@ -120,15 +121,21 @@ func CheckPermissions(ctx context.Context, l fs.Fs) error {
 		if err != nil {
 			return asOperationError("open", l, err)
 		}
-		defer r.Close()
-		if _, err := io.Copy(io.Discard, r); err != nil {
-			return asOperationError("copy", l, err)
+		_, readErr := io.Copy(io.Discard, r)
+		if err := stderr.Join(readErr, r.Close()); err != nil {
+			return asOperationError("read", l, err)
+		}
+		if err := o.Remove(ctx); err != nil {
+			return asOperationError("remove", l, err)
 		}
 	}
 
-	// Remove remote dir.
+	// Cleanup.
 	if err := operations.Purge(ctx, l, testDirName); err != nil {
-		return asOperationError("purge", l, err)
+		// As we already verified all permissions needed by SM to perform
+		// a successful backup, we can just log an error here to allow
+		// backup to proceed in case of unexpected and not critical error.
+		fs.Errorf(l, "failed to remove test directory %q: %v", testDirName, err)
 	}
 
 	return nil