make-directory now returns t if dir already exists

This new feature will help fix a copy-directory bug (Bug#58919).
Its implementation does not rely on make-directory handlers
supporting the new feature, as it no longer uses a make-directory
handler H in any way other than (funcall H DIR), thus using
only the intersection of the old and new behavior for handlers.
This will give us time to fix handlers at our leisure.
* lisp/files.el (files--ensure-directory): New arg MKDIR.
All uses changed.
(files--ensure-directory, make-directory):
Return non-nil if DIR is already a directory.  All uses changed.
* test/lisp/files-tests.el (files-tests-make-directory):
Test new return-value convention.

1 files changed, 3 insertions, 0 deletions

diff --git a/doc/lispref/files.texi b/doc/lispref/files.texi
index b3f63b8f32a..a767f9c28d5 100644
--- a/doc/lispref/files.texi
+++ b/doc/lispref/files.texi
@@ -3209,6 +3209,9 @@ This command creates a directory named @var{dirname}.  If
 @var{parents} is non-@code{nil}, as is always the case in an
 interactive call, that means to create the parent directories first,
 if they don't already exist.
+As a function, @code{make-directory} returns non-@code{nil} if @var{dirname}
+already exists as a directory and @var{parents} is non-@code{nil},
+and returns @code{nil} if it successfully created @var{dirname}.
 @code{mkdir} is an alias for this.
 @end deffn