bug#63648: 29.0.90; project.el: with switch-use-entire-map, switch-project errors on non-project commands

[Juri Linkov]

>> 1. the current buffer should remain the same for the next command;
>> 2. the buffer-local value of 'default-directory' should remain the same;
>> 3. the next command should have a new value of 'default-directory'.
>>
>> I see no way to satisfy all these requirements.
>
> Except adding a variable like 'project-current-directory-override'
> somewhere inside 'command_loop_1'.

And indeed with the following patch replacing the current definition of
'project-switch-project' with just:

  (defun project-switch-project (dir)
    (interactive (list (funcall project-prompter)))
    (setq next-default-directory dir))

'C-x p p (select project ...) RET M-& pwd RET' confirms that
the command runs in the selected project directory.
Whereas the original buffer keeps its previous buffer-local value
of 'default-directory'.

diff --git a/src/buffer.c b/src/buffer.c
index 0c46b201586..ca81db21894 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -5360,6 +5360,10 @@ syms_of_buffer (void)
 these names start with `/' or `~' and end with `/'.
 To interactively change the default directory, use command `cd'. */);
 
+  DEFVAR_LISP ("next-default-directory", Vnext_default_directory,
+              doc: /* Default directory for the next command.  */);
+  Vnext_default_directory = Qnil;
+
   DEFVAR_PER_BUFFER ("auto-fill-function", &BVAR (current_buffer, 
auto_fill_function),
                     Qnil,
                     doc: /* Function called (if non-nil) to perform auto-fill.
diff --git a/src/keyboard.c b/src/keyboard.c
index 14c55666768..dfe939750c9 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -1500,7 +1500,19 @@ command_loop_1 (void)
            update_redisplay_ticks (0, NULL);
            display_working_on_window_p = false;
 
+            Lisp_Object next_dir = Vnext_default_directory;
+            specpdl_ref count = SPECPDL_INDEX ();
+            if (!NILP (next_dir))
+              specbind (Qdefault_directory, next_dir);
+
             call1 (Qcommand_execute, Vthis_command);
+
+            if (!NILP (next_dir))
+              {
+                unbind_to (count, Qnil);
+                Vnext_default_directory = Qnil;
+              }
+
            display_working_on_window_p = false;
 
 #ifdef HAVE_WINDOW_SYSTEM