[TCLCORE] Atomicity in Tcl and Tk

[Marc C. <cul...@gm...>]

I have a question for the experts.  It has to do with when the interpreter,
during execution of a script, allows idle tasks and timer tasks to be run.

There is an expectation in may situations that idle tasks will not be run
between two consecutive commands in the script.  For example, in
macosx/README it says:

The TkAqua-specific command [tk::unsupported::MacWindowStyle style] is used
to
get and set macOS-specific toplevel window class and attributes. Note that
the window class and many attributes have to be set before the window is
first
mapped for the change to have any effect.

That means that the following code is expected to work:
  toplevel .t
  tk::unsupported::MacWindowStyle style .t modal

but something like this:

  toplevel .t
  < lots of code here>
  tk::unsupported::MacWindowStyle modal

will probably not work because the window will get mapped during the <lots
of code>.  The window actually gets mapped in an idle task that is created
by the toplevel command.  The first code block works because that idle task
gets run after the MacWindowStyle command has run.

My question is essentially "How large can the <lots of code> be and have
this still work?"  What are the rules?  When running a script, when does
the interpreter pause and "return to the event loop"?  Is there some way to
ensure that a given block of code will run completely before any additional
idle tasks get run?

- Marc