bug#31981: Fix 'flymake-proc-legacy-flymake' temporary file deletion bug

[Noam Postavsky]

Joel Rosdahl <joel@rosdahl.net> writes:

> 1. The user modifies the buffer, making flymake-proc start a process A
> which reads from a temporary file T containing the buffer content.
> 2. Before process A has finished, the user modifies the buffer again,
> making flymake-proc start a new process B reading from the same
> temporary file T.

If I'm reading flymake-proc-init-create-temp-buffer-copy correctly, it
actually makes a new temporary file (say T1), but the file name is
stored in a single variable, so Emacs effectively forgets about the
original file T.

> 3. A is marked as obsolete.
> 4. When A's sentinel detects that A has died, it runs A's cleanup
> function which deletes T.

So here it deletes T1, leaving the original T.  And then B will fail,
since it needs T1, not T.

> But I see no other quick fix for the issue since the name of the
> temporary file is kept in a single buffer-local variable and there is
> no way for the cleanup function to know that it's being called for an
> obsolete process.

Would it work to store the filename in the process object instead, with
process-put (and then the sentinel can retreive it with process-get)?