gh-35195: Workaround for an ecl race in maxima init 
    
### 📚 Description

When maxima is initialized a bug in ecl implementation of
`ensure-directories-exist` might result in a runtime error.

As a workaround, in case we get a runtime error we use python to create
the directory and continue with maxima initialization.

Note that for normal usage the directory will already exist within the
user's `DOT_SAGE` so this code will almost never run. However, when
running doctests on CI this occasionally triggers.

#### New doctest

The first commit introduces a doctest to try to catch this race.
We run a new instance of sage in a subprocess to ensure maxima is
not already initialized. We use a temporary `MAXIMA_USERDIR` so its
empty,
and we try to initialize maxima twice in parallel to entice the race.

This temporary dir is placed within `DOT_SAGE` so it is easy
to try different filesystems.

The bug triggers more frequently if `DOT_SAGE` is in a high
latency filesystem (e.g. sshfs on a non-local host).

Closes #26968.

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->
<!-- If your change requires a documentation PR, please link it
appropriately -->
<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->

- [x] I have made sure that the title is self-explanatory and the
description concisely explains the PR.
- [x] I have linked an issue or discussion.
- [x] I have created tests covering the changes.
    
URL: #35195
Reported by: Gonzalo Tornaría
Reviewer(s): Gonzalo Tornaría, Matthias Köppe

src/sage/interfaces/maxima_lib.py

@@ -77,6 +77,26 @@
     sage: bar == foo
     True
 
+TESTS:
+
+Check our workaround for a race in ecl works, see :trac:`26968`.
+We use a temporary `MAXIMA_USERDIR` so it's empty; we place it
+in `DOT_SAGE` since we expect it to have more latency than `/tmp`.
+
+    sage: import tempfile, subprocess
+    sage: tmpdir = tempfile.TemporaryDirectory(dir=DOT_SAGE)
+    sage: _ = subprocess.run(['sage', '-c',  # long time
+    ....: f'''
+    ....: import os
+    ....: os.environ["MAXIMA_USERDIR"] = "{tmpdir.name}"
+    ....: if not os.fork():
+    ....:     import sage.interfaces.maxima_lib
+    ....: else:
+    ....:     import sage.interfaces.maxima_lib
+    ....:     os.wait()
+    ....: '''])
+    sage: tmpdir.cleanup()
+
 """
 
 # ****************************************************************************
@@ -116,7 +136,23 @@
 ecl_eval("(setq $nolabels t))")
 ecl_eval("(defvar *MAXIMA-LANG-SUBDIR* NIL)")
 ecl_eval("(set-locale-subdir)")
-ecl_eval("(set-pathnames)")
+
+try:
+    ecl_eval("(set-pathnames)")
+except RuntimeError:
+    # Recover from :trac:`26968` by creating `*maxima-objdir*` here.
+    # This cannot be done before calling `(set-pathnames)` since
+    # `*maxima-objdir*` is computed there.
+    # We use python `os.makedirs()` which is immune to the race.
+    # Using `(ensure-directories-exist ...)` in lisp would be
+    # subject to the same race condition and since `*maxima-objdir*`
+    # has multiple components this is quite plausible to happen.
+    maxima_objdir = ecl_eval("*maxima-objdir*").python()[1:-1]
+    import os
+    os.makedirs(maxima_objdir, exist_ok=True)
+    # Call `(set-pathnames)` again to complete its job.
+    ecl_eval("(set-pathnames)")
+
 ecl_eval("(defun add-lineinfo (x) x)")
 ecl_eval('(defun principal nil (cond ($noprincipal (diverg)) ((not pcprntd) (merror "Divergent Integral"))))')
 ecl_eval("(remprop 'mfactorial 'grind)")  # don't use ! for factorials (#11539)