sagemath#26968: add a doctest to catch ecl race in maxima init

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).

The next commit introduces a workaround for the bug.

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()
+
 """
 
 # ****************************************************************************