src/sage/doctest: use CPU time for --warn-long

Our --warn-long flag used during testing currently takes "wall time"
as its argument. This is somewhat nice from a user perspective, since
a normal user is quite likely to know what what wall time is, but it's
also a bit nonsensical given the purpose of the flag. The flag's
intent is to warn developers about tests that take too long, so
whether or not it triggers should not depend on the developer's
machine, or what else he's doing while the tests are running -- both
of which do affect the wall time.

CPU time provides a more reliable measurement. (In fact, when no value
for --wall-time was given, we were already using a heuristic to mimic
CPU time.) This commit changes the --warn-long flag to use CPU time
instead of wall time, obviating the need for the heuristic.

Issue: sagemathGH-32981

src/sage/doctest/control.py

@@ -588,36 +588,32 @@ def __del__(self):
 
     def _init_warn_long(self):
         """
-        Pick a suitable default for the ``--warn-long`` option if not specified.
+        Pick a suitable default for the ``--warn-long`` option if not
+        specified.
 
         It is desirable to have all tests (even ``# long`` ones)
         finish in less than about 5 seconds. Longer tests typically
         don't add coverage, they just make testing slow.
 
-        The default used here is 60 seconds on a modern computer. It
-        should eventually be lowered to 5 seconds, but its best to
-        boil the frog slowly.
+        The default used here, for all tests, is 60 seconds.
 
-        The stored timings are used to adjust this limit according to
-        the machine running the tests.
+        TESTS:
 
-        EXAMPLES::
+        Ensure that the user's command-line options are not changed::
 
-            sage: from sage.doctest.control import DocTestDefaults, DocTestController
+            sage: from sage.doctest.control import (DocTestDefaults,
+            ....:                                   DocTestController)
             sage: DC = DocTestController(DocTestDefaults(), [])
             sage: DC.options.warn_long = 5.0
             sage: DC._init_warn_long()
-            sage: DC.options.warn_long    # existing command-line options are not changed
+            sage: DC.options.warn_long
             5.00000000000000
         """
         # default is -1.0
         if self.options.warn_long >= 0:     # Specified on the command line
             return
-        try:
-            self.options.warn_long = 60.0 * self.second_on_modern_computer()
-        except RuntimeError as err:
-            if not sage.doctest.DOCTEST_MODE:
-                print(err)   # No usable timing information
+
+        self.options.warn_long = 60.0
 
     def second_on_modern_computer(self):
         """

src/sage/doctest/forker.py

@@ -729,7 +729,7 @@ def compiler(example):
             finally:
                 if self.debugger is not None:
                     self.debugger.set_continue()  # ==== Example Finished ====
-            check_starttime = walltime()
+            check_timer = Timer().start()
             got = self._fakeout.getvalue()
 
             outcome = FAILURE   # guilty until proved innocent or insane
@@ -803,22 +803,22 @@ def compiler(example):
                                 f"and it succeeded (raised an exception as expected).")
                         outcome = SUCCESS
 
-            check_duration = walltime(check_starttime)
-            self.total_walltime += example.walltime + check_duration
+            check_timer.stop()
+            self.total_walltime += example.walltime + check_timer.walltime
 
             # Report the outcome.
             if example.warnings:
                 for warning in example.warnings:
                     out(self._failure_header(test, example, f'Warning: {warning}'))
             if outcome is SUCCESS:
-                if self.options.warn_long > 0 and example.walltime + check_duration > self.options.warn_long:
+                if self.options.warn_long > 0 and example.cputime + check_timer.cputime > self.options.warn_long:
                     self.report_overtime(out, test, example, got,
-                                         check_duration=check_duration)
+                                         check_duration=check_timer.cputime)
                 elif example.warnings:
                     pass
                 elif not quiet:
                     self.report_success(out, test, example, got,
-                                        check_duration=check_duration)
+                                        check_duration=check_timer.cputime)
             elif probed_tags:
                 pass
             elif outcome is FAILURE:
@@ -1528,17 +1528,17 @@ def report_overtime(self, out, test, example, got, *, check_duration=0):
             sage: FDS = FileDocTestSource(filename, DD)
             sage: doctests, extras = FDS.create_doctests(globals())
             sage: ex = doctests[0].examples[0]
-            sage: ex.walltime = 1.23r
+            sage: ex.cputime = 1.23
             sage: DTR.report_overtime(sys.stdout.write, doctests[0], ex, 'BAD ANSWER\n', check_duration=2.34r)
             **********************************************************************
             File ".../sage/doctest/forker.py", line 12, in sage.doctest.forker
             Warning, slow doctest:
                 doctest_var = 42; doctest_var^2
-            Test ran for 1.23 s, check ran for 2.34 s
+            Test ran for 1.23 cpu seconds, check ran for 2.34 cpu seconds
         """
         out(self._failure_header(test, example, 'Warning, slow doctest:') +
-            ('Test ran for %.2f s, check ran for %.2f s\n'
-             % (example.walltime, check_duration)))
+            ('Test ran for %.2f cpu seconds, check ran for %.2f cpu seconds\n'
+             % (example.cputime, check_duration)))
 
     def report_unexpected_exception(self, out, test, example, exc_info):
         r"""