Trac #33907: interfaces/expect.py random test failure

I've been getting this very frequently when running tests under heavy
CPU load. This happens on Sage 9.5 too.

{{{
sage -t --long --random-seed=233407191301051614325254530123917684827
/nix/store/fmcggxb3m1xvlsdqmi045p87xml9paca-sage-
src-9.6/src/sage/interfaces/expect.py
**********************************************************************
File "/nix/store/fmcggxb3m1xvlsdqmi045p87xml9paca-sage-
src-9.6/src/sage/interfaces/expect.py", line 920, in
sage.interfaces.expect.Expect._eval_line
Failed example:
    singular.interrupt()
Expected:
    True
Got:
    False
**********************************************************************
File "/nix/store/fmcggxb3m1xvlsdqmi045p87xml9paca-sage-
src-9.6/src/sage/interfaces/expect.py", line 926, in
sage.interfaces.expect.Expect._eval_line
Failed example:
    singular('2+3')
Expected:
    Singular crashed -- automatically restarting.
    5
Got:
    5
}}}

URL: https://trac.sagemath.org/33907
Reported by: gh-collares
Ticket author(s): Gonzalo Tornaría
Reviewer(s): Matthias Koeppe

src/sage/interfaces/expect.py

@@ -941,7 +941,6 @@ def _eval_line(self, line, allow_use_file=True, wait_for_prompt=True, restart_if
         The interface still works after this interrupt::
 
             sage: singular('2+3')
-            Singular crashed -- automatically restarting.
             5
 
         Last, we demonstrate that by default the execution of a command

src/sage/interfaces/singular.py

@@ -341,6 +341,7 @@
 import sys
 import pexpect
 import shlex
+import time
 
 from .expect import Expect, ExpectElement, FunctionElement, ExpectFunction
 
@@ -508,6 +509,52 @@ def _quit_string(self):
         """
         return 'quit;'
 
+    def _send_interrupt(self):
+        """
+        Send an interrupt to Singular. If needed, additional
+        semi-colons are sent until we get back at the prompt.
+
+        TESTS:
+
+        The following works without restarting Singular::
+
+            sage: a = singular(1)
+            sage: _ = singular._expect.sendline('while(1){};')
+            sage: singular.interrupt()
+            True
+
+        We can still access a::
+
+            sage: 2*a
+            2
+
+        Interrupting nothing or unfinished input also works::
+
+            sage: singular.interrupt()
+            True
+            sage: _ = singular._expect.sendline('1+')
+            sage: singular.interrupt()
+            True
+            sage: 3*a
+            3
+
+        """
+        # Work around for Singular bug
+        # http://www.singular.uni-kl.de:8002/trac/ticket/727
+        time.sleep(0.1)
+
+        E = self._expect
+        E.sendline(chr(3))
+        # The following is needed so interrupt() works even when
+        # there is no computation going on.
+        for i in range(5):
+            try:
+                E.expect_upto(self._prompt, timeout=0.1)
+                return
+            except pexpect.TIMEOUT:
+                pass
+            E.sendline(";")
+
     def _read_in_file_command(self, filename):
         r"""
         EXAMPLES::