Fix interrupting Singular

src/sage/interfaces/expect.py

@@ -822,10 +822,15 @@ def _eval_line(self, line, allow_use_file=True, wait_for_prompt=True, restart_if
             sage: singular._eval_using_file_cutoff = 4
             sage: singular._eval_line('for(int i=1;i<=3;i++){i=1;};', wait_for_prompt=False)
             ''
-            sage: singular.interrupt(timeout=3)  # sometimes very slow (up to 60s on sage.math, 2012)
-            False
+            sage: singular.interrupt()
+            True
             sage: singular._eval_using_file_cutoff = cutoff
 
+        The interface still works after this interrupt::
+
+            sage: singular('2+3')
+            5
+
         Last, we demonstrate that by default the execution of a command
         is tried twice if it fails the first time due to a crashed
         interface::

src/sage/interfaces/sagespawn.pyx

@@ -109,6 +109,42 @@ class SageSpawn(spawn):
         """
         Py_INCREF(self)
 
+    def expect_peek(self, *args, **kwds):
+        """
+        Like :meth:`expect` but restore the read buffer such that it
+        looks like nothing was actually read. The next reading will
+        continue at the current position.
+
+        EXAMPLES::
+
+            sage: from sage.interfaces.sagespawn import SageSpawn
+            sage: E = SageSpawn("sh", ["-c", "echo hello world"])
+            sage: _ = E.expect_peek("w")
+            sage: E.read()
+            'hello world\r\n'
+        """
+        ret = self.expect(*args, **kwds)
+        self.buffer = self.before + self.after + self.buffer
+        return ret
+
+    def expect_upto(self, *args, **kwds):
+        """
+        Like :meth:`expect` but restore the read buffer starting from
+        the matched string. The next reading will continue starting
+        with the matched string.
+
+        EXAMPLES::
+
+            sage: from sage.interfaces.sagespawn import SageSpawn
+            sage: E = SageSpawn("sh", ["-c", "echo hello world"])
+            sage: _ = E.expect_upto("w")
+            sage: E.read()
+            'world\r\n'
+        """
+        ret = self.expect(*args, **kwds)
+        self.buffer = self.after + self.buffer
+        return ret
+
     def close(self):
         """
         Quit the child process: send the quit string, close the

src/sage/interfaces/singular.py

@@ -304,37 +304,22 @@
     ''
 """
 
-
-
-#We could also do these calculations without using the singular
-#interface (behind the scenes the interface is used by Sage):
-#    sage: x, y = PolynomialRing(RationalField(), 2, names=['x','y']).gens()
-#    sage: C = ProjectivePlaneCurve(y**9 - x**2*(x-1)**9)
-#    sage: C.genus()
-#    0
-#    sage: C = ProjectivePlaneCurve(y**9 - x**2*(x-1)**9 + x)
-#    sage: C.genus()
-#    40
-
 #*****************************************************************************
 #       Copyright (C) 2005 David Joyner and William Stein
 #
-#  Distributed under the terms of the GNU General Public License (GPL)
-#
-#    This code is distributed in the hope that it will be useful,
-#    but WITHOUT ANY WARRANTY; without even the implied warranty of
-#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
-#    General Public License for more details.
-#
-#  The full text of the GPL is available at:
-#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 2 of the License, or
+# (at your option) any later version.
 #                  http://www.gnu.org/licenses/
 #*****************************************************************************
 
+
 import os
 import re
 import sys
 import pexpect
+from time import sleep
 
 from expect import Expect, ExpectElement, FunctionElement, ExpectFunction
 
@@ -386,7 +371,9 @@ def __init__(self, maxread=1000, script_subdirectory=None,
                         terminal_echo=False,
                         name = 'singular',
                         prompt = prompt,
-                        command = "Singular -t --ticks-per-sec 1000", #no tty and fine grained cputime()
+                        # no tty, fine grained cputime()
+                        # and do not display CTRL-C prompt
+                        command = "Singular -t --ticks-per-sec 1000 --cntrlc=a",
                         maxread = maxread,
                         server = server,
                         server_tmpdir = server_tmpdir,
@@ -468,6 +455,43 @@ 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('1+')  # unfinished input
+            sage: try:
+            ....:     alarm(0.5)
+            ....:     singular._expect_expr('>')  # interrupt this
+            ....: except KeyboardInterrupt:
+            ....:     pass
+            Control-C pressed.  Interrupting Singular. Please wait a few seconds...
+
+        We can still access a::
+
+            sage: 2*a
+            2
+        """
+        # Work around for Singular bug
+        # http://www.singular.uni-kl.de:8002/trac/ticket/727
+        sleep(0.1)
+
+        E = self._expect
+        E.sendline(chr(3))
+        for i in range(5):
+            try:
+                E.expect_upto(self._prompt, timeout=1.0)
+                return
+            except Exception:
+                pass
+            E.sendline(";")
+
     def _read_in_file_command(self, filename):
         r"""
         EXAMPLES::