[interactive] Fix handling of Ctrl-C.

Addresses issue #239.

- We almost always ignore Ctrl-C in the main shell.
  - But we restore Python's SIGINT handler for the readline binding, so
    it raises KeyboardInterrupt.
- Child processes use the default signal handling SIG_DFL.

This seems to be correct but I encourage testing :)

Author: Andy Chu

bin/oil.py

@@ -512,8 +512,9 @@ def ShellMain(lang, argv0, argv, login_shell):
     else:  # Without readline module
       display = comp_ui.MinimalDisplay(comp_ui_state, prompt_state, debug_f)
 
-    # NOTE: SIGINT is handled in frontend/reader.py.  It interacts with
-    # readline.
+    # NOTE: SIGINT is temporarily enabled during readline() by
+    # frontend/reader.py.
+    signal.signal(signal.SIGINT, signal.SIG_IGN)
 
     # The shell itself should ignore Ctrl-\.
     signal.signal(signal.SIGQUIT, signal.SIG_IGN)

core/process.py

@@ -550,6 +550,8 @@ def Start(self):
     elif pid == 0:  # child
       # Respond to Ctrl-\ (core dump)
       signal.signal(signal.SIGQUIT, signal.SIG_DFL)
+      # Respond to Ctrl-C
+      signal.signal(signal.SIGINT, signal.SIG_DFL)
 
       # This doesn't make the child respond to Ctrl-Z?  Why not?  Is there
       # something at the Python level?  signalmodule.c has PyOS_AfterFork but

frontend/reader.py

@@ -65,7 +65,6 @@ def _GetLine(self):
     #sys.stderr.write(self.prompt_str)
 
     signal.signal(signal.SIGINT, self.orig_handler)  # raise KeyboardInterrupt
-
     try:
       line = raw_input(self.prompt_str) + '\n'  # newline required
     except EOFError:
@@ -83,7 +82,7 @@ def _GetLine(self):
       # When we're not waiting for input, ignore Ctrl-C so we don't get
       # KeyboardInterrupt in weird places.  NOTE: This can't be SIG_IGN,
       # because that affects the child process.
-      signal.signal(signal.SIGINT, _DoNothing)
+      signal.signal(signal.SIGINT, signal.SIG_IGN)
       # TODO: Should we restore the user-registered handler?
 
     # Add the line if it's not EOL, the same as the previous line, and we have