Trac #18908: Fix math-readline script

{{{
sage: mathematica(1)
}}}
hangs indefinitely if `Mathematica` is not installed.

The reason is that internally the script `$SAGE_LOCAL/bin/math-readline`
is called by `Expect()`, and the latter hangs. This script should check
whether the `math` command has finished: currently the `math-readline`
script keeps running even after the subprocess has exited.

URL: http://trac.sagemath.org/18908
Reported by: dimpase
Ticket author(s): Jeroen Demeyer
Reviewer(s): Nathann Cohen, Sébastien Labbé, Salvatore Stella, Dima
Pasechnik

src/bin/math-readline

@@ -5,21 +5,27 @@
 # See
 #    http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/363500
 
-import os, sys
-import six
-f1 = os.popen('math ', 'w')
-f1.flush()
-try:
-    while True:
-        sys.stdout.write('')
-        try:
-            line = six.moves.input()
-            f1.writelines(line+'\n')
-            f1.flush()
-        except KeyboardInterrupt:
-            f1.close()
-            break
-except EOFError:
-    pass
+import sys, signal, subprocess
+import readline
+from six.moves import input
+
+def child_exited(*args):
+    global child
+    status = child.poll()
+    if status is not None:
+        sys.exit(status)
+
+signal.signal(signal.SIGCHLD, child_exited)
+
+child = subprocess.Popen('math', shell=True, stdin=subprocess.PIPE)
+pipe = child.stdin
+while True:
+    try:
+        line = input()
+        pipe.write(line + '\n')
+        pipe.flush()
+    except KeyboardInterrupt:
+        pipe.close()
+    except EOFError:
+        break
 sys.stdout.write('\n')
-sys.exit()