diff --git a/Lib/test/test_signal.py b/Lib/test/test_signal.py
index 202dc53..94ee4d1 100644
--- a/Lib/test/test_signal.py
+++ b/Lib/test/test_signal.py
@@ -255,48 +255,105 @@
 
 class SiginterruptTest(unittest.TestCase):
     signum = signal.SIGUSR1
-    def readpipe_interrupted(self, cb):
+
+    def setUp(self):
+        """Install a no-op signal handler that can be set to allow
+        interrupts or not, and arrange for the original signal handler to be
+        re-installed when the test is finished.
+        """
+        oldhandler = signal.signal(self.signum, lambda x,y: None)
+        self.addCleanup(signal.signal, self.signum, oldhandler)
+
+    def readpipe_interrupted(self):
+        """Perform a read during which a signal will arrive.  Return True if the
+        read is interrupted by the signal and raises an exception.  Return False
+        if it returns normally.
+        """
+        # Create a pipe that can be used for the read.  Also clean it up
+        # when the test is over, since nothing else will (but see below for
+        # the write end).
         r, w = os.pipe()
+        self.addCleanup(os.close, r)
+
+        # Create another process which can send a signal to this one to try
+        # to interrupt the read.
         ppid = os.getpid()
         pid = os.fork()
 
-        oldhandler = signal.signal(self.signum, lambda x,y: None)
-        cb()
-        if pid==0:
-            # child code: sleep, kill, sleep. and then exit,
-            # which closes the pipe from which the parent process reads
+        if pid == 0:
+            # Child code: sleep to give the parent enough time to enter the
+            # read() call (there's a race here, but it's really tricky to
+            # eliminate it); then signal the parent process.  Also, sleep
+            # again to make it likely that the signal is delivered to the
+            # parent process before the child exits.  If the child exits
+            # first, the write end of the pipe will be closed and the test
+            # is invalid.
             try:
                 time.sleep(0.2)
                 os.kill(ppid, self.signum)
                 time.sleep(0.2)
             finally:
+                # No matter what, just exit as fast as possible now.
                 exit_subprocess()
+        else:
+            # Parent code.
+            # Make sure the child is eventually reaped, else it'll be a
+            # zombie for the rest of the test suite run.
+            self.addCleanup(os.waitpid, pid, 0)
 
-        try:
+            # Close the write end of the pipe.  The child has a copy, so
+            # it's not really closed until the child exits.  We need it to
+            # close when the child exits so that in the non-interrupt case
+            # the read eventually completes, otherwise we could just close
+            # it *after* the test.
             os.close(w)
 
+            # Try the read and report whether it is interrupted or not to
+            # the caller.
             try:
-                d=os.read(r, 1)
+                d = os.read(r, 1)
                 return False
             except OSError, err:
                 if err.errno != errno.EINTR:
                     raise
                 return True
-        finally:
-            signal.signal(self.signum, oldhandler)
-            os.waitpid(pid, 0)
 
     def test_without_siginterrupt(self):
-        i=self.readpipe_interrupted(lambda: None)
-        self.assertEquals(i, True)
+        """If a signal handler is installed and siginterrupt is not called
+        at all, when that signal arrives, it interrupts a syscall that's in
+        progress.
+        """
+        i = self.readpipe_interrupted()
+        self.assertTrue(i)
+        # Arrival of the signal shouldn't have changed anything.
+        i = self.readpipe_interrupted()
+        self.assertTrue(i)
 
     def test_siginterrupt_on(self):
-        i=self.readpipe_interrupted(lambda: signal.siginterrupt(self.signum, 1))
-        self.assertEquals(i, True)
+        """If a signal handler is installed and siginterrupt is called with
+        a true value for the second argument, when that signal arrives, it
+        interrupts a syscall that's in progress.
+        """
+        signal.siginterrupt(self.signum, 1)
+        i = self.readpipe_interrupted()
+        self.assertTrue(i)
+        # Arrival of the signal shouldn't have changed anything.
+        i = self.readpipe_interrupted()
+        self.assertTrue(i)
 
     def test_siginterrupt_off(self):
-        i=self.readpipe_interrupted(lambda: signal.siginterrupt(self.signum, 0))
-        self.assertEquals(i, False)
+        """If a signal handler is installed and siginterrupt is called with
+        a false value for the second argument, when that signal arrives, it
+        does not interrupt a syscall that's in progress.
+        """
+        signal.siginterrupt(self.signum, 0)
+        i = self.readpipe_interrupted()
+        self.assertFalse(i)
+        # Arrival of the signal shouldn't have changed anything.
+        i = self.readpipe_interrupted()
+        self.assertFalse(i)
+
+
 
 class ItimerTest(unittest.TestCase):
     def setUp(self):