Add Self-Pipe Trick demo code

README.rst

@@ -228,6 +228,11 @@ The following steps describe how to implement it:
    execute any actions that have to be done in response to the signal
    delivery.
 
+Start the
+`selfpipe.py <https://github.com/rspivak/csdesign/blob/master/selfpipe.py>`_
+and send a *USR1* signal to it. You should see it output a message
+*Got signal* and exit.
+
 
 Roadmap
 -------

selfpipe.py

@@ -0,0 +1,115 @@
+###############################################################################
+#
+# Copyright (c) 2012 Ruslan Spivak
+#
+# Permission is hereby granted, free of charge, to any person obtaining a copy
+# of this software and associated documentation files (the "Software"), to deal
+# in the Software without restriction, including without limitation the rights
+# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+# copies of the Software, and to permit persons to whom the Software is
+# furnished to do so, subject to the following conditions:
+#
+# The above copyright notice and this permission notice shall be included in
+# all copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+# THE SOFTWARE.
+#
+###############################################################################
+
+"""
+Demonstrates SELF-PIPE Trick that is used to
+avoid race condtions when waiting for signals and calling select() on
+a set of descriptors.
+"""
+
+__author__ = 'Ruslan Spivak <ruslan.spivak@gmail.com>'
+
+import os
+import sys
+import fcntl
+import time
+import errno
+import select
+import signal
+
+# read, write pipe descriptors
+RD, WD = None, None
+
+
+def handler(signum, frame):
+    try:
+        os.write(WD, 'x')
+    except OSError as e:
+        if e.errno != errno.EAGAIN:
+            print 'Signal handler write error'
+            os._exit(1)
+
+
+def main():
+    print 'PID: %s' % os.getpid()
+
+    # 1. Create a pipe and make both ends nonblocking
+    global RD, WD
+    RD, WD = os.pipe()
+
+    flags = fcntl.fcntl(RD, fcntl.F_GETFL)
+    fcntl.fcntl(RD, fcntl.F_SETFL, flags | os.O_NONBLOCK)
+
+    flags = fcntl.fcntl(WD, fcntl.F_GETFL)
+    fcntl.fcntl(WD, fcntl.F_SETFL, flags | os.O_NONBLOCK)
+
+
+    # 2. Add read end of the pipe to the read list of descriptors
+    # read, write, exception lists with descriptors to poll
+    rlist, wlist, elist = [RD], [], []
+
+    # 3. Install signal handler for USR1 signal
+    signal.signal(signal.SIGUSR1, handler)
+
+    # A racing can happen at this point if the signal arrives before
+    # the call to 'select' - but we're ready for it, bring it on!
+
+    # call 'select' in loop - this way we determine the signal arrival
+    # through examinination of the read end of the pipe (RD descriptor)
+    while True:
+        try:
+            readables, writables, exceptions = select.select(
+                rlist, wlist, elist)
+        except select.error as e:
+            code, msg = e.args
+            # restart if interrupted by a signal
+            if code == errno.EINTR:
+                continue
+            else:
+                raise
+        else:
+            # we have result - break the loop
+            break
+
+    # 4. Check if our pipe's read end descriptor is in the result set
+    if RD in readables:
+        print 'Got signal'
+        # 5. Read all the bytes, because multiple
+        # signal may have arrived
+        while True:
+            try:
+                os.read(RD, 1)
+            except OSError as e:
+                if e.errno == errno.EAGAIN:
+                    # the pipe is drained - exit
+                    break
+                else:
+                    raise
+    else:
+        print 'Something unexpected happened'
+
+
+if __name__ == '__main__':
+    main()
+