1 files changed, 106 insertions, 0 deletions
diff --git a/demo/premise.py b/demo/premise.py
new file mode 100755
index 0000000..98dd30c
--- /dev/null
+++ b/demo/premise.py
@@ -0,0 +1,106 @@
+# Python modules
+import time
+import os
+import sys
+PY_MAJOR_VERSION = sys.version_info[0]
+# hashlib is only available in Python >= 2.5. I still want to support 
+# older Pythons so I import md5 if hashlib is not available. Fortunately
+# md5 can masquerade as hashlib for my purposes.
+try:
+    import hashlib
+except ImportError:
+    import md5 as hashlib
+
+# 3rd party modules
+import sysv_ipc
+
+# Utils for this demo
+import utils
+if PY_MAJOR_VERSION > 2:
+    import utils_for_3 as flex_utils
+else:
+    import utils_for_2 as flex_utils
+
+utils.say("Oooo 'ello, I'm Mrs. Premise!")
+
+params = utils.read_params()
+
+# Create the semaphore & shared memory. I read somewhere that semaphores 
+# and shared memory have separate key spaces, so one can safely use the 
+# same key for each. This seems to be true in my experience.
+
+# For purposes of simplicity, this demo code makes no allowance for the 
+# failure of the semaphore or memory constructors. This is unrealistic 
+# because one can never predict whether or not a given key will be available,
+# so your code must *always* be prepared for these functions to fail. 
+
+semaphore = sysv_ipc.Semaphore(params["KEY"], sysv_ipc.IPC_CREX)
+memory = sysv_ipc.SharedMemory(params["KEY"], sysv_ipc.IPC_CREX)
+
+# I seed the shared memory with a random value which is the current time.
+what_i_wrote = time.asctime()
+s = what_i_wrote
+
+utils.write_to_memory(memory, what_i_wrote)
+
+for i in range(0, params["ITERATIONS"]):
+    utils.say("iteration %d" % i)
+    if not params["LIVE_DANGEROUSLY"]:
+        # Releasing the semaphore...
+        utils.say("releasing the semaphore")
+        semaphore.release()
+        # ...and wait for it to become available again. In real code it'd be 
+        # wise to sleep briefly before calling .acquire() in order to be 
+        # polite and give other processes an opportunity to grab the semaphore
+        # while it is free and thereby avoid starvation. But this code is meant
+        # to be a stress test that maximizes the opportunity for shared memory
+        # corruption, and politeness has no place in that.
+        utils.say("acquiring the semaphore...")
+        semaphore.acquire()
+
+    s = utils.read_from_memory(memory)
+
+    # I keep checking the shared memory until something new has been written.
+    while s == what_i_wrote:
+        if not params["LIVE_DANGEROUSLY"]:
+            utils.say("releasing the semaphore")
+            semaphore.release()
+            utils.say("acquiring the semaphore...")
+            semaphore.acquire()
+
+        # Once the call to .acquire() completes, I own the shared resource and
+        # I'm free to read from the memory.
+        s = utils.read_from_memory(memory)
+
+    # What I read must be the md5 of what I wrote or something's gone wrong.
+    if PY_MAJOR_VERSION > 2:
+        what_i_wrote = what_i_wrote.encode()
+
+    try:
+        assert(s == hashlib.md5(what_i_wrote).hexdigest())
+    except AssertionError:
+        flex_utils.raise_error(AssertionError, 
+                        "Shared memory corruption after %d iterations." % i)
+
+    # MD5 the reply and write back to Mrs. Conclusion.
+    if PY_MAJOR_VERSION > 2:
+        s = s.encode()
+    what_i_wrote = hashlib.md5(s).hexdigest()
+    utils.write_to_memory(memory, what_i_wrote)
+
+
+# Announce for one last time that the semaphore is free again so that 
+# Mrs. Conclusion can exit.
+if not params["LIVE_DANGEROUSLY"]:
+    utils.say("Final release of the semaphore followed by a 5 second pause")
+    semaphore.release()
+    time.sleep(5)
+    # ...before beginning to wait until it is free again.
+    utils.say("Final acquisition of the semaphore")
+    semaphore.acquire()
+
+utils.say("Destroying semaphore and shared memory")
+# It'd be more natural to call memory.remove() and semaphore.remove() here, 
+# but I'll use the module-level functions instead to demonstrate their use.
+sysv_ipc.remove_shared_memory(memory.id)
+sysv_ipc.remove_semaphore(semaphore.id)