Added optional lock parameter to condition class.

Added mrsw (multiple-reader-single-writer) lock.

Demo/threads/sync.py

 # Defines classes that provide synchronization objects.  Note that use of
 # this module requires that your Python support threads.
 #
-#    condition()   # a POSIX-like condition-variable object
-#    barrier(n)    # an n-thread barrier
-#    event()       # an event object
-#    semaphore(n=1)# a semaphore object, with initial count n
+#    condition(lock=None)       # a POSIX-like condition-variable object
+#    barrier(n)                 # an n-thread barrier
+#    event()                    # an event object
+#    semaphore(n=1)             # a semaphore object, with initial count n
+#    mrsw()                     # a multiple-reader single-writer lock
 #
 # CONDITIONS
 #
 # A condition object is created via
 #   import this_module
-#   your_condition_object = this_module.condition()
+#   your_condition_object = this_module.condition(lock=None)
+#
+# As explained below, a condition object has a lock associated with it,
+# used in the protocol to protect condition data.  You can specify a
+# lock to use in the constructor, else the constructor will allocate
+# an anonymous lock for you.  Specifying a lock explicitly can be useful
+# when more than one condition keys off the same set of shared data.
 #
 # Methods:
 #   .acquire()
@@ -209,13 +216,63 @@
 #      any) blocked by a .p().  It's an (detected) error for a .v() to
 #      increase the semaphore's count to a value larger than the initial
 #      count.
+#
+# MULTIPLE-READER SINGLE-WRITER LOCKS
+#
+# A mrsw lock is created via
+#   import this_module
+#   your_mrsw_lock = this_module.mrsw()
+#
+# This kind of lock is often useful with complex shared data structures.
+# The object lets any number of "readers" proceed, so long as no thread
+# wishes to "write".  When a (one or more) thread declares its intention
+# to "write" (e.g., to update a shared structure), all current readers
+# are allowed to finish, and then a writer gets exclusive access; all
+# other readers & writers are blocked until the current writer completes.
+# Finally, if some thread is waiting to write and another is waiting to
+# read, the writer takes precedence.
+#
+# Methods:
+#
+#   .read_in()
+#      If no thread is writing or waiting to write, returns immediately.
+#      Else blocks until no thread is writing or waiting to write.  So
+#      long as some thread has completed a .read_in but not a .read_out,
+#      writers are blocked.
+#
+#   .read_out()
+#      Use sometime after a .read_in to declare that the thread is done
+#      reading.  When all threads complete reading, a writer can proceed.
+#
+#   .write_in()
+#      If no thread is writing (has completed a .write_in, but hasn't yet
+#      done a .write_out) or reading (similarly), returns immediately.
+#      Else blocks the calling thread, and threads waiting to read, until
+#      the current writer completes writing or all the current readers
+#      complete reading; if then more than one thread is waiting to
+#      write, one of them is allowed to proceed, but which one is not
+#      specified.
+#
+#   .write_out()
+#      Use sometime after a .write_in to declare that the thread is done
+#      writing.  Then if some other thread is waiting to write, it's
+#      allowed to proceed.  Else all threads (if any) waiting to read are
+#      allowed to proceed.
 
 import thread
 
 class condition:
-    def __init__(self):
+    def __init__(self, lock=None):
         # the lock actually used by .acquire() and .release()
-        self.mutex = thread.allocate_lock()
+        if lock is None:
+            self.mutex = thread.allocate_lock()
+        else:
+            if hasattr(lock, 'acquire') and \
+               hasattr(lock, 'release'):
+                self.mutex = lock
+            else:
+                raise TypeError, 'condition constructor requires ' \
+                                 'a lock argument'
 
         # lock used to block threads until a signal
         self.checkout = thread.allocate_lock()
@@ -357,6 +414,56 @@ class semaphore:
         self.nonzero.signal()
         self.nonzero.release()
 
+class mrsw:
+    def __init__(self):
+        # critical-section lock & the data it protects
+        self.rwOK = thread.allocate_lock()
+        self.nr = 0  # number readers actively reading (not just waiting)
+        self.nw = 0  # number writers either waiting to write or writing
+        self.writing = 0  # 1 iff some thread is writing
+
+        # conditions
+        self.readOK  = condition(self.rwOK)  # OK to unblock readers
+        self.writeOK = condition(self.rwOK)  # OK to unblock writers
+
+    def read_in(self):
+        self.rwOK.acquire()
+        while self.nw:
+            self.readOK.wait()
+        self.nr = self.nr + 1
+        self.rwOK.release()
+
+    def read_out(self):
+        self.rwOK.acquire()
+        if self.nr <= 0:
+            raise ValueError, \
+                  '.read_out() invoked without an active reader'
+        self.nr = self.nr - 1
+        if self.nr == 0:
+            self.writeOK.signal()
+        self.rwOK.release()
+
+    def write_in(self):
+        self.rwOK.acquire()
+        self.nw = self.nw + 1
+        while self.writing or self.nr:
+            self.writeOK.wait()
+        self.writing = 1
+        self.rwOK.release()
+
+    def write_out(self):
+        self.rwOK.acquire()
+        if not self.writing:
+            raise ValueError, \
+                  '.write_out() invoked without an active writer'
+        self.writing = 0
+        self.nw = self.nw - 1
+        if self.nw:
+            self.writeOK.signal()
+        else:
+            self.readOK.broadcast()
+        self.rwOK.release()
+
 # The rest of the file is a test case, that runs a number of parallelized
 # quicksorts in parallel.  If it works, you'll get about 600 lines of
 # tracing output, with a line like