new version by Tim

Demo/threads/sync.py

@@ -258,6 +258,15 @@
 #      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.
+#
+#   .write_to_read()
+#      Use instead of a .write_in to declare that the thread is done
+#      writing but wants to continue reading without other writers
+#      intervening.  If there are other threads waiting to write, they
+#      are allowed to proceed only if the current thread calls
+#      .read_out; threads waiting to read are only allowed to proceed
+#      if there are are no threads waiting to write.  (This is a
+#      weakness of the interface!)
 
 import thread
 
@@ -464,6 +473,18 @@ class mrsw:
             self.readOK.broadcast()
         self.rwOK.release()
 
+    def write_to_read(self):
+	self.rwOK.acquire()
+	if not self.writing:
+	    raise ValueError, \
+		  '.write_to_read() invoked without an active writer'
+	self.writing = 0
+	self.nw = self.nw - 1
+	self.nr = self.nr + 1
+	if not self.nw:
+	    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