Add comments explaining thread unsafety of this code.

diff --git a/Lib/whrandom.py b/Lib/whrandom.py
index 95e88f3399b53683c21901fc6f8fc49e5db1886b..7f33fd60799d4dca632a491a610cc2f0afe4c4bf 100644 (file)
--- a/Lib/whrandom.py
+++ b/Lib/whrandom.py
@@ -29,6 +29,13 @@
 #      Adrian Baddeley.
 
 
+# Multi-threading note: the random number generator used here is not
+# thread-safe; it is possible that nearly simultaneous calls in
+# different theads return the same random value.  To avoid this, you
+# have to use a lock around all calls.  (I didn't want to slow this
+# down in the serial case by using a lock here.)
+
+
 class whrandom:
        #
        # Initialize an instance.
@@ -60,6 +67,8 @@ class whrandom:
        # Get the next random number in the range [0.0, 1.0).
        #
        def random(self):
+               # This part is thread-unsafe:
+               # BEGIN CRITICAL SECTION
                x, y, z = self._seed
                #
                x = (171 * x) % 30269
@@ -67,6 +76,7 @@ class whrandom:
                z = (170 * z) % 30323
                #
                self._seed = x, y, z
+               # END CRITICAL SECTION
                #
                return (x/30269.0 + y/30307.0 + z/30323.0) % 1.0
        #