Add comments explaining thread unsafety of this code.

gvanrossum · gvanrossum · commit cc20b76ad00d · 1998-05-29T17:51:59.000Z
diff --git 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.
@@ -60,13 +67,16 @@ def seed(self, x = 0, y = 0, z = 0):
 	# 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
 		y = (172 * y) % 30307
 		z = (170 * z) % 30323
 		#
 		self._seed = x, y, z
+		# END CRITICAL SECTION
 		#
 		return (x/30269.0 + y/30307.0 + z/30323.0) % 1.0
 	#

Original file line number	Diff line number	Diff line change
`@@ -29,6 +29,13 @@`
`29`	`29`	`# Adrian Baddeley.`
`30`	`30`
`31`	`31`
	`32`	`+# Multi-threading note: the random number generator used here is not`
	`33`	`+# thread-safe; it is possible that nearly simultaneous calls in`
	`34`	`+# different theads return the same random value. To avoid this, you`
	`35`	`+# have to use a lock around all calls. (I didn't want to slow this`
	`36`	`+# down in the serial case by using a lock here.)`
	`37`	`+`
	`38`	`+`
`32`	`39`	`class whrandom:`
`33`	`40`	`#`
`34`	`41`	`# Initialize an instance.`
`@@ -60,13 +67,16 @@ def seed(self, x = 0, y = 0, z = 0):`
`60`	`67`	`# Get the next random number in the range [0.0, 1.0).`
`61`	`68`	`#`
`62`	`69`	`def random(self):`
	`70`	`+ # This part is thread-unsafe:`
	`71`	`+ # BEGIN CRITICAL SECTION`
`63`	`72`	`x, y, z = self._seed`
`64`	`73`	`#`
`65`	`74`	`x = (171 * x) % 30269`
`66`	`75`	`y = (172 * y) % 30307`
`67`	`76`	`z = (170 * z) % 30323`
`68`	`77`	`#`
`69`	`78`	`self._seed = x, y, z`
	`79`	`+ # END CRITICAL SECTION`
`70`	`80`	`#`
`71`	`81`	`return (x/30269.0 + y/30307.0 + z/30323.0) % 1.0`
`72`	`82`	`#`