[Checkins] SVN: zc.twist/trunk/src/zc/twist/ Fix floating-point additive accumulation error in the tests.

[Zvezdan Petkovic]

Log message for revision 104624:
  Fix floating-point additive accumulation error in the tests.
  
  The tests were failing 1 out of 10 times on average when run consecutively.
  With this patch
  
  	./bin/test -N 1000 --all
  
  passes without a single failure!
  
  

Changed:
  U   zc.twist/trunk/src/zc/twist/README.txt
  U   zc.twist/trunk/src/zc/twist/__init__.py

-=-
Modified: zc.twist/trunk/src/zc/twist/README.txt
===================================================================
--- zc.twist/trunk/src/zc/twist/README.txt	2009-09-29 19:13:11 UTC (rev 104623)
+++ zc.twist/trunk/src/zc/twist/README.txt	2009-09-29 20:01:40 UTC (rev 104624)
@@ -215,6 +215,10 @@
 reactor; we'll use a `time_flies` function, which takes seconds to move
 ahead, to simulate time passing in the reactor.
 
+We use powers of 2 for the floating-points numbers (e.g. 0.125) to avoid
+a floating-point additive accumulation error that happened in the tests
+when values such as 0.1 were used.
+
     >>> db.setPoolSize(1)
     >>> db.getPoolSize()
     1
@@ -226,7 +230,7 @@
     >>> d = deferred.addCallback(get_result)
     >>> call.attempt_count
     0
-    >>> time_flies(.1) >= 1 # returns number of connection attempts
+    >>> time_flies(.125) >= 1 # returns number of connection attempts
     True
     >>> call.attempt_count
     0
@@ -234,7 +238,7 @@
     >>> db.setPoolSize(2)
     >>> db.getPoolSize()
     2
-    >>> time_flies(.2) >= 1
+    >>> time_flies(.25) >= 1
     True
     >>> call.attempt_count > 0
     True
@@ -256,12 +260,12 @@
     >>> d = deferred.addCallback(get_result)
     >>> call.attempt_count
     0
-    >>> time_flies(.1) >= 1
+    >>> time_flies(.125) >= 1
     True
     >>> call.attempt_count
     0
     >>> res # None
-    >>> time_flies(2.0) >= 2 # for a total of at least 3
+    >>> time_flies(2) >= 2 # for a total of at least 3
     True
     >>> res
     2
@@ -800,7 +804,7 @@
     >>> d = deferred.addCallback(get_result)
     >>> call.attempt_count
     0
-    >>> time_flies(.1) >= 1 # returns number of connection attempts
+    >>> time_flies(.125) >= 1 # returns number of connection attempts
     True
     >>> call.attempt_count
     0
@@ -808,7 +812,7 @@
     >>> db.setPoolSize(2)
     >>> db.getPoolSize()
     2
-    >>> time_flies(.2) >= 1
+    >>> time_flies(.25) >= 1
     True
     >>> call.attempt_count > 0
     True
@@ -830,12 +834,12 @@
     >>> d = deferred.addCallback(get_result)
     >>> call.attempt_count
     0
-    >>> time_flies(.1) >= 1
+    >>> time_flies(.125) >= 1
     True
     >>> call.attempt_count
     0
     >>> res # None
-    >>> time_flies(1.9) >= 2 # for a total of at least 3
+    >>> time_flies(2) >= 2 # for a total of at least 3
     True
     >>> res
     2

Modified: zc.twist/trunk/src/zc/twist/__init__.py
===================================================================
--- zc.twist/trunk/src/zc/twist/__init__.py	2009-09-29 19:13:11 UTC (rev 104623)
+++ zc.twist/trunk/src/zc/twist/__init__.py	2009-09-29 20:01:40 UTC (rev 104624)
@@ -96,7 +96,7 @@
 def get_connection(db, deferred=None, backoff=0, reactor=None):
     if deferred is None:
         deferred = twisted.internet.defer.Deferred()
-    backoff += random.random() / 20.0 + .05 # .05 to .10 of a second
+    backoff += random.random() / 20.0 + .0625 # 1/16 second (USE POWERS OF 2!)
     # if this is taking too long (i.e., the cumulative backoff is taking
     # more than half a second) then we'll just take one.  This might be
     # a bad idea: we'll have to see in practice.  Otherwise, if the