test/chan: avoid wrap-around in memstats comparison

The select2.go test assumed that the memory allocated between
its two samplings of runtime.ReadMemStats is strictly
increasing.  To avoid failing the tests when this is not true,
a greater-than check is introduced before computing the
difference in allocated memory.

R=golang-dev, r, cshapiro
CC=golang-dev
https://golang.org/cl/13701046

diff --git a/test/chan/select2.go b/test/chan/select2.go
index 4a08139126192f87e72a39fed7ceb41a6ab90112..ccf9dab81bc2aab83c599ee173ed3123dcf31f34 100644 (file)
--- a/test/chan/select2.go
+++ b/test/chan/select2.go
@@ -47,7 +47,8 @@ func main() {
        runtime.GC()
        runtime.ReadMemStats(memstats)
 
-       if memstats.Alloc-alloc > 1.1e5 {
+       // Be careful to avoid wraparound.
+       if memstats.Alloc > alloc && memstats.Alloc-alloc > 1.1e5 {
                println("BUG: too much memory for 100,000 selects:", memstats.Alloc-alloc)
        }
 }