math/rand: document that NewSource sources race

While it was previously explicitly documented that "the default Source"
is safe for concurrent use, a careless reader can interpret that as
meaning "the implementation of the Source interface created by functions
in this package" rather than "the default shared Source used by
top-level functions". Be explicit that the Source returned by NewSource
is not safe for use by multiple goroutines.

Fixes #3611.

Change-Id: Iae4bc04c3887ad6e2491e36e38feda40324022c5
Reviewed-on: https://go-review.googlesource.com/25501
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

diff --git a/src/math/rand/rand.go b/src/math/rand/rand.go
index dd8d43cca1c18d81fd57419d802aa7e29c96d790..5b271e62a21a8fbc5a218f1dbee6e3499bfc7d4c 100644 (file)
--- a/src/math/rand/rand.go
+++ b/src/math/rand/rand.go
@@ -24,6 +24,8 @@ type Source interface {
 }
 
 // NewSource returns a new pseudo-random Source seeded with the given value.
+// Unlike the default Source used by top-level functions, this source is not
+// safe for concurrent use by multiple goroutines.
 func NewSource(seed int64) Source {
        var rng rngSource
        rng.Seed(seed)