Use global rand.Rand instance in mathrand.

common/data/rand/mathrand/impl.go

 // Copyright 2016 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 package mathrand
 
 import (
         "math/rand"
         "sync"
 )
@@ skipping 76 matching lines @@
         // the output using:
         //
         //  sample = ExpFloat64(ctx) / desiredRateParameter
         //
         ExpFloat64() float64
 
         // WithGoRand invokes the supplied "fn" while holding an exclusive lock
         // for it. This can be used by callers to pull and use a *rand.Rand instance
         // out of the Context safely.
         //
+        // Other "mathrand" functions and objects MUST NOT BE USED inside the
+        // callback, as WithGoRand holds the lock to the current Rand instance, so any
+        // additional function call will deadlock.
+        //
         // The callback's r must not be retained or used outside of hte scope of the
         // callback.
         WithGoRand(fn func(r *rand.Rand) error) error
 }
 
 // Locking wraps a Rand instance in a layer that locks around all of its
 // methods.
 //
 // A user must hold Locking's Mutex if the want to directly access and use
 // Locking's R member safely.
@@ skipping 153 matching lines @@
 
 // wrapped is a simple wrapper to a *math.Rand instance.
 type wrapped struct {
         *rand.Rand
 }
 
 // Wrap wraps a Rand instance, allowing it to satisfy the Rand interface.
 func wrapRand(r *rand.Rand) Rand { return wrapped{r} }
 
 func (w wrapped) WithGoRand(fn func(r *rand.Rand) error) error { return fn(w.Rand) }