Refactor a couple parts of the boundary random sourcing

Signed-off-by: James Hillyerd <james@hillyerd.com>

builder.go

@@ -400,21 +400,15 @@ func (p MailBuilder) Build() (*Part, error) {
 		}
 	}
 	if r := p.randSource; r != nil {
-		root.propagateRand(r)
+		// Traverse all parts, discard match result.
+		_ = root.DepthMatchAll(func(part *Part) bool {
+			part.randSource = r
+			return false
+		})
 	}
 	return root, nil
 }
 
-func (p *Part) propagateRand(rand rand.Source) {
-	p.randSource = rand
-	for _, x := range []*Part{p.FirstChild, p.NextSibling} {
-		if x == nil {
-			continue
-		}
-		x.propagateRand(rand)
-	}
-}
-
 // SendWithReversePath encodes the message and sends it via the specified Sender.
 func (p MailBuilder) SendWithReversePath(sender Sender, from string) error {
 	buf := &bytes.Buffer{}

internal/stringutil/rand_source.go

@@ -15,13 +15,12 @@ func init() {
 // NewLockedSource creates a source of randomness using the given seed.
 func NewLockedSource(seed int64) rand.Source64 {
 	return &lockedSource{
-		lock: new(sync.Mutex),
-		s:    rand.NewSource(seed).(rand.Source64),
+		s: rand.NewSource(seed).(rand.Source64),
 	}
 }
 
 type lockedSource struct {
-	lock sync.Locker
+	lock sync.Mutex
 	s    rand.Source64
 }