[MM-29296] model/user: add lock to source (#16054)

* model/user: add lock to source

* guard rand creation with locking

* use locked random

* remove assignment

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

model/user_test.go

@@ -353,7 +353,9 @@ func TestUserSlice(t *testing.T) {
 }
 
 func TestGeneratePassword(t *testing.T) {
-	passwordRandomSource = rand.NewSource(12345)
+	passwordRandom = lockedRand{
+		rn: rand.New(rand.NewSource(12345)),
+	}
 
 	t.Run("Should be the minimum length or 4, whichever is less", func(t *testing.T) {
 		password1 := GeneratePassword(5)
@@ -372,4 +374,10 @@ func TestGeneratePassword(t *testing.T) {
 		assert.Contains(t, []rune(passwordLowerCaseLetters), []rune(password)[2])
 		assert.Contains(t, []rune(passwordSpecialChars), []rune(password)[3])
 	})
+
+	t.Run("Should not fail on concurrent calls", func(t *testing.T) {
+		for i := 0; i < 10; i++ {
+			go GeneratePassword(10)
+		}
+	})
 }