internal/globalconfig: make statsTags un-changeable from outside of the globalconfig package

internal/globalconfig/globalconfig.go

@@ -80,15 +80,21 @@ func SetDogstatsdAddr(addr string) {
 func StatsTags() []string {
 	cfg.mu.RLock()
 	defer cfg.mu.RUnlock()
-	return cfg.statsTags
+	// Copy the slice before returning it, so that callers cannot pollute the underlying array
+	tags := make([]string, len(cfg.statsTags))
+	copy(tags, cfg.statsTags)
+	return tags
 }
 
 // SetStatsTags configures the list of tags that should be applied to contribs' statsd.Client as global tags
 // It should only be called by the tracer package
 func SetStatsTags(tags []string) {
 	cfg.mu.Lock()
 	defer cfg.mu.Unlock()
-	cfg.statsTags = tags
+	// Copy the slice before setting it, so that any changes to the slice provided to SetStatsTags does not pollute the underlying array of statsTags
+	statsTags := make([]string, len(tags))
+	copy(statsTags, tags)
+	cfg.statsTags = statsTags
 }
 
 // RuntimeID returns this process's unique runtime id.

internal/globalconfig/globalconfig_test.go

@@ -18,3 +18,17 @@ func TestHeaderTag(t *testing.T) {
 	assert.Equal(t, "tag1", cfg.headersAsTags.Get("header1"))
 	assert.Equal(t, "tag2", cfg.headersAsTags.Get("header2"))
 }
+
+// Assert that APIs to access cfg.statsTags protect against pollution from external changes
+func TestStatsTags(t *testing.T) {
+	array := [6]string{"aaa", "bbb", "ccc"}
+	slice1 := array[:]
+	SetStatsTags(slice1)
+	slice1 = append(slice1, []string{"ddd", "eee", "fff"}...)
+	slice1[0] = "zzz"
+	assert.Equal(t, cfg.statsTags[:3], []string{"aaa", "bbb", "ccc"})
+
+	tags := StatsTags()
+	tags[1] = "yyy"
+	assert.Equal(t, cfg.statsTags[1], "bbb")
+}