fix(11-04): keep earlier overrides when compass Persist saves

Persist rewrote overrides.yaml with only this process's runtime values,
so saving one key (for example websockets:generate-vapid-keys --update)
dropped every key persisted earlier. It now starts from the saved file
and lets runtime values win.

modules/compass/persist_test.go

@@ -115,3 +115,44 @@ func TestPersistRejectsInvalidEnvName(t *testing.T) {
 		t.Fatalf("error should not echo unsanitized env path: %v", err)
 	}
 }
+
+func TestPersistKeepsEarlierOverrides(t *testing.T) {
+	dir := writeConfigTree(t, map[string]string{
+		"app.yaml":                       "name: base\n",
+		"env/development/overrides.yaml": "app:\n  name: persisted\n",
+	})
+	cfg, err := Open(Options{Dir: dir, Env: "development", Environ: []string{}})
+	if err != nil {
+		t.Fatalf("Open: %v", err)
+	}
+	if err := cfg.Set("push.public_key", "saved-key"); err != nil {
+		t.Fatalf("Set: %v", err)
+	}
+	if err := cfg.Persist(); err != nil {
+		t.Fatalf("Persist: %v", err)
+	}
+	again, err := Open(Options{Dir: dir, Env: "development", Environ: []string{}})
+	if err != nil {
+		t.Fatalf("reopen: %v", err)
+	}
+	if got := again.String("push.public_key"); got != "saved-key" {
+		t.Fatalf("push.public_key = %q, want saved-key", got)
+	}
+	if got := again.String("app.name"); got != "persisted" {
+		t.Fatalf("app.name = %q, want the earlier persisted value", got)
+	}
+	// A runtime value replaces the persisted value of the same key.
+	if err := again.Set("app.name", "runtime"); err != nil {
+		t.Fatal(err)
+	}
+	if err := again.Persist(); err != nil {
+		t.Fatal(err)
+	}
+	third, err := Open(Options{Dir: dir, Env: "development", Environ: []string{}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if third.String("app.name") != "runtime" || third.String("push.public_key") != "saved-key" {
+		t.Fatalf("after second Persist: app.name=%q push.public_key=%q", third.String("app.name"), third.String("push.public_key"))
+	}
+}