Harden configuration validation

internal/app/test_environment_test.go

@@ -0,0 +1,31 @@
+package app
+
+import (
+	"os"
+	"testing"
+)
+
+// restoreEnvAfterTest clears names for a test and restores both their prior
+// values and their set/unset state during cleanup.
+func restoreEnvAfterTest(t *testing.T, names ...string) {
+	t.Helper()
+	originals := make(map[string]string, len(names))
+	present := make(map[string]bool, len(names))
+	for _, name := range names {
+		value, ok := os.LookupEnv(name)
+		originals[name] = value
+		present[name] = ok
+		if err := os.Unsetenv(name); err != nil {
+			t.Fatalf("Unsetenv(%q): %v", name, err)
+		}
+	}
+	t.Cleanup(func() {
+		for _, name := range names {
+			if present[name] {
+				_ = os.Setenv(name, originals[name])
+			} else {
+				_ = os.Unsetenv(name)
+			}
+		}
+	})
+}