Report configuration and HTTP startup failures

Signed-off-by: Sean Du <do4suki@gmail.com>

config/config_test.go

@@ -0,0 +1,50 @@
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestLoadCreatesDefaultConfig(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "config.json")
+
+	conf, err := Load(path)
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if conf.Settings.ListenPort != "8080" {
+		t.Fatalf("ListenPort = %q, want 8080", conf.Settings.ListenPort)
+	}
+	if _, err := os.Stat(path); err != nil {
+		t.Fatalf("default config was not written: %v", err)
+	}
+}
+
+func TestLoadBacksUpInvalidConfig(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "config.json")
+	if err := os.WriteFile(path, []byte("{"), 0644); err != nil {
+		t.Fatalf("write invalid config: %v", err)
+	}
+
+	conf, err := Load(path)
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if conf.Settings.ListenPort != "8080" {
+		t.Fatalf("ListenPort = %q, want 8080", conf.Settings.ListenPort)
+	}
+
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		t.Fatalf("ReadDir: %v", err)
+	}
+	for _, entry := range entries {
+		if strings.HasPrefix(entry.Name(), "config.json.backup") {
+			return
+		}
+	}
+	t.Fatal("invalid config backup was not created")
+}