feat: support default values in tls.NewConfigFromEnv

tls/config.go

@@ -23,6 +23,10 @@ import (
 	"strings"
 )
 
+// DefaultMinTLSVersion is the minimum TLS version used when no override
+// is provided via environment variable or caller-supplied defaults.
+const DefaultMinTLSVersion = cryptotls.VersionTLS13
+
 // Environment variable name suffixes for TLS configuration.
 // Use with a prefix to namespace them, e.g. "WEBHOOK_" + MinVersionEnvKey
 // reads the WEBHOOK_TLS_MIN_VERSION variable.
@@ -46,9 +50,19 @@ type Config struct {
 // returns a Config. The prefix is prepended to each standard env-var suffix;
 // for example with prefix "WEBHOOK_" the function reads
 // WEBHOOK_TLS_MIN_VERSION, WEBHOOK_TLS_MAX_VERSION, etc.
-// Fields whose corresponding env var is unset are left at their zero value.
-func NewConfigFromEnv(prefix string) (*Config, error) {
+//
+// An optional defaults Config may be supplied; its values are used for any
+// field whose corresponding environment variable is unset. When no defaults
+// are provided, unset fields remain at their zero value.
+func NewConfigFromEnv(prefix string, defaults ...Config) (*Config, error) {
+	if len(defaults) > 1 {
+		return nil, fmt.Errorf("at most one defaults Config may be provided, got %d", len(defaults))
+	}
+
 	var cfg Config
+	if len(defaults) == 1 {
+		cfg = defaults[0]
+	}
 
 	if v := os.Getenv(prefix + MinVersionEnvKey); v != "" {
 		ver, err := parseVersion(v)

tls/config_test.go

@@ -349,6 +349,68 @@ func TestNewConfigFromEnv(t *testing.T) {
 			t.Fatal("expected error for invalid curve")
 		}
 	})
+
+	t.Run("defaults used when env vars are unset", func(t *testing.T) {
+		defaults := Config{
+			MinVersion:   DefaultMinTLSVersion,
+			MaxVersion:   cryptotls.VersionTLS13,
+			CipherSuites: []uint16{cryptotls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256},
+			CurvePreferences: []cryptotls.CurveID{
+				cryptotls.X25519,
+			},
+		}
+		cfg, err := NewConfigFromEnv("", defaults)
+		if err != nil {
+			t.Fatal("unexpected error:", err)
+		}
+		if cfg.MinVersion != DefaultMinTLSVersion {
+			t.Errorf("MinVersion = %d, want %d", cfg.MinVersion, DefaultMinTLSVersion)
+		}
+		if cfg.MaxVersion != cryptotls.VersionTLS13 {
+			t.Errorf("MaxVersion = %d, want %d", cfg.MaxVersion, cryptotls.VersionTLS13)
+		}
+		if len(cfg.CipherSuites) != 1 || cfg.CipherSuites[0] != cryptotls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 {
+			t.Errorf("CipherSuites = %v, want [%d]", cfg.CipherSuites, cryptotls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256)
+		}
+		if len(cfg.CurvePreferences) != 1 || cfg.CurvePreferences[0] != cryptotls.X25519 {
+			t.Errorf("CurvePreferences = %v, want [%d]", cfg.CurvePreferences, cryptotls.X25519)
+		}
+	})
+
+	t.Run("env vars override defaults", func(t *testing.T) {
+		t.Setenv(MinVersionEnvKey, "1.2")
+		t.Setenv(CurvePreferencesEnvKey, "CurveP256")
+
+		defaults := Config{
+			MinVersion: DefaultMinTLSVersion,
+			CurvePreferences: []cryptotls.CurveID{
+				cryptotls.X25519,
+			},
+		}
+		cfg, err := NewConfigFromEnv("", defaults)
+		if err != nil {
+			t.Fatal("unexpected error:", err)
+		}
+		if cfg.MinVersion != cryptotls.VersionTLS12 {
+			t.Errorf("MinVersion = %d, want %d (env should override default)", cfg.MinVersion, cryptotls.VersionTLS12)
+		}
+		if len(cfg.CurvePreferences) != 1 || cfg.CurvePreferences[0] != cryptotls.CurveP256 {
+			t.Errorf("CurvePreferences = %v, want [%d] (env should override default)", cfg.CurvePreferences, cryptotls.CurveP256)
+		}
+	})
+
+	t.Run("multiple defaults returns error", func(t *testing.T) {
+		_, err := NewConfigFromEnv("", Config{}, Config{})
+		if err == nil {
+			t.Fatal("expected error when passing multiple defaults")
+		}
+	})
+
+	t.Run("DefaultMinTLSVersion equals TLS 1.3", func(t *testing.T) {
+		if DefaultMinTLSVersion != cryptotls.VersionTLS13 {
+			t.Errorf("DefaultMinTLSVersion = %d, want %d (TLS 1.3)", DefaultMinTLSVersion, cryptotls.VersionTLS13)
+		}
+	})
 }
 
 func TestConfig_TLSConfig(t *testing.T) {

webhook/webhook.go

@@ -191,20 +191,17 @@ func New(
 
 	logger := logging.FromContext(ctx)
 
-	tlsCfg, err := knativetls.NewConfigFromEnv("WEBHOOK_")
+	tlsCfg, err := knativetls.NewConfigFromEnv("WEBHOOK_", knativetls.Config{
+		MinVersion: knativetls.DefaultMinTLSVersion,
+	})
 	if err != nil {
 		return nil, fmt.Errorf("reading TLS configuration from environment: %w", err)
 	}
 
-	// Replace the TLS configuration with the one from the environment if not set.
-	// Default to TLS 1.3 as the minimum version when neither the caller nor the
-	// environment specifies one.
-	if opts.TLSMinVersion == 0 {
-		if tlsCfg.MinVersion != 0 {
-			opts.TLSMinVersion = tlsCfg.MinVersion
-		} else {
-			opts.TLSMinVersion = tls.VersionTLS13
-		}
+	// Apply environment / default TLS settings for any fields the caller
+	// has not already configured.
+	if opts.TLSMinVersion == 0 && tlsCfg.MinVersion != 0 {
+		opts.TLSMinVersion = tlsCfg.MinVersion
 	}
 	if opts.TLSMaxVersion == 0 && tlsCfg.MaxVersion != 0 {
 		opts.TLSMaxVersion = tlsCfg.MaxVersion