fix(oidc): retry issuer discovery with bounded back-off at start-up

A 503 from the issuer's discovery endpoint made `serve` exit and the pod
restart. New now retries discovery 5 times (2+4+8+16 s back-off, ~30 s)
before failing as before; a cancelled ctx ends the back-off at once.
WithDiscoveryRetry lets tests shrink the schedule.

Closes #59

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/web/oidc/oidc.go

@@ -57,6 +57,10 @@ const (
 	pendingTTL        = 10 * time.Minute
 	sessionCookie     = "tapir_session"
 	loginPath         = "/auth/login"
+
+	// Discovery retry (#59): 5 tries, 2+4+8+16 s back-off ≈ 30 s, then fail.
+	defaultDiscoveryAttempts = 5
+	defaultDiscoveryDelay    = 2 * time.Second
 )
 
 // DexAuth implements web.Auth against Dex.
@@ -70,6 +74,9 @@ type DexAuth struct {
 	sessionTTL time.Duration
 	now        func() time.Time
 	insecure   bool // tests only: drop the Secure attribute so cookies flow over http
+
+	discoveryAttempts int
+	discoveryDelay    time.Duration
 }
 
 var _ web.Auth = (*DexAuth)(nil)
@@ -96,14 +103,55 @@ func WithSessionTTL(ttl time.Duration) Option {
 	}
 }
 
+// WithDiscoveryRetry overrides the discovery attempt count and the initial
+// back-off delay (doubled after each failed attempt).
+func WithDiscoveryRetry(attempts int, delay time.Duration) Option {
+	return func(d *DexAuth) {
+		if attempts > 0 {
+			d.discoveryAttempts = attempts
+		}
+		if delay > 0 {
+			d.discoveryDelay = delay
+		}
+	}
+}
+
 // WithInsecureCookies drops the Secure cookie attribute. TEST ONLY — it lets
 // the session cookie travel over plain http; never use it in production.
 func WithInsecureCookies() Option {
 	return func(d *DexAuth) { d.insecure = true }
 }
 
-// New discovers the issuer and returns a configured DexAuth. ctx bounds the
-// discovery request only.
+// discover runs OIDC discovery up to attempts times, doubling delay between
+// tries (#59: an issuer briefly unavailable behind its proxy answered 503 and
+// serve exited). A cancelled ctx ends the back-off immediately with ctx's error.
+func discover(ctx context.Context, issuer string, attempts int, delay time.Duration) (*oidc.Provider, error) {
+	var lastErr error
+	for i := range attempts {
+		if i > 0 {
+			t := time.NewTimer(delay)
+			select {
+			case <-ctx.Done():
+				t.Stop()
+				return nil, fmt.Errorf("after %d attempts (last: %v): %w", i, lastErr, ctx.Err())
+			case <-t.C:
+			}
+			delay *= 2
+		}
+		p, err := oidc.NewProvider(ctx, issuer)
+		if err == nil {
+			return p, nil
+		}
+		lastErr = err
+		if ctx.Err() != nil {
+			return nil, fmt.Errorf("after %d attempts: %w", i+1, ctx.Err())
+		}
+	}
+	return nil, fmt.Errorf("after %d attempts: %w", attempts, lastErr)
+}
+
+// New discovers the issuer and returns a configured DexAuth. ctx bounds discovery,
+// including its retry back-off.
 func New(ctx context.Context, cfg Config, opts ...Option) (*DexAuth, error) {
 	for name, val := range map[string]string{
 		"issuer":         cfg.Issuer,
@@ -117,29 +165,31 @@ func New(ctx context.Context, cfg Config, opts ...Option) (*DexAuth, error) {
 		}
 	}
 
-	provider, err := oidc.NewProvider(ctx, cfg.Issuer)
-	if err != nil {
-		return nil, fmt.Errorf("oidc: discover issuer: %w", err)
-	}
-
 	d := &DexAuth{
-		cfg:      cfg,
-		verifier: provider.Verifier(&oidc.Config{ClientID: cfg.ClientID}),
-		oauth: &oauth2.Config{
-			ClientID:     cfg.ClientID,
-			ClientSecret: cfg.ClientSecret,
-			Endpoint:     provider.Endpoint(),
-			RedirectURL:  cfg.RedirectURL,
-			Scopes:       []string{oidc.ScopeOpenID, "profile", "email"},
-		},
-		pending:    newPendingStore(),
-		secret:     []byte(cfg.SessionSecret),
-		sessionTTL: defaultSessionTTL,
-		now:        time.Now,
+		cfg:               cfg,
+		pending:           newPendingStore(),
+		secret:            []byte(cfg.SessionSecret),
+		sessionTTL:        defaultSessionTTL,
+		now:               time.Now,
+		discoveryAttempts: defaultDiscoveryAttempts,
+		discoveryDelay:    defaultDiscoveryDelay,
 	}
 	for _, o := range opts {
 		o(d)
 	}
+
+	provider, err := discover(ctx, cfg.Issuer, d.discoveryAttempts, d.discoveryDelay)
+	if err != nil {
+		return nil, fmt.Errorf("oidc: discover issuer: %w", err)
+	}
+	d.verifier = provider.Verifier(&oidc.Config{ClientID: cfg.ClientID})
+	d.oauth = &oauth2.Config{
+		ClientID:     cfg.ClientID,
+		ClientSecret: cfg.ClientSecret,
+		Endpoint:     provider.Endpoint(),
+		RedirectURL:  cfg.RedirectURL,
+		Scopes:       []string{oidc.ScopeOpenID, "profile", "email"},
+	}
 	return d, nil
 }