harden(auth/session+oidc): 503/401 split + go-oidc string pin (LOW-6 + Nit-2)

Audit 2026-05-10 — close LOW-6 + Nit-2 from the HANDOFF.md backend
batch (items 8 + 9).

LOW-6: introduce ErrSessionTransient sentinel in session.Service.
session.Validate now distinguishes:
  - errors.Is(err, repository.ErrSessionNotFound) → ErrSessionInvalidCookie (401)
  - All other repo errors                         → ErrSessionTransient (503)
The session middleware maps ErrSessionTransient to HTTP 503 with
Retry-After: 1. Pre-fix, every DB hiccup looked like a forged-cookie
401 and forced the user to re-authenticate on a transient outage.
Two new regression tests pin the wire shape:
  - TestService_Validate_TransientSessionGetError (service layer)
  - TestService_Validate_SessionNotFoundMapsToInvalidCookie (negative
    leg: not-found stays 401)
  - TestSessionMiddleware_TransientErrorMappedTo503 (middleware-level
    503 + Retry-After header)

Nit-2: isJWKSFetchError documentation now pins go-oidc/v3 v3.18.0 as
the source-of-truth string set. v3.18.0 exposes only
*oidc.TokenExpiredError as a typed error; JWKS-fetch failures bubble
up as fmt.Errorf-wrapped strings. New regression test
TestIsJWKSFetchError_GoOIDCV318Strings pins the canonical substrings
emitted by go-oidc's jwks.go — a future upstream bump that changes
the wording trips the test and forces the matcher to be re-derived.
The test caught a real gap: 'oidc: failed to decode keys' (emitted
when the IdP returns non-JSON at the jwks_uri — broken proxy, gateway
HTML error page, etc.) was previously misclassified as a generic 500
instead of 503 ErrJWKSUnreachable. Added 'decode keys' substring to
the matcher.

Status: LOW-6 + Nit-2 marked CLOSED in audit-doc table.

Refs: cowork/auth-bundles-fixes-2026-05-10/HANDOFF.md items 8, 9
      cowork/auth-bundles-audit-2026-05-10.md LOW-6, Nit-2

internal/auth/session/middleware.go

@@ -90,6 +90,20 @@ func NewSessionMiddleware(svc SessionValidator) func(http.Handler) http.Handler
 				UserAgent:   r.UserAgent(),
 			})
 			if verr != nil {
+				// Audit 2026-05-10 LOW-6 closure — ErrSessionTransient
+				// means the backend hit a retryable error (DB hiccup,
+				// connection reset, etc.) rather than the cookie being
+				// malformed. Surface 503 + Retry-After so well-behaved
+				// clients (curl --retry, browser fetch automatic retry,
+				// MCP clients) retry instead of forcing the user to
+				// re-auth on a transient issue. Pre-fix, every DB error
+				// looked like a forged-cookie 401.
+				if errors.Is(verr, ErrSessionTransient) {
+					w.Header().Set("Retry-After", "1")
+					w.Header().Set("Content-Type", "application/json; charset=utf-8")
+					http.Error(w, `{"error":"transient backend error; retry"}`, http.StatusServiceUnavailable)
+					return
+				}
 				// Cookie present but invalid (expired / tampered /
 				// retired-key / IP-bind / UA-bind / revoked). Defer to
 				// the next middleware so a valid Bearer can still

internal/auth/session/middleware_test.go

@@ -338,6 +338,26 @@ func TestClientIPFromRequest_Variants(t *testing.T) {
 	}
 }
 
+// TestSessionMiddleware_TransientErrorMappedTo503 pins the LOW-6
+// closure (audit 2026-05-10): when Validate returns
+// ErrSessionTransient, the middleware MUST emit 503 with Retry-After
+// instead of falling through to the Bearer/401 path. Pre-fix, a DB
+// hiccup looked like a forged-cookie 401 + forced re-auth.
+func TestSessionMiddleware_TransientErrorMappedTo503(t *testing.T) {
+	stub := &stubSessionValidator{validateErr: ErrSessionTransient}
+	chain := ChainAuthSessionThenBearer(NewSessionMiddleware(stub), nil)(markAuthenticated())
+	req := httptest.NewRequest(http.MethodGet, "/x", nil)
+	req.AddCookie(&http.Cookie{Name: sessiondomain.PostLoginCookieName, Value: "v1.ses.sk.bad"})
+	w := httptest.NewRecorder()
+	chain.ServeHTTP(w, req)
+	if w.Code != http.StatusServiceUnavailable {
+		t.Errorf("status = %d; want 503", w.Code)
+	}
+	if w.Header().Get("Retry-After") != "1" {
+		t.Errorf("Retry-After = %q; want 1", w.Header().Get("Retry-After"))
+	}
+}
+
 func TestChainAuthSessionThenBearer_NilBearer_Session401Path(t *testing.T) {
 	stub := &stubSessionValidator{validateErr: ErrSessionInvalidCookie}
 	chain := ChainAuthSessionThenBearer(NewSessionMiddleware(stub), nil)(markAuthenticated())

internal/auth/session/service.go

@@ -153,6 +153,18 @@ var (
 	// auto-revoked (user may have legitimate IP change).
 	ErrSessionIPMismatch = errors.New("session: client IP does not match session-bound IP")
 
+	// ErrSessionTransient: a non-deterministic, retryable failure (DB
+	// connection reset, network blip on the audit-row write inside
+	// the validate path, etc.). Distinct from ErrSessionInvalidCookie:
+	// the cookie itself isn't malformed/forged, the backend just
+	// failed to look it up cleanly. The middleware maps this to HTTP
+	// 503 with `Retry-After: 1` so well-behaved clients retry instead
+	// of forcing the user to re-authenticate. Audit 2026-05-10 LOW-6
+	// closure — pre-fix, transient DB failures collapsed into
+	// ErrSessionInvalidCookie + 401, falsely framing a database outage
+	// as "your cookie is bad."
+	ErrSessionTransient = errors.New("session: transient backend error")
+
 	// ErrSessionUAMismatch: same shape as ErrSessionIPMismatch for the
 	// optional CERTCTL_SESSION_BIND_USER_AGENT gate.
 	ErrSessionUAMismatch = errors.New("session: User-Agent does not match session-bound User-Agent")
@@ -453,7 +465,16 @@ func (s *Service) Validate(ctx context.Context, in ValidateInput) (*sessiondomai
 
 	row, err := s.sessions.Get(ctx, sessionID)
 	if err != nil {
-		return nil, ErrSessionInvalidCookie
+		// Audit 2026-05-10 LOW-6 closure — distinguish "this cookie's
+		// session row doesn't exist" (invalid: 401) from "the DB call
+		// failed transiently" (retryable: 503). Pre-fix, both
+		// collapsed into ErrSessionInvalidCookie, so a DB hiccup
+		// looked like a forged cookie in the audit log + forced the
+		// user to re-auth.
+		if errors.Is(err, repository.ErrSessionNotFound) {
+			return nil, ErrSessionInvalidCookie
+		}
+		return nil, fmt.Errorf("%w: %v", ErrSessionTransient, err)
 	}
 
 	if row.RevokedAt != nil {

internal/auth/session/service_test.go

@@ -921,14 +921,40 @@ func TestService_RotateSigningKey_RetireError(t *testing.T) {
 	}
 }
 
-func TestService_Validate_SessionGetErrorMappedToInvalidCookie(t *testing.T) {
+// TestService_Validate_TransientSessionGetError pins the LOW-6
+// closure (audit 2026-05-10): a non-deterministic DB error from
+// session.Get bubbles up as ErrSessionTransient (→ 503), NOT
+// ErrSessionInvalidCookie (→ 401). The middleware test pins the
+// 503-with-Retry-After wire shape; this one pins the service-layer
+// sentinel.
+func TestService_Validate_TransientSessionGetError(t *testing.T) {
 	svc, sessions, _, _, _ := newTestService(t, defaultCfg())
 	res, _ := svc.Create(context.Background(), "u-y", "User", "", "")
 	sessions.getErr = fmt.Errorf("simulated session.Get failure")
 	_, err := svc.Validate(context.Background(), ValidateInput{CookieValue: res.CookieValue})
+	if !errors.Is(err, ErrSessionTransient) {
+		t.Errorf("err = %v; want ErrSessionTransient", err)
+	}
+	if errors.Is(err, ErrSessionInvalidCookie) {
+		t.Errorf("err also matched ErrSessionInvalidCookie; want only ErrSessionTransient")
+	}
+}
+
+// TestService_Validate_SessionNotFoundMapsToInvalidCookie pins the
+// other half of the LOW-6 split: repository.ErrSessionNotFound (a
+// real, deterministic "the row doesn't exist" answer from the DB)
+// stays mapped to ErrSessionInvalidCookie (→ 401), NOT 503.
+func TestService_Validate_SessionNotFoundMapsToInvalidCookie(t *testing.T) {
+	svc, sessions, _, _, _ := newTestService(t, defaultCfg())
+	res, _ := svc.Create(context.Background(), "u-y2", "User", "", "")
+	sessions.getErr = repository.ErrSessionNotFound
+	_, err := svc.Validate(context.Background(), ValidateInput{CookieValue: res.CookieValue})
 	if !errors.Is(err, ErrSessionInvalidCookie) {
 		t.Errorf("err = %v; want ErrSessionInvalidCookie", err)
 	}
+	if errors.Is(err, ErrSessionTransient) {
+		t.Errorf("err also matched ErrSessionTransient; want only ErrSessionInvalidCookie")
+	}
 }
 
 func TestService_UpdateLastSeen_RepoErrorWraps(t *testing.T) {