Refuse an OIDC sign-in from creating the very first user

Closes the race terdut-operator#1 found: /api/bootstrap and OIDC
auto-provisioning both key off the same signal (SELECT COUNT(*) FROM
users) with no coordination between them, so an otherwise-ordinary OIDC
sign-in against a freshly-created, not-yet-bootstrapped install could
create user #1 itself and take the one slot /api/bootstrap expects to win
uncontested (terdut-operator's own design, DESIGN.md §1/§6, assumes it is
the only caller). The operator has no way to recover from losing that
race -- it never gets a credential, and nothing it owns can clear the
occupying user row.

resolveSSOUser now checks the same gate handleBootstrap already does,
right where it's about to create a brand-new user (an identity nobody has
linked yet, that also matches no existing local account by email) -- not
anywhere else, since every other sign-in on an already-bootstrapped
install is unaffected. New sso_error code `not_bootstrapped`: the person
sees "this install is still setting up, try again in a moment" and a
second attempt once something has actually bootstrapped succeeds
normally, same as any other first sign-in.

Does not fix the other half of that issue (BootstrapStateLost's own
"delete and recreate" instructions still don't work once something has
occupied the slot some other way) -- this closes the specific race, not
every path to that state.

internal/api/oidc_test.go

@@ -543,6 +543,45 @@ func TestSSO_DisabledUserIsRefused(t *testing.T) {
 	}
 }
 
+// TestSSO_FirstUserIsRefusedUntilBootstrap is terdut-operator#1: an
+// otherwise-ordinary OIDC sign-in against a brand-new, not-yet-bootstrapped
+// install must not be allowed to create the first user and win the race
+// POST /api/bootstrap expects to win uncontested. Built directly over
+// api.NewRouter rather than newSSOTS/newTS, both of which bootstrap before
+// a test body ever runs -- exactly the state this test needs to not have yet.
+func TestSSO_FirstUserIsRefusedUntilBootstrap(t *testing.T) {
+	idp := newFakeIdP(t)
+	database := newTestDB(t)
+	srv := httptest.NewServer(api.NewRouter(database, api.NotifyConfig{PublicURL: "http://terdut.test"}, ssoConfig(idp), "test"))
+	t.Cleanup(srv.Close)
+
+	first := newBrowser(t, srv.URL)
+	first.CheckRedirect = func(*http.Request, []*http.Request) error { return http.ErrUseLastResponse }
+	if loc := signInSSO(t, idp, first, alice); loc != "/?sso_error=not_bootstrapped" {
+		t.Fatalf("sent to %q, want not_bootstrapped", loc)
+	}
+
+	// Bootstrap the install for real, the way terdut-operator's own
+	// reconcileBootstrap does.
+	resp, err := http.Post(srv.URL+"/api/bootstrap", "application/json",
+		strings.NewReader(`{"username":"admin","email":"admin@test.com"}`))
+	if err != nil {
+		t.Fatal(err)
+	}
+	resp.Body.Close()
+	if resp.StatusCode != http.StatusCreated {
+		t.Fatalf("bootstrap: %d", resp.StatusCode)
+	}
+
+	// The same identity, signing in again, is this install's ordinary first
+	// SSO user now -- no longer refused.
+	second := newBrowser(t, srv.URL)
+	second.CheckRedirect = func(*http.Request, []*http.Request) error { return http.ErrUseLastResponse }
+	if loc := signInSSO(t, idp, second, alice); loc != "/" {
+		t.Errorf("sent to %q after bootstrap, want success", loc)
+	}
+}
+
 func TestSSO_NoEmailIsRefused(t *testing.T) {
 	idp := newFakeIdP(t)
 	s := newSSOTS(t, idp)