visionA/visionA-backend/internal/oidc/provider_test.go

package oidc

import (
	"context"
	"crypto/rand"
	"crypto/rsa"
	"encoding/base64"
	"encoding/json"
	"errors"
	"fmt"
	"net/http"
	"net/http/httptest"
	"net/url"
	"strings"
	"testing"
	"time"

	"github.com/go-jose/go-jose/v4"
	"github.com/go-jose/go-jose/v4/jwt"
	"github.com/stretchr/testify/assert"
	"github.com/stretchr/testify/require"
)

// fakeOIDC 是一個用 httptest 起來的最小化 OIDC 模擬器：
//   - GET /.well-known/openid-configuration → discovery doc
//   - GET /jwks                              → JWKS（含 1 把 RSA public key）
//   - POST /token                            → 接 authorization_code，回 token set
//
// 簽 id_token 用 go-jose（coreos/go-oidc 的內部依賴，已在 go.sum 中），不需引額外 lib。
type fakeOIDC struct {
	server     *httptest.Server
	signingKey *rsa.PrivateKey
	keyID      string
	clientID   string

	// 可由各測試修改的「下一個 token 行為」控制旗標
	mu             chan struct{} // 簡單以 buffered chan 當 mutex（避免 import sync）
	expectVerifier string        // POST /token 時驗 code_verifier 是否相符；空字串=不驗
	respondCode    int           // POST /token 回應 status code（0 = 200）
	respondBody    string        // 非空時直接回此 body 取代正常 token response
	idTokenClaims  jwt.Claims    // 自訂簽 token 的 standard claims（zero = 預設）
	idTokenExtra   map[string]any
	idTokenAlg     jose.SignatureAlgorithm // 預設 RS256
	skipIDToken    bool                    // true 時 token response 不含 id_token

	// 觀測：最後一次 /token 收到的 form / Authorization header（A1 加：驗 public client mode）
	lastTokenForm    url.Values
	lastTokenAuthHdr string
}

func newFakeOIDC(t *testing.T, clientID string) *fakeOIDC {
	t.Helper()

	priv, err := rsa.GenerateKey(rand.Reader, 2048)
	require.NoError(t, err, "產 RSA key 失敗")

	f := &fakeOIDC{
		signingKey: priv,
		keyID:      "test-key-1",
		clientID:   clientID,
		mu:         make(chan struct{}, 1),
		idTokenAlg: jose.RS256,
	}
	f.mu <- struct{}{} // 初始化「鎖可用」

	mux := http.NewServeMux()
	// discovery doc 必須在 server 起來後才知道 issuer URL，所以用 closure 延遲組
	mux.HandleFunc("/.well-known/openid-configuration", func(w http.ResponseWriter, r *http.Request) {
		base := "http://" + r.Host
		doc := map[string]any{
			"issuer":                 base,
			"authorization_endpoint": base + "/authorize",
			"token_endpoint":         base + "/token",
			"jwks_uri":               base + "/jwks",
			"response_types_supported": []string{
				"code",
			},
			"id_token_signing_alg_values_supported": []string{"RS256"},
			"subject_types_supported":               []string{"public"},
			"scopes_supported":                      []string{"openid", "email", "profile"},
		}
		writeJSON(w, doc)
	})

	mux.HandleFunc("/jwks", func(w http.ResponseWriter, r *http.Request) {
		jwks := map[string]any{
			"keys": []map[string]any{
				rsaPublicKeyToJWK(&priv.PublicKey, f.keyID),
			},
		}
		writeJSON(w, jwks)
	})

	mux.HandleFunc("/token", func(w http.ResponseWriter, r *http.Request) {
		f.handleToken(t, w, r)
	})

	f.server = httptest.NewServer(mux)
	t.Cleanup(f.server.Close)
	return f
}

func (f *fakeOIDC) issuer() string { return f.server.URL }

// withState 是 helper：在 chan-as-mutex 保護下安全修改控制旗標。
func (f *fakeOIDC) withState(fn func()) {
	<-f.mu
	defer func() { f.mu <- struct{}{} }()
	fn()
}

// snapshot 是 helper：原子讀取所有控制旗標。
func (f *fakeOIDC) snapshot() (verifier string, code int, body string, claims jwt.Claims, extra map[string]any, alg jose.SignatureAlgorithm, skipID bool) {
	<-f.mu
	defer func() { f.mu <- struct{}{} }()
	return f.expectVerifier, f.respondCode, f.respondBody, f.idTokenClaims, f.idTokenExtra, f.idTokenAlg, f.skipIDToken
}

func (f *fakeOIDC) handleToken(t *testing.T, w http.ResponseWriter, r *http.Request) {
	t.Helper()

	if err := r.ParseForm(); err != nil {
		http.Error(w, "bad form", http.StatusBadRequest)
		return
	}

	// 觀測：抄一份 form / Authorization header 給測試驗 public vs confidential mode
	f.withState(func() {
		f.lastTokenForm = make(url.Values, len(r.PostForm))
		for k, vv := range r.PostForm {
			f.lastTokenForm[k] = append([]string(nil), vv...)
		}
		f.lastTokenAuthHdr = r.Header.Get("Authorization")
	})

	expectVerifier, code, body, claims, extra, alg, skipID := f.snapshot()

	// caller 可指定強制錯誤 / 自訂 body
	if body != "" {
		if code == 0 {
			code = http.StatusBadRequest
		}
		w.Header().Set("Content-Type", "application/json")
		w.WriteHeader(code)
		_, _ = w.Write([]byte(body))
		return
	}

	if expectVerifier != "" && r.Form.Get("code_verifier") != expectVerifier {
		// PKCE proof 失敗：回 RFC 6749 §5.2 規範的 invalid_grant
		w.Header().Set("Content-Type", "application/json")
		w.WriteHeader(http.StatusBadRequest)
		_, _ = w.Write([]byte(`{"error":"invalid_grant","error_description":"PKCE verifier mismatch"}`))
		return
	}

	// 簽 id_token
	now := time.Now()
	if claims.Issuer == "" {
		claims = jwt.Claims{
			Issuer:    f.issuer(),
			Subject:   "sub-fake-user-001",
			Audience:  jwt.Audience{f.clientID},
			IssuedAt:  jwt.NewNumericDate(now),
			Expiry:    jwt.NewNumericDate(now.Add(5 * time.Minute)),
			NotBefore: jwt.NewNumericDate(now),
		}
	}
	if extra == nil {
		extra = map[string]any{
			"email": "fake-user@example.com",
			"name":  "Fake User",
			"nonce": r.Form.Get("__test_nonce_passthrough"), // 不會由真 OIDC server 帶；測試以另路徑注入
		}
	}

	rawIDToken, err := signIDToken(f.signingKey, f.keyID, alg, claims, extra)
	if err != nil {
		http.Error(w, "sign error: "+err.Error(), http.StatusInternalServerError)
		return
	}

	resp := map[string]any{
		"access_token": "fake-access-token",
		"token_type":   "Bearer",
		"expires_in":   3600,
	}
	if !skipID {
		resp["id_token"] = rawIDToken
	}
	writeJSON(w, resp)
}

// signIDToken 用 RSA private key 簽一個 OIDC id_token（JWS / RS256）。
func signIDToken(priv *rsa.PrivateKey, kid string, alg jose.SignatureAlgorithm, std jwt.Claims, extra map[string]any) (string, error) {
	signerOpts := (&jose.SignerOptions{}).WithType("JWT")
	signerOpts.WithHeader("kid", kid)
	signer, err := jose.NewSigner(jose.SigningKey{Algorithm: alg, Key: priv}, signerOpts)
	if err != nil {
		return "", err
	}

	builder := jwt.Signed(signer).Claims(std)
	if len(extra) > 0 {
		builder = builder.Claims(extra)
	}
	return builder.Serialize()
}

// rsaPublicKeyToJWK 把 RSA public key 編成 JWKS spec 的 key 物件。
func rsaPublicKeyToJWK(pub *rsa.PublicKey, kid string) map[string]any {
	return map[string]any{
		"kty": "RSA",
		"alg": "RS256",
		"use": "sig",
		"kid": kid,
		"n":   base64.RawURLEncoding.EncodeToString(pub.N.Bytes()),
		"e":   base64.RawURLEncoding.EncodeToString(bigIntBytes(pub.E)),
	}
}

func bigIntBytes(e int) []byte {
	// RSA exponent 通常是 65537（0x010001）= 3 bytes。手動編碼。
	out := []byte{}
	for e > 0 {
		out = append([]byte{byte(e & 0xff)}, out...)
		e >>= 8
	}
	if len(out) == 0 {
		out = []byte{0}
	}
	return out
}

func writeJSON(w http.ResponseWriter, v any) {
	w.Header().Set("Content-Type", "application/json")
	_ = json.NewEncoder(w).Encode(v)
}

// =====================================================================
// 測試
// =====================================================================

const (
	testClientID     = "visiona-backend-test"
	testClientSecret = "test-secret"
	testRedirect     = "http://localhost:8080/api/auth/callback"
)

func newProviderForTest(t *testing.T, fake *fakeOIDC) Provider {
	t.Helper()
	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
	defer cancel()
	p, err := NewProvider(ctx, ProviderConfig{
		IssuerURL:    fake.issuer(),
		ClientID:     testClientID,
		ClientSecret: testClientSecret,
		RedirectURL:  testRedirect,
	})
	require.NoError(t, err)
	return p
}

func TestNewProvider_Discovery(t *testing.T) {
	fake := newFakeOIDC(t, testClientID)
	p := newProviderForTest(t, fake)
	require.NotNil(t, p)
}

func TestNewProvider_RejectInvalidConfig(t *testing.T) {
	// A1（2026-05-01）：ClientSecret 為選填，因此 "missing secret" case 已移除。
	cases := []struct {
		name string
		cfg  ProviderConfig
	}{
		{"missing issuer", ProviderConfig{ClientID: "x", ClientSecret: "y", RedirectURL: "http://z"}},
		{"missing client id", ProviderConfig{IssuerURL: "http://x", ClientSecret: "y", RedirectURL: "http://z"}},
		{"missing redirect", ProviderConfig{IssuerURL: "http://x", ClientID: "y", ClientSecret: "z"}},
	}
	for _, c := range cases {
		t.Run(c.name, func(t *testing.T) {
			_, err := NewProvider(context.Background(), c.cfg)
			assert.ErrorIs(t, err, ErrInvalidConfig)
		})
	}
}

func TestNewProvider_DiscoveryFailure(t *testing.T) {
	// 用一個立刻關掉的 server 模擬 IdP 不可達
	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		http.NotFound(w, r)
	}))
	defer srv.Close()
	_, err := NewProvider(context.Background(), ProviderConfig{
		IssuerURL:    srv.URL,
		ClientID:     "c",
		ClientSecret: "s",
		RedirectURL:  "http://r",
	})
	require.Error(t, err)
	assert.ErrorIs(t, err, ErrDiscoveryFetch)
}

func TestAuthorizationURL_Format(t *testing.T) {
	fake := newFakeOIDC(t, testClientID)
	p := newProviderForTest(t, fake)

	state, _ := GenerateState()
	nonce, _ := GenerateNonce()
	verifier, _ := GenerateCodeVerifier()
	challenge := CodeChallenge(verifier)

	raw := p.AuthorizationURL(state, nonce, challenge)
	u, err := url.Parse(raw)
	require.NoError(t, err)

	q := u.Query()
	assert.Equal(t, "code", q.Get("response_type"))
	assert.Equal(t, testClientID, q.Get("client_id"))
	assert.Equal(t, testRedirect, q.Get("redirect_uri"))
	assert.Equal(t, state, q.Get("state"))
	assert.Equal(t, nonce, q.Get("nonce"))
	assert.Equal(t, challenge, q.Get("code_challenge"))
	assert.Equal(t, "S256", q.Get("code_challenge_method"))

	// scope 應含 openid email profile（順序由 oauth2 lib 決定，用 contains 驗）
	scope := q.Get("scope")
	for _, s := range []string{"openid", "email", "profile"} {
		assert.Truef(t, strings.Contains(scope, s), "scope 應含 %q，得 %q", s, scope)
	}

	// authorization_endpoint 應指向 fake server 的 /authorize
	assert.Equal(t, fake.issuer()+"/authorize", u.Scheme+"://"+u.Host+u.Path)
}

func TestExchangeCode_Success(t *testing.T) {
	fake := newFakeOIDC(t, testClientID)
	p := newProviderForTest(t, fake)

	verifier, _ := GenerateCodeVerifier()
	fake.withState(func() { fake.expectVerifier = verifier })

	tok, err := p.ExchangeCode(context.Background(), "fake-code", verifier)
	require.NoError(t, err)
	assert.Equal(t, "fake-access-token", tok.AccessToken)
	assert.NotEmpty(t, tok.IDToken, "id_token 應有值")
	assert.Equal(t, "Bearer", tok.TokenType)
	assert.Greater(t, tok.ExpiresIn, 0)
}

func TestExchangeCode_PKCEMismatch(t *testing.T) {
	fake := newFakeOIDC(t, testClientID)
	p := newProviderForTest(t, fake)

	fake.withState(func() { fake.expectVerifier = "the-real-verifier" })

	_, err := p.ExchangeCode(context.Background(), "fake-code", "wrong-verifier")
	require.Error(t, err)
	assert.ErrorIs(t, err, ErrInvalidGrant, "PKCE 不符應對應到 ErrInvalidGrant")
}

func TestExchangeCode_ServerError(t *testing.T) {
	fake := newFakeOIDC(t, testClientID)
	p := newProviderForTest(t, fake)

	fake.withState(func() {
		fake.respondCode = http.StatusInternalServerError
		fake.respondBody = `{"error":"server_error"}`
	})

	_, err := p.ExchangeCode(context.Background(), "fake-code", "any")
	require.Error(t, err)
	assert.ErrorIs(t, err, ErrTokenExchange)
	assert.NotErrorIs(t, err, ErrInvalidGrant, "5xx 不應被分類為 invalid_grant")
}

func TestExchangeCode_MissingIDToken(t *testing.T) {
	fake := newFakeOIDC(t, testClientID)
	p := newProviderForTest(t, fake)

	fake.withState(func() { fake.skipIDToken = true })

	_, err := p.ExchangeCode(context.Background(), "fake-code", "any")
	require.Error(t, err)
	assert.ErrorIs(t, err, ErrTokenExchange, "缺 id_token 應視為 token_exchange 失敗")
}

func TestVerifyIDToken_HappyPath(t *testing.T) {
	fake := newFakeOIDC(t, testClientID)
	p := newProviderForTest(t, fake)

	const expectedNonce = "nonce-happy-path"
	now := time.Now()
	fake.withState(func() {
		fake.idTokenClaims = jwt.Claims{
			Issuer:   fake.issuer(),
			Subject:  "user-123",
			Audience: jwt.Audience{testClientID},
			IssuedAt: jwt.NewNumericDate(now),
			Expiry:   jwt.NewNumericDate(now.Add(5 * time.Minute)),
		}
		fake.idTokenExtra = map[string]any{
			"email": "alice@example.com",
			"name":  "Alice",
			"nonce": expectedNonce,
		}
	})

	tok, err := p.ExchangeCode(context.Background(), "code", "verifier")
	require.NoError(t, err)

	claims, err := p.VerifyIDToken(context.Background(), tok.IDToken, expectedNonce)
	require.NoError(t, err)
	assert.Equal(t, "user-123", claims.Subject)
	assert.Equal(t, "alice@example.com", claims.Email)
	assert.Equal(t, "Alice", claims.Name)
	assert.Equal(t, fake.issuer(), claims.Issuer)
	assert.Equal(t, testClientID, claims.Audience)
	assert.Equal(t, expectedNonce, claims.Nonce)
	assert.False(t, claims.ExpiresAt.IsZero())
	assert.NotEmpty(t, claims.Raw)
}

func TestVerifyIDToken_WrongNonce(t *testing.T) {
	fake := newFakeOIDC(t, testClientID)
	p := newProviderForTest(t, fake)

	now := time.Now()
	fake.withState(func() {
		fake.idTokenClaims = jwt.Claims{
			Issuer:   fake.issuer(),
			Subject:  "user-x",
			Audience: jwt.Audience{testClientID},
			IssuedAt: jwt.NewNumericDate(now),
			Expiry:   jwt.NewNumericDate(now.Add(5 * time.Minute)),
		}
		fake.idTokenExtra = map[string]any{"nonce": "actual-nonce"}
	})
	tok, err := p.ExchangeCode(context.Background(), "code", "verifier")
	require.NoError(t, err)

	_, err = p.VerifyIDToken(context.Background(), tok.IDToken, "expected-different-nonce")
	require.Error(t, err)
	assert.ErrorIs(t, err, ErrInvalidNonce)
}

func TestVerifyIDToken_WrongAudience(t *testing.T) {
	fake := newFakeOIDC(t, testClientID)
	p := newProviderForTest(t, fake)

	now := time.Now()
	fake.withState(func() {
		fake.idTokenClaims = jwt.Claims{
			Issuer:   fake.issuer(),
			Subject:  "user-x",
			Audience: jwt.Audience{"some-other-client"}, // 故意錯
			IssuedAt: jwt.NewNumericDate(now),
			Expiry:   jwt.NewNumericDate(now.Add(5 * time.Minute)),
		}
		fake.idTokenExtra = map[string]any{"nonce": "n1"}
	})
	tok, err := p.ExchangeCode(context.Background(), "code", "verifier")
	require.NoError(t, err)

	_, err = p.VerifyIDToken(context.Background(), tok.IDToken, "n1")
	require.Error(t, err)
	// 涵蓋 audience 失敗 → 應映射到 ErrInvalidAudience 或至少 ErrInvalidIDToken
	assert.True(t, errors.Is(err, ErrInvalidAudience) || errors.Is(err, ErrInvalidIDToken),
		"audience 錯誤應對應到 ErrInvalidAudience（或 fallback ErrInvalidIDToken），得 %v", err)
}

func TestVerifyIDToken_Expired(t *testing.T) {
	fake := newFakeOIDC(t, testClientID)
	p := newProviderForTest(t, fake)

	past := time.Now().Add(-1 * time.Hour)
	fake.withState(func() {
		fake.idTokenClaims = jwt.Claims{
			Issuer:   fake.issuer(),
			Subject:  "user-x",
			Audience: jwt.Audience{testClientID},
			IssuedAt: jwt.NewNumericDate(past.Add(-5 * time.Minute)),
			Expiry:   jwt.NewNumericDate(past), // 已過期
		}
		fake.idTokenExtra = map[string]any{"nonce": "n1"}
	})
	tok, err := p.ExchangeCode(context.Background(), "code", "verifier")
	require.NoError(t, err)

	_, err = p.VerifyIDToken(context.Background(), tok.IDToken, "n1")
	require.Error(t, err)
	assert.True(t, errors.Is(err, ErrTokenExpired) || errors.Is(err, ErrInvalidIDToken),
		"過期應對應到 ErrTokenExpired（或 fallback），得 %v", err)
}

func TestVerifyIDToken_BadSignature(t *testing.T) {
	fake := newFakeOIDC(t, testClientID)
	p := newProviderForTest(t, fake)

	now := time.Now()
	fake.withState(func() {
		fake.idTokenClaims = jwt.Claims{
			Issuer:   fake.issuer(),
			Subject:  "u",
			Audience: jwt.Audience{testClientID},
			IssuedAt: jwt.NewNumericDate(now),
			Expiry:   jwt.NewNumericDate(now.Add(5 * time.Minute)),
		}
		fake.idTokenExtra = map[string]any{"nonce": "n1"}
	})
	tok, err := p.ExchangeCode(context.Background(), "code", "verifier")
	require.NoError(t, err)

	// 把簽章部分整段替換成「保證無效」的 base64url 字串。
	//
	// 為什麼不用「翻最後一個字元」：base64url 末字元只承載部分原始 bit（取決於整段
	// 長度對 3 取餘的結果），翻轉某些字元時 padding bit 不變，仍然會解碼出相同的
	// 原始 bytes → 簽章值不變 → test 偶發 fail。改翻「中間字元」雖大幅降低風險
	// 仍非 0；最穩定的作法是直接替換成完全不同的合法 base64url 字串，確保解碼出
	// 來的 bytes 與原簽章不同。
	parts := strings.Split(tok.IDToken, ".")
	require.Len(t, parts, 3)
	// 用相同長度的 'A' 串覆寫，base64url('A' * n) 解碼結果與原簽章 bytes 不同的機率近乎 100%
	// （唯有原簽章本身就剛好全 0 才會碰撞，RSA 簽章機率為 0）。
	parts[2] = strings.Repeat("A", len(parts[2]))
	tampered := strings.Join(parts, ".")

	_, err = p.VerifyIDToken(context.Background(), tampered, "n1")
	require.Error(t, err)
	assert.ErrorIs(t, err, ErrInvalidIDToken)
}

func TestVerifyIDToken_EmptyInputs(t *testing.T) {
	fake := newFakeOIDC(t, testClientID)
	p := newProviderForTest(t, fake)

	_, err := p.VerifyIDToken(context.Background(), "", "n1")
	require.Error(t, err)
	assert.ErrorIs(t, err, ErrInvalidIDToken)

	// 空 nonce 也應拒絕（caller 必須提供）
	now := time.Now()
	fake.withState(func() {
		fake.idTokenClaims = jwt.Claims{
			Issuer:   fake.issuer(),
			Subject:  "u",
			Audience: jwt.Audience{testClientID},
			IssuedAt: jwt.NewNumericDate(now),
			Expiry:   jwt.NewNumericDate(now.Add(5 * time.Minute)),
		}
		fake.idTokenExtra = map[string]any{"nonce": "actual"}
	})
	tok, err := p.ExchangeCode(context.Background(), "code", "verifier")
	require.NoError(t, err)

	_, err = p.VerifyIDToken(context.Background(), tok.IDToken, "")
	require.Error(t, err)
	assert.ErrorIs(t, err, ErrInvalidNonce)
}

// 確保 NewProvider 套用預設 Scopes（caller 沒填時）
func TestProviderConfig_DefaultScopes(t *testing.T) {
	fake := newFakeOIDC(t, testClientID)
	ctx := context.Background()
	cfg := ProviderConfig{
		IssuerURL:    fake.issuer(),
		ClientID:     testClientID,
		ClientSecret: testClientSecret,
		RedirectURL:  testRedirect,
		// 不填 Scopes
	}
	p, err := NewProvider(ctx, cfg)
	require.NoError(t, err)

	state, _ := GenerateState()
	nonce, _ := GenerateNonce()
	verifier, _ := GenerateCodeVerifier()
	raw := p.AuthorizationURL(state, nonce, CodeChallenge(verifier))
	u, _ := url.Parse(raw)
	scope := u.Query().Get("scope")
	for _, s := range DefaultScopes {
		assert.Truef(t, strings.Contains(scope, s), "預設 scope 應含 %q，得 %q", s, scope)
	}
}

// =====================================================================
// A1：Public PKCE-only client mode 測試
// =====================================================================

// TestNewProvider_PublicClient 驗 ClientSecret 留空時能成功初始化 Provider，
// 且行為與 confidential client 等價（除了 token request 的 auth method 不同）。
func TestNewProvider_PublicClient(t *testing.T) {
	fake := newFakeOIDC(t, testClientID)

	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
	defer cancel()
	p, err := NewProvider(ctx, ProviderConfig{
		IssuerURL: fake.issuer(),
		ClientID:  testClientID,
		// ClientSecret 故意留空 — A1：public PKCE-only client mode
		RedirectURL: testRedirect,
	})
	require.NoError(t, err)
	require.NotNil(t, p)

	// AuthorizationURL 應含 PKCE 參數但**不含** client_secret（OAuth 規格上 authorize 端點本來就不該帶）
	state, _ := GenerateState()
	nonce, _ := GenerateNonce()
	verifier, _ := GenerateCodeVerifier()
	authURL := p.AuthorizationURL(state, nonce, CodeChallenge(verifier))
	u, err := url.Parse(authURL)
	require.NoError(t, err)
	assert.Empty(t, u.Query().Get("client_secret"), "authorize URL 不應出現 client_secret")
	assert.Equal(t, "S256", u.Query().Get("code_challenge_method"))
}

// TestNewProvider_ConfidentialClient 是 baseline：確認既有 confidential mode 仍能初始化。
// 既有 TestNewProvider_Discovery 其實已涵蓋此情境，這個測試明示「兩種 mode 共存」。
func TestNewProvider_ConfidentialClient(t *testing.T) {
	fake := newFakeOIDC(t, testClientID)

	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
	defer cancel()
	p, err := NewProvider(ctx, ProviderConfig{
		IssuerURL:    fake.issuer(),
		ClientID:     testClientID,
		ClientSecret: testClientSecret, // 有值 → confidential
		RedirectURL:  testRedirect,
	})
	require.NoError(t, err)
	require.NotNil(t, p)
}

// TestExchangeCode_PublicClientNoSecretSent 驗 public client mode 下，
// /token request 的 form 不含 client_secret 欄位、且 Authorization header 不是 Basic auth。
//
// 這是 A1 改造的核心驗證：oauth2 lib 在 ClientSecret="" + AuthStyleInParams 時，
// 完全不送 client_secret（符合 RFC 6749 §2.3.1 對 public client 的規範）。
func TestExchangeCode_PublicClientNoSecretSent(t *testing.T) {
	fake := newFakeOIDC(t, testClientID)

	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
	defer cancel()
	p, err := NewProvider(ctx, ProviderConfig{
		IssuerURL: fake.issuer(),
		ClientID:  testClientID,
		// ClientSecret 留空 → public PKCE-only client
		RedirectURL: testRedirect,
	})
	require.NoError(t, err)

	verifier, _ := GenerateCodeVerifier()
	fake.withState(func() { fake.expectVerifier = verifier })

	tok, err := p.ExchangeCode(context.Background(), "fake-code", verifier)
	require.NoError(t, err)
	assert.NotEmpty(t, tok.IDToken)

	// 取出 fake server 觀察到的 token request
	var (
		form   url.Values
		auth   string
		ok     bool
	)
	fake.withState(func() {
		form = fake.lastTokenForm
		auth = fake.lastTokenAuthHdr
		ok = form != nil
	})
	require.True(t, ok, "fake server 應該已收到一次 token request")

	// public client 應該：
	//   - form 帶 client_id 但**不帶** client_secret
	//   - 不送 Authorization: Basic header（Authorization 應為空字串）
	//   - 帶 code_verifier（PKCE proof）
	assert.Equal(t, testClientID, form.Get("client_id"), "public client 仍需在 form 帶 client_id")
	assert.Empty(t, form.Get("client_secret"), "public client 不應送 client_secret form 欄位")
	assert.Empty(t, auth, "public client 不應送 Authorization header（更不應是 Basic）")
	assert.Equal(t, verifier, form.Get("code_verifier"), "PKCE verifier 必須帶")
}

// TestExchangeCode_ConfidentialClientSendsSecret 對照組：
// confidential client mode 下，/token request 必須帶 client_secret（或 Basic auth）。
func TestExchangeCode_ConfidentialClientSendsSecret(t *testing.T) {
	fake := newFakeOIDC(t, testClientID)
	p := newProviderForTest(t, fake) // 用既有 helper：帶 testClientSecret

	verifier, _ := GenerateCodeVerifier()
	fake.withState(func() { fake.expectVerifier = verifier })

	_, err := p.ExchangeCode(context.Background(), "fake-code", verifier)
	require.NoError(t, err)

	var (
		form url.Values
		auth string
	)
	fake.withState(func() {
		form = fake.lastTokenForm
		auth = fake.lastTokenAuthHdr
	})

	// confidential 可以走兩種：Basic auth header，或 form 帶 client_secret。
	// oauth2 lib 預設先試 InHeader → 第一輪通常是 Basic。
	// 我們不挑路徑，只要「至少一邊」帶 secret 就算正確。
	hasFormSecret := form.Get("client_secret") == testClientSecret
	hasBasicAuth := strings.HasPrefix(auth, "Basic ")
	assert.Truef(t, hasFormSecret || hasBasicAuth,
		"confidential client 應透過 form 或 Basic auth 帶 secret；form=%v auth=%q",
		form, auth)
}

// sanity check：fakeOIDC 自身沒寫錯（簽出來的 token coreos 能驗）
func TestFakeOIDC_SelfSignedTokenIsValid(t *testing.T) {
	fake := newFakeOIDC(t, testClientID)
	now := time.Now()
	std := jwt.Claims{
		Issuer:   fake.issuer(),
		Subject:  "self-test",
		Audience: jwt.Audience{testClientID},
		IssuedAt: jwt.NewNumericDate(now),
		Expiry:   jwt.NewNumericDate(now.Add(time.Minute)),
	}
	tok, err := signIDToken(fake.signingKey, fake.keyID, jose.RS256, std,
		map[string]any{"nonce": "test-nonce"})
	require.NoError(t, err)
	assert.True(t, strings.Count(tok, ".") == 2, "JWT 應有 3 段")
	// 確保非空 payload
	parts := strings.SplitN(tok, ".", 3)
	require.Len(t, parts, 3)
	payload, err := base64.RawURLEncoding.DecodeString(parts[1])
	require.NoError(t, err)
	assert.Contains(t, string(payload), "self-test")
	assert.Contains(t, string(payload), "test-nonce")
	// 也驗證 fmt 可印（避免 unused import 'fmt' 在小幅 refactor 後消失）
	_ = fmt.Sprintf("%s", tok)
}