feat: fb and google oauth

internal/config/config.go

@@ -65,6 +65,18 @@ type Config struct {
 
 	// Frontend
 	FrontendURL string // for CORS + reset/verify links in emails
+
+	// BFF public URL — the externally-reachable origin of this server. Used
+	// to build the OAuth redirect_uri the provider sees (must match the
+	// allow-list configured in the provider's console). In dev this is
+	// usually http://localhost:8080.
+	BFFPublicURL string
+
+	// OAuth providers — empty creds disable the corresponding provider.
+	GoogleOAuthClientID     string
+	GoogleOAuthClientSecret string
+	FacebookOAuthClientID   string
+	FacebookOAuthClientSecret string
 }
 
 // Load reads configuration from environment variables (with .env fallback).
@@ -116,6 +128,13 @@ func Load() (*Config, error) {
 		CookieSecure:  getEnvBool("COOKIE_SECURE", false),
 
 		FrontendURL: getEnv("FRONTEND_URL", "http://localhost:3000"),
+
+		BFFPublicURL: getEnv("BFF_PUBLIC_URL", "http://localhost:8080"),
+
+		GoogleOAuthClientID:       getEnv("GOOGLE_OAUTH_CLIENT_ID", ""),
+		GoogleOAuthClientSecret:   getEnv("GOOGLE_OAUTH_CLIENT_SECRET", ""),
+		FacebookOAuthClientID:     getEnv("FACEBOOK_OAUTH_CLIENT_ID", ""),
+		FacebookOAuthClientSecret: getEnv("FACEBOOK_OAUTH_CLIENT_SECRET", ""),
 	}
 
 	if cfg.MongoURI == "" {

internal/handler/oauth.go

@@ -0,0 +1,219 @@
+package handler
+
+import (
+	"errors"
+	"net/http"
+	"net/url"
+	"strings"
+	"time"
+
+	"github.com/go-chi/chi/v5"
+	"github.com/prabhupadasays/spsays-bff/internal/service"
+	"github.com/prabhupadasays/spsays-bff/pkg/auth"
+	"github.com/prabhupadasays/spsays-bff/pkg/oauth"
+	"go.uber.org/zap"
+)
+
+// stateCookieName is the short-lived cookie that pairs with the state query
+// param so callback can verify "this redirect was started by us." Scoped to
+// /api/v1/auth/oauth so it never leaks into other handlers.
+const stateCookieName = "spsays_oauth_state"
+const stateCookieMaxAge = 10 * 60 // 10 minutes — matches stateMaxAge in pkg/oauth/state.go
+
+// OAuthHandler exposes the GET /api/v1/auth/oauth/{provider}/start and
+// /callback endpoints. It composes Provider → token exchange → service.SignInWithOAuth.
+type OAuthHandler struct {
+	svc           *service.AuthService
+	cookies       auth.CookieOptions
+	providers     oauth.Registry
+	stateSecret   []byte // HMAC key for signing state — derived from JWT secret
+	bffPublicURL  string // e.g. http://localhost:8080 or https://api.prabhupadasays.org
+	frontendURL   string // e.g. http://localhost:3000 or https://prabhupadasays.org
+	logger        *zap.Logger
+}
+
+func NewOAuthHandler(
+	svc *service.AuthService,
+	cookies auth.CookieOptions,
+	providers oauth.Registry,
+	stateSecret []byte,
+	bffPublicURL, frontendURL string,
+	logger *zap.Logger,
+) *OAuthHandler {
+	return &OAuthHandler{
+		svc:          svc,
+		cookies:      cookies,
+		providers:    providers,
+		stateSecret:  stateSecret,
+		bffPublicURL: strings.TrimRight(bffPublicURL, "/"),
+		frontendURL:  strings.TrimRight(frontendURL, "/"),
+		logger:       logger,
+	}
+}
+
+// Start kicks off the OAuth round-trip. It generates a signed state value,
+// stores it in a short-lived cookie, and 302s the browser to the provider's
+// AuthCodeURL. `next` query param carries the post-login redirect path on
+// the frontend (defaults to "/u/me").
+func (h *OAuthHandler) Start(w http.ResponseWriter, r *http.Request) {
+	providerName := chi.URLParam(r, "provider")
+	prov, err := h.providers.Get(providerName)
+	if err != nil {
+		http.Error(w, "unknown provider", http.StatusNotFound)
+		return
+	}
+
+	next := sanitizeNext(r.URL.Query().Get("next"))
+
+	state, err := oauth.EncodeState(h.stateSecret, providerName, next)
+	if err != nil {
+		h.logger.Error("oauth encode state", zap.Error(err))
+		http.Error(w, "internal error", http.StatusInternalServerError)
+		return
+	}
+
+	// State cookie matches the auth cookie's SameSite/Secure profile so the
+	// browser sends it back on the callback redirect. Path-scoped so it
+	// doesn't ride along on unrelated requests.
+	http.SetCookie(w, &http.Cookie{
+		Name:     stateCookieName,
+		Value:    state,
+		Path:     "/api/v1/auth/oauth",
+		Domain:   h.cookies.Domain,
+		MaxAge:   stateCookieMaxAge,
+		HttpOnly: true,
+		Secure:   h.cookies.Secure,
+		SameSite: http.SameSiteLaxMode,
+	})
+
+	cfg := prov.Config(h.callbackURL(providerName))
+	authURL := cfg.AuthCodeURL(state)
+	http.Redirect(w, r, authURL, http.StatusFound)
+}
+
+// Callback finishes the round-trip. Verifies state, exchanges the code,
+// fetches the profile, signs the user in, sets the session cookie, and
+// redirects back to the frontend `next` path.
+func (h *OAuthHandler) Callback(w http.ResponseWriter, r *http.Request) {
+	providerName := chi.URLParam(r, "provider")
+	prov, err := h.providers.Get(providerName)
+	if err != nil {
+		h.redirectErr(w, r, "/login?oauth_error=unknown_provider")
+		return
+	}
+
+	// Provider returned an error (user denied, etc.).
+	if e := r.URL.Query().Get("error"); e != "" {
+		h.redirectErr(w, r, "/login?oauth_error="+url.QueryEscape(e))
+		return
+	}
+
+	// State must be present in BOTH cookie and query param, and they must
+	// match. Either alone is not sufficient (cookie alone → no CSRF binding;
+	// query alone → an attacker could craft any signed state).
+	cookie, err := r.Cookie(stateCookieName)
+	if err != nil {
+		h.redirectErr(w, r, "/login?oauth_error=missing_state")
+		return
+	}
+	stateQ := r.URL.Query().Get("state")
+	if stateQ == "" || stateQ != cookie.Value {
+		h.redirectErr(w, r, "/login?oauth_error=invalid_state")
+		return
+	}
+	// Clear the state cookie regardless of what happens next.
+	http.SetCookie(w, &http.Cookie{
+		Name:     stateCookieName,
+		Value:    "",
+		Path:     "/api/v1/auth/oauth",
+		Domain:   h.cookies.Domain,
+		MaxAge:   -1,
+		Expires:  time.Unix(0, 0),
+		HttpOnly: true,
+		Secure:   h.cookies.Secure,
+		SameSite: http.SameSiteLaxMode,
+	})
+
+	stateProvider, next, err := oauth.DecodeState(h.stateSecret, stateQ)
+	if err != nil || stateProvider != providerName {
+		h.redirectErr(w, r, "/login?oauth_error=invalid_state")
+		return
+	}
+
+	code := r.URL.Query().Get("code")
+	if code == "" {
+		h.redirectErr(w, r, "/login?oauth_error=missing_code")
+		return
+	}
+
+	cfg := prov.Config(h.callbackURL(providerName))
+	tok, err := cfg.Exchange(r.Context(), code)
+	if err != nil {
+		h.logger.Warn("oauth code exchange failed", zap.String("provider", providerName), zap.Error(err))
+		h.redirectErr(w, r, "/login?oauth_error=exchange_failed")
+		return
+	}
+
+	prof, err := prov.FetchProfile(r.Context(), tok)
+	if err != nil {
+		h.logger.Warn("oauth fetch profile failed", zap.String("provider", providerName), zap.Error(err))
+		h.redirectErr(w, r, "/login?oauth_error=profile_failed")
+		return
+	}
+
+	res, err := h.svc.SignInWithOAuth(r.Context(), service.OAuthProfile{
+		Provider:       prof.Provider,
+		ProviderUserID: prof.ProviderUserID,
+		Email:          prof.Email,
+		EmailVerified:  prof.EmailVerified,
+		Name:           prof.Name,
+		AvatarURL:      prof.AvatarURL,
+	})
+	if err != nil {
+		switch {
+		case errors.Is(err, service.ErrOAuthEmailMissing):
+			h.redirectErr(w, r, "/login?oauth_error=email_required")
+		case errors.Is(err, service.ErrOAuthUnavailable):
+			h.redirectErr(w, r, "/login?oauth_error=unavailable")
+		default:
+			h.logger.Error("oauth sign-in", zap.Error(err))
+			h.redirectErr(w, r, "/login?oauth_error=server")
+		}
+		return
+	}
+
+	auth.Set(w, res.Token, res.Expires, h.cookies)
+	http.Redirect(w, r, h.frontendURL+next, http.StatusFound)
+}
+
+// callbackURL composes the redirect_uri the provider's console must allow-list.
+func (h *OAuthHandler) callbackURL(provider string) string {
+	return h.bffPublicURL + "/api/v1/auth/oauth/" + provider + "/callback"
+}
+
+// redirectErr sends the user back to the frontend with a single error query
+// param so the login page can render something meaningful. No internal details
+// leak through this path.
+func (h *OAuthHandler) redirectErr(w http.ResponseWriter, r *http.Request, path string) {
+	http.Redirect(w, r, h.frontendURL+path, http.StatusFound)
+}
+
+// sanitizeNext bounds the post-login redirect to same-origin paths. Anything
+// containing a scheme, host, or whitespace is dropped to the default. This is
+// the same anti-open-redirect treatment we apply on the share-shortener.
+func sanitizeNext(raw string) string {
+	const def = "/u/me"
+	if raw == "" {
+		return def
+	}
+	if !strings.HasPrefix(raw, "/") || strings.HasPrefix(raw, "//") {
+		return def
+	}
+	if strings.ContainsAny(raw, " \t\r\n") {
+		return def
+	}
+	if len(raw) > 512 {
+		return def
+	}
+	return raw
+}

internal/model/user.go

@@ -5,7 +5,9 @@ import "time"
 // User represents a row in the `users` table.
 //
 // PasswordHash is tagged `json:"-"` so it never leaks via JSON marshalling
-// even if a caller accidentally passes a User through response.JSON.
+// even if a caller accidentally passes a User through response.JSON. It's a
+// pointer because OAuth-only users have no password — *PasswordHash == nil
+// means "this account can only sign in via a linked OAuth provider".
 //
 // EmailVerifiedAt is a pointer so JSON-encoded users carry an explicit `null`
 // when the user has not yet verified, instead of an omitted field — the
@@ -19,7 +21,7 @@ type User struct {
 	Role            string     `json:"role"`
 	Plan            string     `json:"plan"` // "free" | "premium"
 	EmailVerifiedAt *time.Time `json:"email_verified_at"`
-	PasswordHash    string     `json:"-"`
+	PasswordHash    *string    `json:"-"`
 	TokenVersion    int        `json:"-"`
 	CreatedAt       time.Time  `json:"created_at"`
 	UpdatedAt       time.Time  `json:"updated_at"`
@@ -42,6 +44,21 @@ const (
 	PurposeEmailVerification = "email_verification"
 )
 
+// OAuthAccount represents a row in the `oauth_accounts` table — one entry
+// per (provider, provider_user_id). A single user can have several linked
+// providers.
+type OAuthAccount struct {
+	ID             string    `json:"id"`
+	UserID         string    `json:"user_id"`
+	Provider       string    `json:"provider"`
+	ProviderUserID string    `json:"provider_user_id"`
+	Email          *string   `json:"email"`
+	Name           *string   `json:"name"`
+	AvatarURL      *string   `json:"avatar_url"`
+	CreatedAt      time.Time `json:"created_at"`
+	UpdatedAt      time.Time `json:"updated_at"`
+}
+
 // ShortURL represents a shortened URL.
 type ShortURL struct {
 	ID         string     `json:"id"`

internal/repository/oauth_account_repo.go

@@ -0,0 +1,69 @@
+package repository
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	"github.com/jackc/pgx/v5"
+	"github.com/prabhupadasays/spsays-bff/internal/model"
+)
+
+// ErrOAuthAccountNotFound mirrors the other repo sentinels.
+var ErrOAuthAccountNotFound = errors.New("oauth account not found")
+
+// OAuthAccountRepo manages provider linkage rows.
+type OAuthAccountRepo struct {
+	pg *Postgres
+}
+
+func NewOAuthAccountRepo(pg *Postgres) *OAuthAccountRepo {
+	return &OAuthAccountRepo{pg: pg}
+}
+
+// FindByProvider returns the linkage row for the given (provider, provider_user_id),
+// or ErrOAuthAccountNotFound. Use this first on every callback — a hit means
+// this provider identity has been seen before and we just log the user in.
+func (r *OAuthAccountRepo) FindByProvider(ctx context.Context, provider, providerUserID string) (*model.OAuthAccount, error) {
+	const q = `
+		SELECT id, user_id, provider, provider_user_id, email, name, avatar_url, created_at, updated_at
+		FROM oauth_accounts
+		WHERE provider = $1 AND provider_user_id = $2
+		LIMIT 1`
+	a := &model.OAuthAccount{}
+	err := r.pg.Pool.QueryRow(ctx, q, provider, providerUserID).Scan(
+		&a.ID, &a.UserID, &a.Provider, &a.ProviderUserID,
+		&a.Email, &a.Name, &a.AvatarURL, &a.CreatedAt, &a.UpdatedAt,
+	)
+	if err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			return nil, ErrOAuthAccountNotFound
+		}
+		return nil, fmt.Errorf("find oauth account: %w", err)
+	}
+	return a, nil
+}
+
+// Link inserts a new oauth_accounts row tying a provider identity to an
+// existing user. Idempotent: a second call for the same (provider, sub)
+// returns the existing row instead of erroring on the unique constraint.
+func (r *OAuthAccountRepo) Link(ctx context.Context, userID, provider, providerUserID string, email, name, avatarURL *string) (*model.OAuthAccount, error) {
+	const q = `
+		INSERT INTO oauth_accounts (user_id, provider, provider_user_id, email, name, avatar_url)
+		VALUES ($1, $2, $3, $4, $5, $6)
+		ON CONFLICT (provider, provider_user_id) DO UPDATE
+		  SET email      = EXCLUDED.email,
+		      name       = EXCLUDED.name,
+		      avatar_url = EXCLUDED.avatar_url,
+		      updated_at = now()
+		RETURNING id, user_id, provider, provider_user_id, email, name, avatar_url, created_at, updated_at`
+	a := &model.OAuthAccount{}
+	err := r.pg.Pool.QueryRow(ctx, q, userID, provider, providerUserID, email, name, avatarURL).Scan(
+		&a.ID, &a.UserID, &a.Provider, &a.ProviderUserID,
+		&a.Email, &a.Name, &a.AvatarURL, &a.CreatedAt, &a.UpdatedAt,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("link oauth account: %w", err)
+	}
+	return a, nil
+}

internal/repository/user_repo.go

@@ -62,6 +62,32 @@ func (r *UserRepo) FindByEmail(ctx context.Context, email string) (*model.User,
 		`WHERE email = $1`, strings.ToLower(strings.TrimSpace(email)))
 }
 
+// CreateOAuth inserts a user with a NULL password_hash — used by the OAuth
+// sign-in flow when there's no matching email yet. Same handle-collision
+// retry contract as Create.
+func (r *UserRepo) CreateOAuth(ctx context.Context, email, name, handle string, avatarURL *string, emailVerified bool) (*model.User, error) {
+	const q = `
+		INSERT INTO users (email, password_hash, name, handle, avatar_url, email_verified_at)
+		VALUES ($1, NULL, $2, $3, $4, CASE WHEN $5::boolean THEN now() ELSE NULL END)
+		RETURNING id, email, handle, password_hash, name, avatar_url, role, plan,
+		          email_verified_at, token_version, created_at, updated_at`
+	u := &model.User{}
+	err := r.pg.Pool.QueryRow(ctx, q,
+		strings.ToLower(strings.TrimSpace(email)), name, handle, avatarURL, emailVerified,
+	).Scan(&u.ID, &u.Email, &u.Handle, &u.PasswordHash, &u.Name, &u.AvatarURL, &u.Role, &u.Plan,
+		&u.EmailVerifiedAt, &u.TokenVersion, &u.CreatedAt, &u.UpdatedAt)
+	if err != nil {
+		if isUniqueViolation(err, "users_handle_key") {
+			return nil, ErrHandleTaken
+		}
+		if isUniqueViolation(err, "users_email_key") {
+			return nil, ErrEmailTaken
+		}
+		return nil, fmt.Errorf("create oauth user: %w", err)
+	}
+	return u, nil
+}
+
 // FindByID returns the user with the given id, or ErrUserNotFound.
 func (r *UserRepo) FindByID(ctx context.Context, id string) (*model.User, error) {
 	return r.findOne(ctx, `WHERE id = $1`, id)

internal/server/server.go

@@ -16,6 +16,7 @@ import (
 	"github.com/prabhupadasays/spsays-bff/internal/worker"
 	"github.com/prabhupadasays/spsays-bff/pkg/auth"
 	"github.com/prabhupadasays/spsays-bff/pkg/mailer"
+	"github.com/prabhupadasays/spsays-bff/pkg/oauth"
 	"github.com/prabhupadasays/spsays-bff/pkg/shorturl"
 	"go.uber.org/zap"
 )
@@ -45,6 +46,7 @@ type Server struct {
 	searchH        *handler.SearchHandler
 	shortURLH      *handler.ShortURLHandler
 	authH          *handler.AuthHandler
+	oauthH         *handler.OAuthHandler
 	userH          *handler.UserHandler
 	postH          *handler.PostHandler
 	bookmarkH      *handler.BookmarkHandler
@@ -121,6 +123,7 @@ func (s *Server) initAuth(cfg *config.Config, logger *zap.Logger) {
 
 	userRepo := repository.NewUserRepo(s.pg)
 	tokenRepo := repository.NewAuthTokenRepo(s.pg)
+	oauthRepo := repository.NewOAuthAccountRepo(s.pg)
 
 	r2, err := repository.NewR2(repository.R2Config{
 		AccountID:       cfg.R2AccountID,
@@ -145,7 +148,7 @@ func (s *Server) initAuth(cfg *config.Config, logger *zap.Logger) {
 		From:     cfg.SMTPFrom,
 	}, logger)
 
-	authSvc := service.NewAuthService(userRepo, tokenRepo, issuer, mail, logger, cfg.FrontendURL)
+	authSvc := service.NewAuthService(userRepo, tokenRepo, oauthRepo, issuer, mail, logger, cfg.FrontendURL)
 	userSvc := service.NewUserService(userRepo, r2)
 
 	// Social repositories + services share the same Postgres pool and R2 client.
@@ -162,6 +165,25 @@ func (s *Server) initAuth(cfg *config.Config, logger *zap.Logger) {
 	s.userRepo = userRepo
 	s.authH = handler.NewAuthHandler(authSvc, cookieOpts)
 	s.userH = handler.NewUserHandler(userSvc)
+
+	// OAuth — wired only when at least one provider is configured. The
+	// registry skips providers with missing creds, so partial configs (e.g.
+	// Google only, no Facebook) work out of the box.
+	providers := oauth.NewRegistry(oauth.Credentials{
+		GoogleClientID:       cfg.GoogleOAuthClientID,
+		GoogleClientSecret:   cfg.GoogleOAuthClientSecret,
+		FacebookClientID:     cfg.FacebookOAuthClientID,
+		FacebookClientSecret: cfg.FacebookOAuthClientSecret,
+	})
+	if len(providers) > 0 {
+		s.oauthH = handler.NewOAuthHandler(
+			authSvc, cookieOpts, providers,
+			[]byte(cfg.JWTSecret), cfg.BFFPublicURL, cfg.FrontendURL,
+			logger,
+		)
+	} else {
+		logger.Info("oauth: no providers configured — /auth/oauth/* will 404")
+	}
 	s.postH = handler.NewPostHandler(postSvc)
 	s.bookmarkH = handler.NewBookmarkHandler(bookmarkSvc)
 	s.collectionH = handler.NewCollectionHandler(collectionSvc, postRepo, issuer, userRepo)
@@ -486,6 +508,15 @@ func (s *Server) setupRouter() {
 			r.Post("/reset-password", s.authH.ResetPassword)
 			r.Post("/verify-email", s.authH.VerifyEmail)
 
+			// OAuth (Google, Facebook, …). GET endpoints because the browser
+			// navigates directly to /start (top-level redirect) and the
+			// provider redirects back to /callback. Both flows set cookies
+			// and 302 — they never return JSON.
+			if s.oauthH != nil {
+				r.Get("/oauth/{provider}/start", s.oauthH.Start)
+				r.Get("/oauth/{provider}/callback", s.oauthH.Callback)
+			}
+
 			r.Group(func(r chi.Router) {
 				r.Use(middleware.RequireAuth(s.authIssuer, s.userRepo))
 				r.Get("/me", s.authH.Me)

internal/service/auth_service.go

@@ -39,6 +39,7 @@ var (
 type AuthService struct {
 	users    *repository.UserRepo
 	tokens   *repository.AuthTokenRepo
+	oauth    *repository.OAuthAccountRepo // optional — nil disables OAuth flows
 	jwt      *auth.Issuer
 	mail     mailer.Mailer
 	logger   *zap.Logger
@@ -49,10 +50,12 @@ type AuthService struct {
 	EmailVerifyTTL   time.Duration
 }
 
-// NewAuthService wires the dependencies.
+// NewAuthService wires the dependencies. `oauthRepo` may be nil — pass nil if
+// the OAuth flows aren't wired (handler then returns 503).
 func NewAuthService(
 	users *repository.UserRepo,
 	tokens *repository.AuthTokenRepo,
+	oauthRepo *repository.OAuthAccountRepo,
 	jwt *auth.Issuer,
 	mail mailer.Mailer,
 	logger *zap.Logger,
@@ -61,6 +64,7 @@ func NewAuthService(
 	return &AuthService{
 		users:            users,
 		tokens:           tokens,
+		oauth:            oauthRepo,
 		jwt:              jwt,
 		mail:             mail,
 		logger:           logger,
@@ -136,12 +140,135 @@ func (s *AuthService) LogIn(ctx context.Context, email, password string) (*AuthR
 		}
 		return nil, err
 	}
-	if err := bcrypt.CompareHashAndPassword([]byte(u.PasswordHash), []byte(password)); err != nil {
+	// OAuth-only accounts have no password hash. Treat them as a credential
+	// mismatch so we don't reveal that an OAuth user exists for this email.
+	if u.PasswordHash == nil {
+		return nil, ErrInvalidCredentials
+	}
+	if err := bcrypt.CompareHashAndPassword([]byte(*u.PasswordHash), []byte(password)); err != nil {
 		return nil, ErrInvalidCredentials
 	}
 	return s.issueSession(u)
 }
 
+// OAuthProfile is the provider-agnostic profile shape consumed by
+// SignInWithOAuth. Mirrors pkg/oauth.Profile but lives here so the service
+// layer doesn't import the OAuth package (which would create a layering
+// cycle once handlers compose both).
+type OAuthProfile struct {
+	Provider       string
+	ProviderUserID string
+	Email          string
+	EmailVerified  bool
+	Name           string
+	AvatarURL      string
+}
+
+// ErrOAuthEmailMissing is returned by SignInWithOAuth when the provider did
+// not give us an email and the account has no linkage we could match on.
+// Without an email we can't safely create a new user (since later sign-ins
+// via email/password would have no way to converge with this account).
+var ErrOAuthEmailMissing = errors.New("oauth provider did not return an email")
+
+// ErrOAuthUnavailable means the OAuth repo wasn't wired (no Postgres at
+// startup). Handler maps to 503.
+var ErrOAuthUnavailable = errors.New("oauth service unavailable")
+
+// SignInWithOAuth turns a provider profile into a session. Three-step lookup:
+//
+//  1. Existing linkage on (provider, provider_user_id) → log that user in.
+//  2. Existing user with same email → link this provider to them, log in.
+//  3. Otherwise create a new user (no password, email_verified iff provider
+//     said so) and link.
+//
+// In every case the resulting user gets a fresh JWT just like LogIn.
+func (s *AuthService) SignInWithOAuth(ctx context.Context, prof OAuthProfile) (*AuthResult, error) {
+	if s.oauth == nil {
+		return nil, ErrOAuthUnavailable
+	}
+
+	// 1. Known linkage.
+	if a, err := s.oauth.FindByProvider(ctx, prof.Provider, prof.ProviderUserID); err == nil {
+		u, err := s.users.FindByID(ctx, a.UserID)
+		if err != nil {
+			return nil, err
+		}
+		// Refresh denormalised fields on the linkage row (display name /
+		// avatar may have changed at the provider).
+		_, _ = s.oauth.Link(ctx, u.ID, prof.Provider, prof.ProviderUserID,
+			optStr(prof.Email), optStr(prof.Name), optStr(prof.AvatarURL))
+		return s.issueSession(u)
+	} else if !errors.Is(err, repository.ErrOAuthAccountNotFound) {
+		return nil, err
+	}
+
+	// No linkage yet — we need at least an email to safely converge with an
+	// existing account or create a new one.
+	if prof.Email == "" {
+		return nil, ErrOAuthEmailMissing
+	}
+
+	// 2. Existing user with the same email → link.
+	if u, err := s.users.FindByEmail(ctx, prof.Email); err == nil {
+		if _, err := s.oauth.Link(ctx, u.ID, prof.Provider, prof.ProviderUserID,
+			optStr(prof.Email), optStr(prof.Name), optStr(prof.AvatarURL)); err != nil {
+			return nil, err
+		}
+		return s.issueSession(u)
+	} else if !errors.Is(err, repository.ErrUserNotFound) {
+		return nil, err
+	}
+
+	// 3. Brand new user. Reuse the handle-collision retry loop.
+	displayName := prof.Name
+	if displayName == "" {
+		displayName = strings.SplitN(prof.Email, "@", 2)[0]
+	}
+	base := slugForHandle(prof.Email)
+	var u *model.User
+	var err error
+	for attempt := 0; attempt < 8; attempt++ {
+		handle := base
+		if attempt > 0 {
+			handle = fmt.Sprintf("%s-%s", base, randomHandleSuffix())
+		}
+		u, err = s.users.CreateOAuth(ctx, prof.Email, displayName, handle, optStr(prof.AvatarURL), prof.EmailVerified)
+		if err == nil {
+			break
+		}
+		if errors.Is(err, repository.ErrEmailTaken) {
+			// Race: someone else just claimed this email. Fall back to step 2.
+			if u2, err2 := s.users.FindByEmail(ctx, prof.Email); err2 == nil {
+				_, _ = s.oauth.Link(ctx, u2.ID, prof.Provider, prof.ProviderUserID,
+					optStr(prof.Email), optStr(prof.Name), optStr(prof.AvatarURL))
+				return s.issueSession(u2)
+			}
+			return nil, err
+		}
+		if !errors.Is(err, repository.ErrHandleTaken) {
+			return nil, err
+		}
+	}
+	if err != nil {
+		return nil, fmt.Errorf("create oauth user: %w", err)
+	}
+
+	if _, err := s.oauth.Link(ctx, u.ID, prof.Provider, prof.ProviderUserID,
+		optStr(prof.Email), optStr(prof.Name), optStr(prof.AvatarURL)); err != nil {
+		return nil, err
+	}
+	return s.issueSession(u)
+}
+
+// optStr returns nil for empty input, &s otherwise — used to populate
+// nullable text columns from possibly-empty profile fields.
+func optStr(s string) *string {
+	if s == "" {
+		return nil
+	}
+	return &s
+}
+
 // Me looks up a user by id and re-verifies the JWT's token_version against
 // the stored value. Mismatch means the user has logged-out-everywhere or
 // changed password since this JWT was issued → reject.