package main

import (
	"context"
	"database/sql"
	_ "embed"
	"fmt"
	"time"

	"github.com/mattn/go-sqlite3"
)

//go:embed schema.sql
var schema string

var errNoDBRows = sql.ErrNoRows

type DB struct {
	db *sql.DB
}

func openDB(filename string) (*DB, error) {
	sqlDB, err := sql.Open("sqlite3", filename)
	if err != nil {
		return nil, err
	}

	db := &DB{sqlDB}
	if err := db.init(context.TODO()); err != nil {
		db.Close()
		return nil, err
	}

	return db, nil
}

func (db *DB) init(ctx context.Context) error {
	var n int
	if err := db.db.QueryRowContext(ctx, "SELECT COUNT(*) FROM sqlite_schema").Scan(&n); err != nil {
		return err
	} else if n != 0 {
		return nil
	}

	if _, err := db.db.ExecContext(ctx, schema); err != nil {
		return err
	}

	// TODO: drop this
	defaultUser := User{Username: "root"}
	if err := defaultUser.SetPassword("root"); err != nil {
		return err
	}
	return db.StoreUser(ctx, &defaultUser)
}

func (db *DB) Close() error {
	return db.db.Close()
}

func (db *DB) FetchUser(ctx context.Context, id ID[*User]) (*User, error) {
	rows, err := db.db.QueryContext(ctx, "SELECT * FROM User WHERE id = ?", id)
	if err != nil {
		return nil, err
	}
	var user User
	err = scanRow(&user, rows)
	return &user, err
}

func (db *DB) FetchUserByUsername(ctx context.Context, username string) (*User, error) {
	rows, err := db.db.QueryContext(ctx, "SELECT * FROM User WHERE username = ?", username)
	if err != nil {
		return nil, err
	}
	var user User
	err = scanRow(&user, rows)
	return &user, err
}

func (db *DB) StoreUser(ctx context.Context, user *User) error {
	return db.db.QueryRowContext(ctx, `
		INSERT INTO User(id, username, password_hash)
		VALUES (:id, :username, :password_hash)
		ON CONFLICT(id) DO UPDATE SET
			username = :username,
			password_hash = :password_hash
		RETURNING id
	`, entityArgs(user)...).Scan(&user.ID)
}

func (db *DB) ListUsers(ctx context.Context) ([]User, error) {
	rows, err := db.db.QueryContext(ctx, "SELECT * FROM User")
	if err != nil {
		return nil, err
	}
	defer rows.Close()

	var l []User
	for rows.Next() {
		var user User
		if err := scan(&user, rows); err != nil {
			return nil, err
		}
		l = append(l, user)
	}

	return l, rows.Close()
}

func (db *DB) FetchClient(ctx context.Context, id ID[*Client]) (*Client, error) {
	rows, err := db.db.QueryContext(ctx, "SELECT * FROM Client WHERE id = ?", id)
	if err != nil {
		return nil, err
	}
	var client Client
	err = scanRow(&client, rows)
	return &client, err
}

func (db *DB) FetchClientByClientID(ctx context.Context, clientID string) (*Client, error) {
	rows, err := db.db.QueryContext(ctx, "SELECT * FROM Client WHERE client_id = ?", clientID)
	if err != nil {
		return nil, err
	}
	var client Client
	err = scanRow(&client, rows)
	return &client, err
}

func (db *DB) StoreClient(ctx context.Context, client *Client) error {
	return db.db.QueryRowContext(ctx, `
		INSERT INTO Client(id, client_id, client_secret_hash, owner,
			redirect_uris, client_name, client_uri)
		VALUES (:id, :client_id, :client_secret_hash, :owner,
			:redirect_uris, :client_name, :client_uri)
		ON CONFLICT(id) DO UPDATE SET
			client_id = :client_id,
			client_secret_hash = :client_secret_hash,
			owner = :owner,
			redirect_uris = :redirect_uris,
			client_name = :client_name,
			client_uri = :client_uri
		RETURNING id
	`, entityArgs(client)...).Scan(&client.ID)
}

func (db *DB) ListClients(ctx context.Context, owner ID[*User]) ([]Client, error) {
	rows, err := db.db.QueryContext(ctx, "SELECT * FROM Client WHERE owner IS ?", owner)
	if err != nil {
		return nil, err
	}
	defer rows.Close()

	var l []Client
	for rows.Next() {
		var client Client
		if err := scan(&client, rows); err != nil {
			return nil, err
		}
		l = append(l, client)
	}

	return l, rows.Close()
}

func (db *DB) ListAuthorizedClients(ctx context.Context, user ID[*User]) ([]AuthorizedClient, error) {
	rows, err := db.db.QueryContext(ctx, `
		SELECT id, client_id, client_name, client_uri, token.expires_at
		FROM Client,
		(
			SELECT client, MAX(expires_at) as expires_at
			FROM AccessToken
			WHERE user = ?
			GROUP BY client
		) AS token
		WHERE Client.id = token.client
	`, user)
	if err != nil {
		return nil, err
	}

	var l []AuthorizedClient
	for rows.Next() {
		var authClient AuthorizedClient
		columns := authClient.Client.columns()
		var expiresAt string
		err := rows.Scan(columns["id"], columns["client_id"], columns["client_name"], columns["client_uri"], &expiresAt)
		if err != nil {
			return nil, err
		}
		authClient.ExpiresAt, err = time.Parse(sqlite3.SQLiteTimestampFormats[0], expiresAt)
		if err != nil {
			return nil, err
		}
		l = append(l, authClient)
	}

	return l, rows.Close()
}

func (db *DB) DeleteClient(ctx context.Context, id ID[*Client]) error {
	_, err := db.db.ExecContext(ctx, "DELETE FROM Client WHERE id = ?", id)
	return err
}

func (db *DB) FetchAccessToken(ctx context.Context, id ID[*AccessToken]) (*AccessToken, error) {
	rows, err := db.db.QueryContext(ctx, "SELECT * FROM AccessToken WHERE id = ?", id)
	if err != nil {
		return nil, err
	}
	var token AccessToken
	err = scanRow(&token, rows)
	return &token, err
}

func (db *DB) CreateAccessToken(ctx context.Context, token *AccessToken) error {
	return db.db.QueryRowContext(ctx, `
		INSERT INTO AccessToken(hash, user, client, scope, issued_at, expires_at)
		VALUES (:hash, :user, :client, :scope, :issued_at, :expires_at)
		RETURNING id
	`, entityArgs(token)...).Scan(&token.ID)
}

func (db *DB) DeleteAccessToken(ctx context.Context, id ID[*AccessToken]) error {
	_, err := db.db.ExecContext(ctx, "DELETE FROM AccessToken WHERE id = ?", id)
	return err
}

func (db *DB) RevokeAccessTokens(ctx context.Context, clientID ID[*Client], userID ID[*User]) error {
	_, err := db.db.ExecContext(ctx, `
		DELETE FROM AccessToken
		WHERE client = ? AND user = ?
	`, clientID, userID)
	return err
}

func (db *DB) CreateAuthCode(ctx context.Context, code *AuthCode) error {
	return db.db.QueryRowContext(ctx, `
		INSERT INTO AuthCode(hash, created_at, user, client, scope)
		VALUES (:hash, :created_at, :user, :client, :scope)
		RETURNING id
	`, entityArgs(code)...).Scan(&code.ID)
}

func (db *DB) PopAuthCode(ctx context.Context, id ID[*AuthCode]) (*AuthCode, error) {
	rows, err := db.db.QueryContext(ctx, `
		DELETE FROM AuthCode
		WHERE id = ?
		RETURNING *
	`, id)
	if err != nil {
		return nil, err
	}
	var authCode AuthCode
	err = scanRow(&authCode, rows)
	return &authCode, err
}

func (db *DB) Maintain(ctx context.Context) error {
	_, err := db.db.ExecContext(ctx, `
		DELETE FROM AccessToken
		WHERE timediff('now', expires_at) > 0
	`)
	if err != nil {
		return err
	}

	_, err = db.db.ExecContext(ctx, `
		DELETE FROM AuthCode
		WHERE timediff(?, created_at) > 0
	`, time.Now().Add(-authCodeExpiration))
	if err != nil {
		return err
	}

	return nil
}

func scan(e entity, rows *sql.Rows) error {
	columns := e.columns()

	keys, err := rows.Columns()
	if err != nil {
		panic(err)
	}
	out := make([]interface{}, len(keys))
	for i, k := range keys {
		v, ok := columns[k]
		if !ok {
			panic(fmt.Errorf("unknown column %q", k))
		}
		out[i] = v
	}

	return rows.Scan(out...)
}

func scanRow(e entity, rows *sql.Rows) error {
	if !rows.Next() {
		return sql.ErrNoRows
	}
	if err := scan(e, rows); err != nil {
		return err
	}
	return rows.Close()
}

func entityArgs(e entity) []interface{} {
	columns := e.columns()

	l := make([]interface{}, 0, len(columns))
	for k, v := range columns {
		l = append(l, sql.Named(k, v))
	}

	return l
}

package main

import (
	"context"
	"embed"
	"flag"
	"html/template"
	"log"
	"net"
	"net/http"
	"time"

	"github.com/go-chi/chi/v5"
)

var (
	//go:embed template
	templateFS embed.FS
	//go:embed static
	staticFS embed.FS
)

func main() {
	var configFilename, listenAddr string
	flag.StringVar(&configFilename, "config", "/etc/sinwon/config", "Configuration filename")
	flag.StringVar(&listenAddr, "listen", "", "HTTP listen address")
	flag.Parse()

	cfg, err := loadConfig(configFilename)
	if err != nil {
		log.Fatalf("Failed to load config file: %v", err)
	}

	if listenAddr == "" {
		listenAddr = cfg.Listen
	}
	if listenAddr == "" {
		log.Fatalf("Missing listen configuration")
	}
	if cfg.Database == "" {
		log.Fatalf("Missing database configuration")
	}

	db, err := openDB(cfg.Database)
	if err != nil {
		log.Fatalf("Failed to open DB: %v", err)
	}

	tpl := template.Must(template.ParseFS(templateFS, "template/*.html"))

	mux := chi.NewRouter()
	mux.Handle("/static/*", http.FileServer(http.FS(staticFS)))
	mux.Get("/", index)
	mux.HandleFunc("/login", login)
	mux.Post("/logout", logout)
	mux.HandleFunc("/client/new", manageClient)
	mux.HandleFunc("/client/{id}", manageClient)
	mux.Post("/client/{id}/revoke", revokeClient)
	mux.HandleFunc("/user/new", manageUser)
	mux.HandleFunc("/user/{id}", manageUser)
	mux.Get("/.well-known/oauth-authorization-server", getOAuthServerMetadata)
	mux.HandleFunc("/authorize", authorize)
	mux.Post("/token", exchangeToken)
	mux.Post("/introspect", introspectToken)

	mux.Post("/revoke", revokeToken)

	go maintainDBLoop(db)

	server := http.Server{
		Addr:    listenAddr,
		Handler: loginTokenMiddleware(mux),
		BaseContext: func(net.Listener) context.Context {
			return newBaseContext(db, tpl)
		},
	}
	log.Printf("OAuth server listening on %v", server.Addr)
	if err := server.ListenAndServe(); err != nil {
		log.Fatalf("Failed to listen and serve: %v", err)
	}
}

func httpError(w http.ResponseWriter, err error) {
	log.Print(err)
	http.Error(w, "Internal server error", http.StatusInternalServerError)
}

func maintainDBLoop(db *DB) {
	ticker := time.NewTicker(15 * time.Minute)
	defer ticker.Stop()

	for range ticker.C {
		ctx := context.Background()
		ctx, cancel := context.WithTimeout(ctx, 30*time.Second)
		if err := db.Maintain(ctx); err != nil {
			log.Printf("Failed to perform database maintenance: %v", err)
		}
		cancel()
	}
}

package main

import (
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"log"
	"mime"
	"net"
	"net/http"
	"net/url"
	"strings"
	"time"

	"git.sr.ht/~emersion/go-oauth2"
)

func getOAuthServerMetadata(w http.ResponseWriter, req *http.Request) {
	issuerURL := url.URL{
		Scheme: "https",
		Host:   req.Host,
	}
	if !isForwardedHTTPS(req) && isLoopback(req) {
		// TODO: add config option for allowed reverse proxy IPs
		issuerURL.Scheme = "http"
	}
	issuer := issuerURL.String()

	w.Header().Set("Content-Type", "application/json")
	json.NewEncoder(w).Encode(&oauth2.ServerMetadata{
		Issuer:                                    issuer,
		AuthorizationEndpoint:                     issuer + "/authorize",
		TokenEndpoint:                             issuer + "/token",
		IntrospectionEndpoint:                     issuer + "/introspect",

		RevocationEndpoint:                        issuer + "/revoke",

		ResponseTypesSupported:                    []oauth2.ResponseType{oauth2.ResponseTypeCode},
		ResponseModesSupported:                    []oauth2.ResponseMode{oauth2.ResponseModeQuery},
		GrantTypesSupported:                       []oauth2.GrantType{oauth2.GrantTypeAuthorizationCode},
		TokenEndpointAuthMethodsSupported:         []oauth2.AuthMethod{oauth2.AuthMethodClientSecretBasic},
		IntrospectionEndpointAuthMethodsSupported: []oauth2.AuthMethod{oauth2.AuthMethodClientSecretBasic},

		RevocationEndpointAuthMethodsSupported:    []oauth2.AuthMethod{oauth2.AuthMethodClientSecretBasic},

	})
}

func isLoopback(req *http.Request) bool {
	host, _, _ := net.SplitHostPort(req.RemoteAddr)
	ip := net.ParseIP(host)
	return ip.IsLoopback()
}

func authorize(w http.ResponseWriter, req *http.Request) {
	ctx := req.Context()
	db := dbFromContext(ctx)
	tpl := templateFromContext(ctx)

	q := req.URL.Query()
	respType := oauth2.ResponseType(q.Get("response_type"))
	clientID := q.Get("client_id")
	rawRedirectURI := q.Get("redirect_uri")
	scope := q.Get("scope")
	state := q.Get("state")

	if clientID == "" {
		http.Error(w, "Missing client ID", http.StatusBadRequest)
		return
	}

	client, err := db.FetchClientByClientID(ctx, clientID)
	if err == errNoDBRows {
		http.Error(w, "Invalid client ID", http.StatusForbidden)
		return
	} else if err != nil {
		httpError(w, fmt.Errorf("failed to fetch client: %v", err))
		return
	}

	var allowedRedirectURIs []*url.URL
	for _, s := range strings.Split(client.RedirectURIs, "\n") {
		if s == "" {
			continue
		}
		u, err := url.Parse(s)
		if err != nil {
			httpError(w, fmt.Errorf("failed to parse client redirect URI"))
			return
		}
		allowedRedirectURIs = append(allowedRedirectURIs, u)
	}

	var redirectURI *url.URL
	if rawRedirectURI != "" {
		redirectURI, err = url.Parse(rawRedirectURI)
		if err != nil {
			http.Error(w, "Invalid redirect URI", http.StatusBadRequest)
			return
		}
		if !validateRedirectURI(redirectURI, allowedRedirectURIs) {
			http.Error(w, "Invalid redirect URI", http.StatusBadRequest)
			return
		}
	} else {
		if len(allowedRedirectURIs) == 0 {
			http.Error(w, "Missing redirect URI", http.StatusBadRequest)
			return
		}
		redirectURI = allowedRedirectURIs[0]
	}

	if respType != oauth2.ResponseTypeCode {
		redirectClientError(w, req, redirectURI, state, &oauth2.Error{
			Code: oauth2.ErrorCodeUnsupportedResponseType,
		})
		return
	}

	// TODO: add support for scope
	if scope != "" {
		redirectClientError(w, req, redirectURI, state, &oauth2.Error{
			Code: oauth2.ErrorCodeInvalidScope,
		})
		return
	}

	loginToken := loginTokenFromContext(ctx)
	if loginToken == nil {
		q := make(url.Values)
		q.Set("redirect_uri", req.URL.String())
		u := url.URL{
			Path:     "/login",
			RawQuery: q.Encode(),
		}
		http.Redirect(w, req, u.String(), http.StatusFound)
		return
	}

	_ = req.ParseForm()
	if _, ok := req.PostForm["deny"]; ok {
		redirectClientError(w, req, redirectURI, state, &oauth2.Error{
			Code: oauth2.ErrorCodeAccessDenied,
		})
		return
	}
	if _, ok := req.PostForm["authorize"]; !ok {
		data := struct {
			Client *Client
		}{
			Client: client,
		}
		if err := tpl.ExecuteTemplate(w, "authorize.html", data); err != nil {
			panic(err)
		}
		return
	}

	authCode, secret, err := NewAuthCode(loginToken.User, client.ID, scope)
	if err != nil {
		httpError(w, fmt.Errorf("failed to generate authentication code: %v", err))
		return
	}

	if err := db.CreateAuthCode(ctx, authCode); err != nil {
		httpError(w, fmt.Errorf("failed to create authentication code: %v", err))
		return
	}

	code := MarshalSecret(authCode.ID, secret)

	values := make(url.Values)
	values.Set("code", code)
	if state != "" {
		values.Set("state", state)
	}
	redirectClient(w, req, redirectURI, values)
}

func exchangeToken(w http.ResponseWriter, req *http.Request) {
	ctx := req.Context()
	db := dbFromContext(ctx)

	values, err := parseRequestBody(req)
	if err != nil {
		oauthError(w, &oauth2.Error{
			Code:        oauth2.ErrorCodeInvalidRequest,
			Description: err.Error(),
		})
		return
	}

	clientID := values.Get("client_id")
	grantType := oauth2.GrantType(values.Get("grant_type"))
	scope := values.Get("scope")

	authClientID, clientSecret, _ := req.BasicAuth()
	if clientID == "" && authClientID == "" {
		oauthError(w, &oauth2.Error{
			Code:        oauth2.ErrorCodeInvalidRequest,
			Description: "Missing client ID",
		})
		return
	} else if clientID == "" {
		clientID = authClientID
	} else if clientID != authClientID {
		oauthError(w, &oauth2.Error{
			Code:        oauth2.ErrorCodeInvalidRequest,
			Description: "Client ID in request body doesn't match Authorization header field",
		})
		return
	}

	client, err := db.FetchClientByClientID(ctx, clientID)
	if err == errNoDBRows {
		oauthError(w, &oauth2.Error{
			Code:        oauth2.ErrorCodeInvalidClient,
			Description: "Invalid client ID",
		})
		return
	} else if err != nil {
		oauthError(w, fmt.Errorf("failed to fetch client: %v", err))
		return
	}

	if !client.IsPublic() {
		if !client.VerifySecret(clientSecret) {
			oauthError(w, &oauth2.Error{
				Code:        oauth2.ErrorCodeAccessDenied,
				Description: "Invalid client secret",
			})
			return
		}
	}

	if grantType != oauth2.GrantTypeAuthorizationCode {
		oauthError(w, &oauth2.Error{
			Code:        oauth2.ErrorCodeUnsupportedGrantType,
			Description: "Unsupported grant type",
		})
		return
	}

	codeID, codeSecret, _ := UnmarshalSecret[*AuthCode](values.Get("code"))
	authCode, err := db.PopAuthCode(ctx, codeID)
	if err == errNoDBRows || (err == nil && !authCode.VerifySecret(codeSecret)) || authCode.Client != client.ID {
		oauthError(w, &oauth2.Error{
			Code:        oauth2.ErrorCodeAccessDenied,
			Description: "Invalid authorization code",
		})
		return
	} else if err != nil {
		oauthError(w, fmt.Errorf("failed to fetch authorization code: %v", err))
		return
	}

	if scope != authCode.Scope {
		oauthError(w, &oauth2.Error{
			Code:        oauth2.ErrorCodeAccessDenied,
			Description: "Invalid scope",
		})
		return
	}

	// TODO: check redirect_uri

	token, secret, err := NewAccessTokenFromAuthCode(authCode)
	if err != nil {
		oauthError(w, err)
		return
	}

	if err := db.CreateAccessToken(ctx, token); err != nil {
		oauthError(w, fmt.Errorf("failed to create access token: %v", err))
		return
	}

	w.Header().Set("Content-Type", "application/json")
	w.Header().Set("Cache-Control", "no-store")
	json.NewEncoder(w).Encode(&oauth2.TokenResp{
		AccessToken: MarshalSecret(token.ID, secret),
		TokenType:   oauth2.TokenTypeBearer,
		ExpiresIn:   time.Until(token.ExpiresAt),
		Scope:       strings.Split(token.Scope, " "),
	})
}

func introspectToken(w http.ResponseWriter, req *http.Request) {
	ctx := req.Context()
	db := dbFromContext(ctx)

	values, err := parseRequestBody(req)
	if err != nil {
		oauthError(w, &oauth2.Error{
			Code:        oauth2.ErrorCodeInvalidRequest,
			Description: err.Error(),
		})
		return
	}

	var client *Client
	if clientID, clientSecret, ok := req.BasicAuth(); ok {
		client, err = db.FetchClientByClientID(ctx, clientID)
		if err == errNoDBRows || (err == nil && !client.VerifySecret(clientSecret)) {
			oauthError(w, &oauth2.Error{
				Code:        oauth2.ErrorCodeInvalidClient,
				Description: "Invalid client ID or secret",
			})
			return
		} else if err != nil {
			oauthError(w, fmt.Errorf("failed to fetch client: %v", err))
			return
		}

	client, err := maybeAuthenticateClient(w, req)
	if err != nil {
		oauthError(w, err)
		return

	}

	tokenID, secret, _ := UnmarshalSecret[*AccessToken](values.Get("token"))
	token, err := db.FetchAccessToken(ctx, tokenID)
	if err == errNoDBRows || (err == nil && !token.VerifySecret(secret)) {
		token = nil
	} else if err != nil {
		oauthError(w, fmt.Errorf("failed to fetch access token: %v", err))
		return
	}

	var resp oauth2.IntrospectionResp
	if token != nil {
		if client == nil {
			client, err = db.FetchClient(ctx, token.Client)
			if err != nil {
				oauthError(w, fmt.Errorf("failed to fetch client: %v", err))
				return
			}

			if !client.IsPublic() {
				oauthError(w, &oauth2.Error{
					Code:        oauth2.ErrorCodeInvalidClient,
					Description: "Missing client ID and secret",
				})
				return
			}
		}

		if client.ID != token.Client {
			oauthError(w, &oauth2.Error{
				Code:        oauth2.ErrorCodeInvalidClient,
				Description: "Invalid client ID or secret",
			})
			return
		}

		user, err := db.FetchUser(ctx, token.User)
		if err != nil {
			oauthError(w, fmt.Errorf("failed to fetch user: %v", err))
			return
		}

		resp.Active = true
		resp.TokenType = oauth2.TokenTypeBearer
		resp.ExpiresAt = token.ExpiresAt
		resp.IssuedAt = token.IssuedAt
		resp.ClientID = client.ClientID
		resp.Username = user.Username
	}

	w.Header().Set("Content-Type", "application/json")
	json.NewEncoder(w).Encode(&resp)
}

func revokeToken(w http.ResponseWriter, req *http.Request) {
	ctx := req.Context()
	db := dbFromContext(ctx)

	values, err := parseRequestBody(req)
	if err != nil {
		oauthError(w, &oauth2.Error{
			Code:        oauth2.ErrorCodeInvalidRequest,
			Description: err.Error(),
		})
		return
	}

	client, err := maybeAuthenticateClient(w, req)
	if err != nil {
		oauthError(w, err)
		return
	}

	tokenID, secret, _ := UnmarshalSecret[*AccessToken](values.Get("token"))
	token, err := db.FetchAccessToken(ctx, tokenID)
	if err == errNoDBRows || (err == nil && !token.VerifySecret(secret)) {
		return // ignore
	} else if err != nil {
		oauthError(w, fmt.Errorf("failed to fetch access token: %v", err))
		return
	}

	if client == nil {
		client, err = db.FetchClient(ctx, token.Client)
		if err != nil {
			oauthError(w, fmt.Errorf("failed to fetch client: %v", err))
			return
		}

		if !client.IsPublic() {
			oauthError(w, &oauth2.Error{
				Code:        oauth2.ErrorCodeInvalidClient,
				Description: "Missing client ID and secret",
			})
			return
		}
	}

	if client.ID != token.Client {
		oauthError(w, &oauth2.Error{
			Code:        oauth2.ErrorCodeInvalidClient,
			Description: "Invalid client ID or secret",
		})
		return
	}

	if err := db.DeleteAccessToken(ctx, token.ID); err != nil {
		oauthError(w, err)
		return
	}
}

func parseRequestBody(req *http.Request) (url.Values, error) {
	ct := req.Header.Get("Content-Type")
	if ct != "" {
		mimeType, _, err := mime.ParseMediaType(req.Header.Get("Content-Type"))
		if err != nil {
			return nil, fmt.Errorf("malformed Content-Type header field")
		} else if mimeType != "application/x-www-form-urlencoded" {
			return nil, fmt.Errorf("unsupported request content type")
		}
	}

	r := io.LimitReader(req.Body, 10<<20)
	b, err := io.ReadAll(r)
	if err != nil {
		return nil, fmt.Errorf("failed to read request body: %v", err)
	}

	values, err := url.ParseQuery(string(b))
	if err != nil {
		return nil, fmt.Errorf("failed to parse request body: %v", err)
	}

	return values, nil
}

func oauthError(w http.ResponseWriter, err error) {
	var oauthErr *oauth2.Error
	if !errors.As(err, &oauthErr) {
		oauthErr = &oauth2.Error{Code: oauth2.ErrorCodeServerError}
		log.Print(err)
	}

	statusCode := http.StatusInternalServerError
	switch oauthErr.Code {
	case oauth2.ErrorCodeInvalidRequest, oauth2.ErrorCodeUnsupportedResponseType, oauth2.ErrorCodeInvalidScope, oauth2.ErrorCodeInvalidClient, oauth2.ErrorCodeInvalidGrant, oauth2.ErrorCodeUnsupportedGrantType:
		statusCode = http.StatusBadRequest
	case oauth2.ErrorCodeUnauthorizedClient, oauth2.ErrorCodeAccessDenied:
		statusCode = http.StatusForbidden
	case oauth2.ErrorCodeTemporarilyUnavailable:
		statusCode = http.StatusServiceUnavailable
	}

	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(statusCode)
	json.NewEncoder(w).Encode(oauthErr)
}

func redirectClient(w http.ResponseWriter, req *http.Request, redirectURI *url.URL, values url.Values) {
	q := redirectURI.Query()
	for k, v := range values {
		q[k] = v
	}

	u := *redirectURI
	u.RawQuery = q.Encode()

	http.Redirect(w, req, u.String(), http.StatusFound)
}

func redirectClientError(w http.ResponseWriter, req *http.Request, redirectURI *url.URL, state string, err error) {
	var oauthErr *oauth2.Error
	if !errors.As(err, &oauthErr) {
		oauthErr = &oauth2.Error{Code: oauth2.ErrorCodeServerError}
		log.Print(err)
	}

	values := make(url.Values)
	values.Set("error", string(oauthErr.Code))
	if oauthErr.Description != "" {
		values.Set("error_description", oauthErr.Description)
	}
	if oauthErr.URI != "" {
		values.Set("error_uri", oauthErr.URI)
	}
	if state != "" {
		values.Set("state", state)
	}
	redirectClient(w, req, redirectURI, values)
}

func validateRedirectURI(u *url.URL, allowedURIs []*url.URL) bool {
	// Loopback interface, see RFC 8252 section 7.3
	host, _, _ := net.SplitHostPort(u.Host)
	ip := net.ParseIP(host)
	if u.Scheme == "http" && ip.IsLoopback() {
		uu := *u
		uu.Host = "localhost"
		u = &uu
	}

	for _, allowed := range allowedURIs {
		if u.String() == allowed.String() {
			return true
		}
	}

	return false
}

func maybeAuthenticateClient(w http.ResponseWriter, req *http.Request) (*Client, error) {
	ctx := req.Context()
	db := dbFromContext(ctx)

	clientID, clientSecret, ok := req.BasicAuth()
	if !ok {
		return nil, nil
	}

	client, err := db.FetchClientByClientID(ctx, clientID)
	if err == errNoDBRows || (err == nil && !client.VerifySecret(clientSecret)) {
		return nil, &oauth2.Error{
			Code:        oauth2.ErrorCodeInvalidClient,
			Description: "Invalid client ID or secret",
		}
	} else if err != nil {
		return nil, fmt.Errorf("failed to fetch client: %v", err)
	}

	return client, nil
}