package auth

import (
	"bytes"
	"context"
	"errors"
	"fmt"
	"io"
	"net"
	"net/http"
	"net/url"
	"regexp"
	"strings"
	"time"

	"github.com/traefik/traefik/v3/pkg/config/dynamic"
	"github.com/traefik/traefik/v3/pkg/middlewares"
	"github.com/traefik/traefik/v3/pkg/middlewares/accesslog"
	"github.com/traefik/traefik/v3/pkg/middlewares/observability"
	"github.com/traefik/traefik/v3/pkg/observability/tracing"
	"github.com/traefik/traefik/v3/pkg/proxy/httputil"
	"github.com/traefik/traefik/v3/pkg/types"
	"github.com/vulcand/oxy/v2/forward"
	"github.com/vulcand/oxy/v2/utils"
	"go.opentelemetry.io/otel/trace"
)

const typeNameForward = "ForwardAuth"

const (
	xForwardedURI    = "X-Forwarded-Uri"
	xForwardedMethod = "X-Forwarded-Method"
)

// hopHeaders Hop-by-hop headers to be removed in the authentication request.
// http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html
// Proxy-Authorization header is forwarded to the authentication server (see https://tools.ietf.org/html/rfc7235#section-4.4).
var hopHeaders = []string{
	forward.Connection,
	forward.KeepAlive,
	forward.Te, // canonicalized version of "TE"
	forward.Trailers,
	forward.TransferEncoding,
	forward.Upgrade,
}

var userAgentHeader = http.CanonicalHeaderKey("User-Agent")

type forwardAuth struct {
	address                  string
	authResponseHeaders      []string
	authResponseHeadersRegex *regexp.Regexp
	next                     http.Handler
	name                     string
	client                   http.Client
	trustForwardHeader       bool
	authRequestHeaders       []string
	maxResponseBodySize      int64
	addAuthCookiesToResponse map[string]struct{}
	headerField              string
	forwardBody              bool
	maxBodySize              int64
	preserveLocationHeader   bool
	preserveRequestMethod    bool
	authSigninURL            string
}

// NewForward creates a forward auth middleware.
func NewForward(ctx context.Context, next http.Handler, config dynamic.ForwardAuth, name string) (http.Handler, error) {
	logger := middlewares.GetLogger(ctx, name, typeNameForward)
	logger.Debug().Msg("Creating middleware")

	addAuthCookiesToResponse := make(map[string]struct{})
	for _, cookieName := range config.AddAuthCookiesToResponse {
		addAuthCookiesToResponse[cookieName] = struct{}{}
	}

	fa := &forwardAuth{
		address:                  config.Address,
		authResponseHeaders:      config.AuthResponseHeaders,
		next:                     next,
		name:                     name,
		trustForwardHeader:       config.TrustForwardHeader,
		authRequestHeaders:       config.AuthRequestHeaders,
		addAuthCookiesToResponse: addAuthCookiesToResponse,
		headerField:              config.HeaderField,
		forwardBody:              config.ForwardBody,
		maxBodySize:              dynamic.ForwardAuthDefaultMaxBodySize,
		preserveLocationHeader:   config.PreserveLocationHeader,
		preserveRequestMethod:    config.PreserveRequestMethod,
		authSigninURL:            config.AuthSigninURL,
	}

	if config.MaxBodySize != nil {
		fa.maxBodySize = *config.MaxBodySize
	} else if fa.forwardBody {
		logger.Warn().Msgf("ForwardAuth 'maxBodySize' is not configured with 'forwardBody: true', allowing unlimited request body size which can lead to DoS attacks and memory exhaustion. Please set an appropriate limit.")
	}

	if config.MaxResponseBodySize != nil {
		fa.maxResponseBodySize = *config.MaxResponseBodySize
	} else {
		fa.maxResponseBodySize = -1
		logger.Warn().Msg("ForwardAuth 'maxResponseBodySize' is not configured, allowing unlimited response body size which can lead to DoS attacks and memory exhaustion. Please set an appropriate limit.")
	}

	// Ensure our request client does not follow redirects
	fa.client = http.Client{
		CheckRedirect: func(r *http.Request, via []*http.Request) error {
			return http.ErrUseLastResponse
		},
		Timeout: 30 * time.Second,
	}

	if config.TLS != nil {
		if config.TLS.CAOptional != nil {
			logger.Warn().Msg("CAOptional option is deprecated, TLS client authentication is a server side option, please remove any usage of this option.")
		}

		clientTLS := &types.ClientTLS{
			CA:                 config.TLS.CA,
			Cert:               config.TLS.Cert,
			Key:                config.TLS.Key,
			InsecureSkipVerify: config.TLS.InsecureSkipVerify,
		}

		tlsConfig, err := clientTLS.CreateTLSConfig(ctx)
		if err != nil {
			return nil, fmt.Errorf("unable to create client TLS configuration: %w", err)
		}

		tr := http.DefaultTransport.(*http.Transport).Clone()
		tr.TLSClientConfig = tlsConfig
		fa.client.Transport = tr
	}

	if config.AuthResponseHeadersRegex != "" {
		re, err := regexp.Compile(config.AuthResponseHeadersRegex)
		if err != nil {
			return nil, fmt.Errorf("error compiling regular expression %s: %w", config.AuthResponseHeadersRegex, err)
		}
		fa.authResponseHeadersRegex = re
	}

	return fa, nil
}

func (fa *forwardAuth) GetTracingInformation() (string, string) {
	return fa.name, typeNameForward
}

func (fa *forwardAuth) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
	logger := middlewares.GetLogger(req.Context(), fa.name, typeNameForward)

	forwardReqMethod := http.MethodGet
	if fa.preserveRequestMethod {
		forwardReqMethod = req.Method
	}

	forwardReq, err := http.NewRequestWithContext(req.Context(), forwardReqMethod, fa.address, nil)
	if err != nil {
		logger.Debug().Err(err).Msgf("Error calling %s", fa.address)
		observability.SetStatusErrorf(req.Context(), "Error calling %s. Cause %s", fa.address, err)

		rw.WriteHeader(http.StatusInternalServerError)
		return
	}

	if fa.forwardBody {
		bodyBytes, err := fa.readBodyBytes(req)
		if errors.Is(err, errBodyTooLarge) {
			logger.Debug().Msgf("Request body is too large, maxBodySize: %d", fa.maxBodySize)

			observability.SetStatusErrorf(req.Context(), "Request body is too large, maxBodySize: %d", fa.maxBodySize)
			rw.WriteHeader(http.StatusUnauthorized)
			return
		}
		if err != nil {
			logger.Debug().Err(err).Msg("Error while reading body")

			observability.SetStatusErrorf(req.Context(), "Error while reading Body: %s", err)
			rw.WriteHeader(http.StatusInternalServerError)
			return
		}

		// bodyBytes is nil when the request has no body.
		if bodyBytes != nil {
			req.Body = io.NopCloser(bytes.NewReader(bodyBytes))
			forwardReq.Body = io.NopCloser(bytes.NewReader(bodyBytes))
		}
	}

	writeHeader(req, forwardReq, fa.trustForwardHeader, fa.authRequestHeaders)

	var forwardSpan trace.Span
	var tracer *tracing.Tracer
	if tracer = tracing.TracerFromContext(req.Context()); tracer != nil && observability.TracingEnabled(req.Context()) {
		var tracingCtx context.Context
		tracingCtx, forwardSpan = tracer.Start(req.Context(), "AuthRequest", trace.WithSpanKind(trace.SpanKindClient))
		defer forwardSpan.End()

		forwardReq = forwardReq.WithContext(tracingCtx)

		tracing.InjectContextIntoCarrier(forwardReq)
		tracer.CaptureClientRequest(forwardSpan, forwardReq)
	}

	forwardResponse, forwardErr := fa.client.Do(forwardReq)
	if forwardErr != nil {
		logger.Error().Err(forwardErr).Msgf("Error calling %s", fa.address)
		observability.SetStatusErrorf(req.Context(), "Error calling %s. Cause: %s", fa.address, forwardErr)

		statusCode := http.StatusInternalServerError
		if errors.Is(forwardErr, context.Canceled) {
			statusCode = httputil.StatusClientClosedRequest
		}

		rw.WriteHeader(statusCode)
		return
	}
	defer forwardResponse.Body.Close()

	body, readError := fa.readResponseBodyBytes(forwardResponse)
	if readError != nil {
		if errors.Is(readError, errResponseBodyTooLarge) {
			logger.Debug().Msgf("Response body is too large, maxResponseBodySize: %d", fa.maxResponseBodySize)

			observability.SetStatusErrorf(req.Context(), "Response body is too large, maxResponseBodySize: %d", fa.maxResponseBodySize)
			rw.WriteHeader(http.StatusUnauthorized)
			return
		}
		logger.Debug().Err(readError).Msgf("Error reading body %s", fa.address)
		observability.SetStatusErrorf(req.Context(), "Error reading body %s. Cause: %s", fa.address, readError)

		rw.WriteHeader(http.StatusInternalServerError)
		return
	}

	// Ending the forward request span as soon as the response is handled.
	// If any errors happen earlier, this span will be close by the defer instruction.
	if forwardSpan != nil {
		forwardSpan.End()
	}

	if fa.headerField != "" {
		if elems := forwardResponse.Header[http.CanonicalHeaderKey(fa.headerField)]; len(elems) > 0 {
			logData := accesslog.GetLogData(req)
			if logData != nil {
				logData.Core[accesslog.ClientUsername] = elems[0]
			}
		}
	}

	// If auth server returns 401 and AuthSigninURL is configured, redirect to signin URL.
	if fa.authSigninURL != "" && forwardResponse.StatusCode == http.StatusUnauthorized {
		logger.Debug().Msgf("Redirecting to signin URL: %s", fa.authSigninURL)

		tracer.CaptureResponse(forwardSpan, forwardResponse.Header, http.StatusFound, trace.SpanKindClient)
		http.Redirect(rw, req, fa.authSigninURL, http.StatusFound)
		return
	}

	// Pass the forward response's body and selected headers if it
	// didn't return a response within the range of [200, 300).
	if forwardResponse.StatusCode < http.StatusOK || forwardResponse.StatusCode >= http.StatusMultipleChoices {
		logger.Debug().Msgf("Remote error %s. StatusCode: %d", fa.address, forwardResponse.StatusCode)

		utils.CopyHeaders(rw.Header(), forwardResponse.Header)
		utils.RemoveHeaders(rw.Header(), hopHeaders...)

		redirectURL, err := fa.redirectURL(forwardResponse)
		if err != nil {
			if !errors.Is(err, http.ErrNoLocation) {
				logger.Debug().Err(err).Msgf("Error reading response location header %s", fa.address)
				observability.SetStatusErrorf(req.Context(), "Error reading response location header %s. Cause: %s", fa.address, err)

				rw.WriteHeader(http.StatusInternalServerError)
				return
			}
		} else if redirectURL.String() != "" {
			// Set the location in our response if one was sent back.
			rw.Header().Set("Location", redirectURL.String())
		}

		tracer.CaptureResponse(forwardSpan, forwardResponse.Header, forwardResponse.StatusCode, trace.SpanKindClient)
		rw.WriteHeader(forwardResponse.StatusCode)

		if _, err = rw.Write(body); err != nil {
			logger.Error().Err(err).Send()
		}
		return
	}

	for _, headerName := range fa.authResponseHeaders {
		headerKey := http.CanonicalHeaderKey(headerName)
		req.Header.Del(headerKey)
		if len(forwardResponse.Header[headerKey]) > 0 {
			req.Header[headerKey] = append([]string(nil), forwardResponse.Header[headerKey]...)
		}
	}

	if fa.authResponseHeadersRegex != nil {
		for headerKey := range req.Header {
			if fa.authResponseHeadersRegex.MatchString(headerKey) {
				req.Header.Del(headerKey)
			}
		}

		for headerKey, headerValues := range forwardResponse.Header {
			if fa.authResponseHeadersRegex.MatchString(headerKey) {
				req.Header[headerKey] = append([]string(nil), headerValues...)
			}
		}
	}

	tracer.CaptureResponse(forwardSpan, forwardResponse.Header, forwardResponse.StatusCode, trace.SpanKindClient)

	req.RequestURI = req.URL.RequestURI()

	authCookies := forwardResponse.Cookies()
	if len(authCookies) == 0 {
		fa.next.ServeHTTP(rw, req)
		return
	}

	fa.next.ServeHTTP(middlewares.NewResponseModifier(rw, req, fa.buildModifier(authCookies)), req)
}

func (fa *forwardAuth) redirectURL(forwardResponse *http.Response) (*url.URL, error) {
	if !fa.preserveLocationHeader {
		return forwardResponse.Location()
	}

	// Preserve the Location header if it exists.
	if lv := forwardResponse.Header.Get("Location"); lv != "" {
		return url.Parse(lv)
	}
	return nil, http.ErrNoLocation
}

func (fa *forwardAuth) buildModifier(authCookies []*http.Cookie) func(res *http.Response) error {
	return func(res *http.Response) error {
		cookies := res.Cookies()
		res.Header.Del("Set-Cookie")

		for _, cookie := range cookies {
			if _, found := fa.addAuthCookiesToResponse[cookie.Name]; !found {
				res.Header.Add("Set-Cookie", cookie.String())
			}
		}

		for _, cookie := range authCookies {
			if _, found := fa.addAuthCookiesToResponse[cookie.Name]; found {
				res.Header.Add("Set-Cookie", cookie.String())
			}
		}

		return nil
	}
}

var errBodyTooLarge = errors.New("request body too large")

func (fa *forwardAuth) readBodyBytes(req *http.Request) ([]byte, error) {
	if fa.maxBodySize < 0 {
		return io.ReadAll(req.Body)
	}

	body := make([]byte, fa.maxBodySize+1)
	n, err := io.ReadFull(req.Body, body)
	if errors.Is(err, io.EOF) {
		return nil, nil
	}
	if err != nil && !errors.Is(err, io.ErrUnexpectedEOF) {
		return nil, fmt.Errorf("reading body bytes: %w", err)
	}
	if errors.Is(err, io.ErrUnexpectedEOF) {
		return body[:n], nil
	}
	return nil, errBodyTooLarge
}

var errResponseBodyTooLarge = errors.New("response body too large")

func (fa *forwardAuth) readResponseBodyBytes(res *http.Response) ([]byte, error) {
	if fa.maxResponseBodySize < 0 {
		return io.ReadAll(res.Body)
	}

	body := make([]byte, fa.maxResponseBodySize+1)
	n, err := io.ReadFull(res.Body, body)
	if errors.Is(err, io.EOF) {
		return nil, nil
	}
	if err != nil && !errors.Is(err, io.ErrUnexpectedEOF) {
		return nil, fmt.Errorf("reading response body bytes: %w", err)
	}
	if errors.Is(err, io.ErrUnexpectedEOF) {
		return body[:n], nil
	}
	return nil, errResponseBodyTooLarge
}

func writeHeader(req, forwardReq *http.Request, trustForwardHeader bool, allowedHeaders []string) {
	utils.CopyHeaders(forwardReq.Header, req.Header)

	RemoveConnectionHeaders(forwardReq)
	utils.RemoveHeaders(forwardReq.Header, hopHeaders...)

	if _, ok := req.Header[userAgentHeader]; !ok {
		// If the incoming request doesn't have a User-Agent header set,
		// don't send the default Go HTTP client User-Agent for the forwarded request.
		forwardReq.Header.Set(userAgentHeader, "")
	}

	forwardReq.Header = filterForwardRequestHeaders(forwardReq.Header, allowedHeaders)

	if clientIP, _, err := net.SplitHostPort(req.RemoteAddr); err == nil {
		if trustForwardHeader {
			if prior, ok := req.Header[forward.XForwardedFor]; ok {
				clientIP = strings.Join(prior, ", ") + ", " + clientIP
			}
		}
		forwardReq.Header.Set(forward.XForwardedFor, clientIP)
	}

	xMethod := req.Header.Get(xForwardedMethod)
	switch {
	case xMethod != "" && trustForwardHeader:
		forwardReq.Header.Set(xForwardedMethod, xMethod)
	case req.Method != "":
		forwardReq.Header.Set(xForwardedMethod, req.Method)
	default:
		forwardReq.Header.Del(xForwardedMethod)
	}

	xfp := req.Header.Get(forward.XForwardedProto)
	switch {
	case xfp != "" && trustForwardHeader:
		forwardReq.Header.Set(forward.XForwardedProto, xfp)
	case req.TLS != nil:
		forwardReq.Header.Set(forward.XForwardedProto, "https")
	default:
		forwardReq.Header.Set(forward.XForwardedProto, "http")
	}

	if xfp := req.Header.Get(forward.XForwardedPort); xfp != "" && trustForwardHeader {
		forwardReq.Header.Set(forward.XForwardedPort, xfp)
	}

	xfh := req.Header.Get(forward.XForwardedHost)
	switch {
	case xfh != "" && trustForwardHeader:
		forwardReq.Header.Set(forward.XForwardedHost, xfh)
	case req.Host != "":
		forwardReq.Header.Set(forward.XForwardedHost, req.Host)
	default:
		forwardReq.Header.Del(forward.XForwardedHost)
	}

	xfURI := req.Header.Get(xForwardedURI)
	switch {
	case xfURI != "" && trustForwardHeader:
		forwardReq.Header.Set(xForwardedURI, xfURI)
	case req.URL.RequestURI() != "":
		forwardReq.Header.Set(xForwardedURI, req.URL.RequestURI())
	default:
		forwardReq.Header.Del(xForwardedURI)
	}
}

func filterForwardRequestHeaders(forwardRequestHeaders http.Header, allowedHeaders []string) http.Header {
	if len(allowedHeaders) == 0 {
		return forwardRequestHeaders
	}

	filteredHeaders := http.Header{}
	for _, headerName := range allowedHeaders {
		values := forwardRequestHeaders.Values(headerName)
		if len(values) > 0 {
			filteredHeaders[http.CanonicalHeaderKey(headerName)] = append([]string(nil), values...)
		}
	}

	return filteredHeaders
}
