// Copyright 2019 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package middleware

import (
	"context"
	"crypto/hmac"
	"crypto/sha256"
	"errors"
	"io"
	"net"
	"net/http"
	"strings"
	"sync"
	"time"

	"github.com/go-redis/redis/v8"
	rrate "github.com/go-redis/redis_rate/v9"
	"github.com/golang/groupcache/lru"
	"go.opencensus.io/stats"
	"go.opencensus.io/stats/view"
	"go.opencensus.io/tag"
	"golang.org/x/pkgsite/internal/config"
	"golang.org/x/pkgsite/internal/log"
	"golang.org/x/time/rate"
)

var (
	keyQuotaBlocked = tag.MustNewKey("quota.blocked")
	quotaResults    = stats.Int64(
		"go-discovery/quota_result_count",
		"The result of a quota check.",
		stats.UnitDimensionless,
	)
	// QuotaResultCount is a counter of quota results, by whether the request was blocked or not.
	QuotaResultCount = &view.View{
		Name:        "go-discovery/quota/result_count",
		Measure:     quotaResults,
		Aggregation: view.Count(),
		Description: "quota results, by blocked or allowed",
		TagKeys:     []tag.Key{keyQuotaBlocked},
	}
)

// LegacyQuota implements a simple IP-based rate limiter. Each set of incoming IP
// addresses with the same low-order byte gets qps requests per second, with the
// given burst.
// Information is kept in an LRU cache of size maxEntries.
//
// If a request is disallowed, a 429 (TooManyRequests) will be served.
func LegacyQuota(settings config.QuotaSettings) Middleware {
	var mu sync.Mutex
	cache := lru.New(settings.MaxEntries)

	return func(h http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			authVal := r.Header.Get(config.BypassQuotaAuthHeader)
			for _, wantVal := range settings.AuthValues {
				if authVal == wantVal {
					recordQuotaMetric(r.Context(), "bypassed")
					log.Infof(r.Context(), "Quota: accepting %q", authVal)
					h.ServeHTTP(w, r)
					return
				}
			}

			header := r.Header.Get("X-Forwarded-For")
			key := ipKey(header)
			// key is empty if we couldn't parse an IP, or there is no IP.
			// Fail open in this case: allow serving.
			var limiter *rate.Limiter
			if key != "" {
				mu.Lock()
				if v, ok := cache.Get(key); ok {
					limiter = v.(*rate.Limiter)
				} else {
					limiter = rate.NewLimiter(rate.Limit(settings.QPS), settings.Burst)
					cache.Add(key, limiter)
				}
				mu.Unlock()
			}
			blocked := limiter != nil && !limiter.Allow()
			var mv string
			switch {
			case header == "":
				mv = "no header"
			case key == "":
				mv = "bad header"
			case blocked:
				mv = "blocked"
			default:
				mv = "allowed"
			}
			recordQuotaMetric(r.Context(), mv)
			if blocked && settings.RecordOnly != nil && !*settings.RecordOnly {
				const tmr = http.StatusTooManyRequests
				http.Error(w, http.StatusText(tmr), tmr)
				return
			}
			h.ServeHTTP(w, r)
		})
	}
}

func recordQuotaMetric(ctx context.Context, blocked string) {
	stats.RecordWithTags(ctx, []tag.Mutator{
		tag.Upsert(keyQuotaBlocked, blocked),
	}, quotaResults.M(1))
}

func ipKey(s string) string {
	fields := strings.SplitN(s, ",", 2)
	// First field is the originating IP address.
	origin := strings.TrimSpace(fields[0])
	ip := net.ParseIP(origin)
	if ip == nil {
		return ""
	}
	// Zero out last byte, to cover ranges of IPv4 addresses. (It's less clear what the
	// effect will be on IPv6 addresses: it will certainly cover a range of them, but we
	// don't know if that range is likely to be allocated to a single organization.)
	ip[len(ip)-1] = 0
	return ip.String()
}

// Quota implements a simple IP-based rate limiter. Each set of incoming IP
// addresses with the same low-order byte gets settings.QPS requests per second.
//
// Information is kept in a redis instance.
//
// If a request is disallowed, a 429 (TooManyRequests) will be served.
func Quota(settings config.QuotaSettings, client *redis.Client) Middleware {
	return func(h http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			ctx := r.Context()
			if settings.Disable {
				recordQuotaMetric(ctx, "disabled")
				h.ServeHTTP(w, r)
				return
			}
			authVal := r.Header.Get(config.BypassQuotaAuthHeader)
			for _, wantVal := range settings.AuthValues {
				if authVal == wantVal {
					recordQuotaMetric(ctx, "bypassed")
					log.Infof(ctx, "Quota: accepting %q", authVal)
					h.ServeHTTP(w, r)
					return
				}
			}
			blocked, reason := enforceQuota(ctx, client, settings.QPS, r.Header.Get("X-Forwarded-For"), settings.HMACKey)
			recordQuotaMetric(ctx, reason)
			if blocked && settings.RecordOnly != nil && !*settings.RecordOnly {
				const tmr = http.StatusTooManyRequests
				http.Error(w, http.StatusText(tmr), tmr)
				return
			}
			h.ServeHTTP(w, r)
		})
	}
}

func enforceQuota(ctx context.Context, client *redis.Client, qps int, header string, hmacKey []byte) (blocked bool, reason string) {
	// Fail open if header is missing or can't be parsed.
	if header == "" {
		return false, "no header"
	}
	key := ipKey(header)
	if key == "" {
		return false, "bad header"
	}
	mac := hmac.New(sha256.New, hmacKey)
	io.WriteString(mac, key)
	rrateKey := string(mac.Sum(nil))
	res, err := rrate.NewLimiter(client.WithTimeout(15*time.Millisecond)).Allow(ctx, rrateKey, rrate.PerSecond(qps))
	if err != nil {
		log.Errorf(ctx, "quota: redis limiter: %v", err)
		if errors.Is(err, context.DeadlineExceeded) {
			return false, "timeout"
		}
		var nerr *net.OpError
		if errors.As(err, &nerr) && nerr.Timeout() {
			return false, "timeout"
		}
		return false, "error"
	}
	if res.Allowed > 0 {
		return false, "allowed"
	}
	return true, "blocked"
}