package discoveryutils

import (
	"context"
	"crypto/tls"
	"flag"
	"fmt"
	"io"
	"net"
	"net/http"
	"net/url"
	"strings"
	"sync"
	"time"

	"github.com/VictoriaMetrics/VictoriaMetrics/lib/promauth"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/proxy"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/timerpool"
	"github.com/VictoriaMetrics/metrics"
)

var (
	maxConcurrency = flag.Int("promscrape.discovery.concurrency", 100, "The maximum number of concurrent requests to Prometheus autodiscovery API (Consul, Kubernetes, etc.)")
	maxWaitTime    = flag.Duration("promscrape.discovery.concurrentWaitTime", time.Minute, "The maximum duration for waiting to perform API requests "+
		"if more than -promscrape.discovery.concurrency requests are simultaneously performed")
)

var defaultClient = &http.Client{
	Timeout: 30 * time.Second,
}

var (
	concurrencyLimitCh     chan struct{}
	concurrencyLimitChOnce sync.Once
)

const (
	// BlockingClientReadTimeout is the maximum duration for waiting the response from GetBlockingAPI*
	BlockingClientReadTimeout = 10 * time.Minute

	// DefaultClientReadTimeout is the maximum duration for waiting the response from GetAPI*
	DefaultClientReadTimeout = time.Minute

	// DefaultClientWriteTimeout is the maximum duration for waiting the request to be sent to GetAPI* and GetBlockingAPI*
	DefaultClientWriteTimeout = 10 * time.Second
)

func concurrencyLimitChInit() {
	concurrencyLimitCh = make(chan struct{}, *maxConcurrency)
}

// GetHTTPClient returns default client for http API requests.
func GetHTTPClient() *http.Client {
	return defaultClient
}

// Client is http client, which talks to the given apiServer passed to NewClient().
type Client struct {
	// client is used for short requests.
	client *HTTPClient

	// blockingClient is used for long-polling requests.
	blockingClient *HTTPClient

	apiServer string

	setHTTPHeaders      func(req *http.Request)
	setHTTPProxyHeaders func(req *http.Request)

	clientCtx    context.Context
	clientCancel context.CancelFunc
}

// HTTPClient is a wrapper around http.Client with timeouts.
type HTTPClient struct {
	client       *http.Client
	ReadTimeout  time.Duration
	WriteTimeout time.Duration
}

var defaultDialer = &net.Dialer{}

// NewClient returns new Client for the given args.
func NewClient(apiServer string, ac *promauth.Config, proxyURL *proxy.URL, proxyAC *promauth.Config) (*Client, error) {
	u, err := url.Parse(apiServer)
	if err != nil {
		return nil, fmt.Errorf("cannot parse apiServer=%q: %w", apiServer, err)
	}

	dialFunc := defaultDialer.DialContext
	if u.Scheme == "unix" {
		// special case for unix socket connection
		dialAddr := u.Path
		apiServer = "http://unix"
		dialFunc = func(ctx context.Context, _, _ string) (net.Conn, error) {
			return defaultDialer.DialContext(ctx, "unix", dialAddr)
		}
	}

	isTLS := u.Scheme == "https"
	var tlsCfg *tls.Config
	if isTLS {
		tlsCfg = ac.NewTLSConfig()
	}

	var proxyURLFunc func(*http.Request) (*url.URL, error)
	if pu := proxyURL.GetURL(); pu != nil {
		proxyURLFunc = http.ProxyURL(pu)
	}

	client := &http.Client{
		Timeout: DefaultClientReadTimeout,
		Transport: &http.Transport{
			TLSClientConfig:       tlsCfg,
			Proxy:                 proxyURLFunc,
			TLSHandshakeTimeout:   10 * time.Second,
			MaxIdleConnsPerHost:   *maxConcurrency,
			ResponseHeaderTimeout: DefaultClientReadTimeout,
			DialContext:           dialFunc,
		},
	}
	blockingClient := &http.Client{
		Timeout: BlockingClientReadTimeout,
		Transport: &http.Transport{
			TLSClientConfig:       tlsCfg,
			Proxy:                 proxyURLFunc,
			TLSHandshakeTimeout:   10 * time.Second,
			MaxIdleConnsPerHost:   1000,
			ResponseHeaderTimeout: BlockingClientReadTimeout,
			DialContext:           dialFunc,
		},
	}

	setHTTPHeaders := func(req *http.Request) {}
	if ac != nil {
		setHTTPHeaders = func(req *http.Request) {
			ac.SetHeaders(req, true)
		}
	}
	setHTTPProxyHeaders := func(req *http.Request) {}
	if proxyAC != nil {
		setHTTPProxyHeaders = func(req *http.Request) {
			proxyURL.SetHeaders(proxyAC, req)
		}
	}
	ctx, cancel := context.WithCancel(context.Background())

	c := &Client{
		client: &HTTPClient{
			client:       client,
			ReadTimeout:  DefaultClientReadTimeout,
			WriteTimeout: DefaultClientWriteTimeout,
		},
		blockingClient: &HTTPClient{
			client:       blockingClient,
			ReadTimeout:  BlockingClientReadTimeout,
			WriteTimeout: DefaultClientWriteTimeout,
		},
		apiServer:           apiServer,
		setHTTPHeaders:      setHTTPHeaders,
		setHTTPProxyHeaders: setHTTPProxyHeaders,
		clientCtx:           ctx,
		clientCancel:        cancel,
	}
	return c, nil
}

// GetAPIResponseWithReqParams returns response for given absolute path with optional callback for request.
func (c *Client) GetAPIResponseWithReqParams(path string, modifyRequest func(request *http.Request)) ([]byte, error) {
	return c.getAPIResponse(path, modifyRequest)
}

// GetAPIResponse returns response for the given absolute path.
func (c *Client) GetAPIResponse(path string) ([]byte, error) {
	return c.getAPIResponse(path, nil)
}

// GetAPIResponse returns response for the given absolute path with optional callback for request.
func (c *Client) getAPIResponse(path string, modifyRequest func(request *http.Request)) ([]byte, error) {
	// Limit the number of concurrent API requests.
	concurrencyLimitChOnce.Do(concurrencyLimitChInit)
	t := timerpool.Get(*maxWaitTime)
	select {
	case concurrencyLimitCh <- struct{}{}:
		timerpool.Put(t)
	case <-t.C:
		timerpool.Put(t)
		return nil, fmt.Errorf("too many outstanding requests to %q; try increasing -promscrape.discovery.concurrentWaitTime=%s or -promscrape.discovery.concurrency=%d",
			c.apiServer, *maxWaitTime, *maxConcurrency)
	}
	defer func() {
		<-concurrencyLimitCh
	}()
	return c.getAPIResponseWithParamsAndClient(c.client, path, modifyRequest, nil)
}

// GetBlockingAPIResponse returns response for given absolute path with blocking client and optional callback for api response,
func (c *Client) GetBlockingAPIResponse(path string, inspectResponse func(resp *http.Response)) ([]byte, error) {
	return c.getAPIResponseWithParamsAndClient(c.blockingClient, path, nil, inspectResponse)
}

// getAPIResponseWithParamsAndClient returns response for the given absolute path with optional callback for request and for response.
func (c *Client) getAPIResponseWithParamsAndClient(client *HTTPClient, path string, modifyRequest func(req *http.Request), inspectResponse func(resp *http.Response)) ([]byte, error) {
	requestURL := c.apiServer + path
	u, err := url.Parse(requestURL)
	if err != nil {
		return nil, fmt.Errorf("cannot parse %q: %w", requestURL, err)
	}

	deadline := time.Now().Add(client.ReadTimeout)
	ctx, cancel := context.WithDeadline(c.clientCtx, deadline)
	defer cancel()
	req, err := http.NewRequestWithContext(ctx, "GET", u.String(), nil)
	if err != nil {
		return nil, fmt.Errorf("cannot create request for %q: %w", requestURL, err)
	}

	c.setHTTPHeaders(req)
	c.setHTTPProxyHeaders(req)
	if modifyRequest != nil {
		modifyRequest(req)
	}

	resp, err := doRequestWithPossibleRetry(client, req)
	if err != nil {
		return nil, fmt.Errorf("cannot fetch %q: %w", requestURL, err)
	}
	data, err := io.ReadAll(resp.Body)
	_ = resp.Body.Close()
	if err != nil {
		return nil, fmt.Errorf("cannot read response from %q: %w", requestURL, err)
	}

	if inspectResponse != nil {
		inspectResponse(resp)
	}
	statusCode := resp.StatusCode
	if statusCode != http.StatusOK {
		return nil, fmt.Errorf("unexpected status code returned from %q: %d; expecting %d; response body: %q",
			requestURL, statusCode, http.StatusOK, data)
	}
	return data, nil
}

// APIServer returns the API server address
func (c *Client) APIServer() string {
	return c.apiServer
}

// Stop cancels all in-flight requests
func (c *Client) Stop() {
	c.clientCancel()
}

func doRequestWithPossibleRetry(hc *HTTPClient, req *http.Request) (*http.Response, error) {
	sleepTime := time.Second
	discoveryRequests.Inc()
	deadline, ok := req.Context().Deadline()
	if !ok {
		deadline = time.Now().Add(hc.WriteTimeout)
	}

	for {
		resp, err := hc.client.Do(req)
		if err == nil {
			statusCode := resp.StatusCode
			if statusCode != http.StatusTooManyRequests {
				return resp, nil
			}
		} else if err != net.ErrClosed && !strings.Contains(err.Error(), "broken pipe") {
			return nil, err
		}
		// Retry request after exponentially increased sleep.
		maxSleepTime := time.Until(deadline)
		if sleepTime > maxSleepTime {
			return nil, fmt.Errorf("the server closes all the connection attempts: %w", err)
		}
		sleepTime += sleepTime
		if sleepTime > maxSleepTime {
			sleepTime = maxSleepTime
		}
		time.Sleep(sleepTime)
		discoveryRetries.Inc()
	}
}

var (
	discoveryRequests = metrics.NewCounter(`vm_promscrape_discovery_requests_total`)
	discoveryRetries  = metrics.NewCounter(`vm_promscrape_discovery_retries_total`)
)