/*
   GoToSocial
   Copyright (C) 2021-2023 GoToSocial Authors admin@gotosocial.org

   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU Affero General Public License as published by
   the Free Software Foundation, either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU Affero General Public License for more details.

   You should have received a copy of the GNU Affero General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

package transport

import (
	"context"
	"crypto"
	"crypto/x509"
	"errors"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"strconv"
	"strings"
	"sync"
	"time"

	errorsv2 "codeberg.org/gruf/go-errors/v2"
	"codeberg.org/gruf/go-kv"
	"github.com/go-fed/httpsig"
	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
	"github.com/superseriousbusiness/gotosocial/internal/httpclient"
	"github.com/superseriousbusiness/gotosocial/internal/log"
)

// Transport implements the pub.Transport interface with some additional functionality for fetching remote media.
//
// Since the transport has the concept of 'shortcuts' for fetching data locally rather than remotely, it is
// not *always* the case that calling a Transport function does an http call, but it usually will for remote
// hosts or resources for which a shortcut isn't provided by the transport controller (also in this package).
//
// For any of the transport functions, if a Fastfail context is passed in as the first parameter, the function
// will return after the first transport failure, instead of retrying + backing off.
type Transport interface {
	/*
		POST functions
	*/

	// Deliver sends an ActivityStreams object.
	Deliver(ctx context.Context, b []byte, to *url.URL) error
	// BatchDeliver sends an ActivityStreams object to multiple recipients.
	BatchDeliver(ctx context.Context, b []byte, recipients []*url.URL) error

	/*
		GET functions
	*/

	// Dereference fetches the ActivityStreams object located at this IRI with a GET request.
	Dereference(ctx context.Context, iri *url.URL) ([]byte, error)
	// DereferenceMedia fetches the given media attachment IRI, returning the reader and filesize.
	DereferenceMedia(ctx context.Context, iri *url.URL) (io.ReadCloser, int64, error)
	// DereferenceInstance dereferences remote instance information, first by checking /api/v1/instance, and then by checking /.well-known/nodeinfo.
	DereferenceInstance(ctx context.Context, iri *url.URL) (*gtsmodel.Instance, error)
	// Finger performs a webfinger request with the given username and domain, and returns the bytes from the response body.
	Finger(ctx context.Context, targetUsername string, targetDomain string) ([]byte, error)
}

// transport implements the Transport interface
type transport struct {
	controller *controller
	pubKeyID   string
	privkey    crypto.PrivateKey

	signerExp  time.Time
	getSigner  httpsig.Signer
	postSigner httpsig.Signer
	signerMu   sync.Mutex
}

// GET will perform given http request using transport client, retrying on certain preset errors, or if status code is among retryOn.
func (t *transport) GET(r *http.Request) (*http.Response, error) {
	if r.Method != http.MethodGet {
		return nil, errors.New("must be GET request")
	}
	return t.do(r, func(r *http.Request) error {
		return t.signGET(r)
	})
}

// POST will perform given http request using transport client, retrying on certain preset errors, or if status code is among retryOn.
func (t *transport) POST(r *http.Request, body []byte) (*http.Response, error) {
	if r.Method != http.MethodPost {
		return nil, errors.New("must be POST request")
	}
	return t.do(r, func(r *http.Request) error {
		return t.signPOST(r, body)
	})
}

func (t *transport) do(r *http.Request, signer func(*http.Request) error) (*http.Response, error) {
	const (
		// max no. attempts
		maxRetries = 5

		// starting backoff duration.
		baseBackoff = 2 * time.Second
	)

	// Get request hostname
	host := r.URL.Hostname()

	// Check if recently reached max retries for this host
	// so we don't need to bother reattempting it. The only
	// errors that are retried upon are server failure and
	// domain resolution type errors, so this cached result
	// indicates this server is likely having issues.
	if t.controller.badHosts.Has(host) {
		return nil, errors.New("too many failed attempts")
	}

	// Check whether request should fast fail, we check this
	// before loop as each context.Value() requires mutex lock.
	fastFail := IsFastfail(r.Context())

	// Start a log entry for this request
	l := log.WithContext(r.Context()).
		WithFields(kv.Fields{
			{"pubKeyID", t.pubKeyID},
			{"method", r.Method},
			{"url", r.URL.String()},
		}...)

	r.Header.Set("User-Agent", t.controller.userAgent)

	for i := 0; i < maxRetries; i++ {
		var backoff time.Duration

		// Reset signing header fields
		now := t.controller.clock.Now().UTC()
		r.Header.Set("Date", now.Format("Mon, 02 Jan 2006 15:04:05")+" GMT")
		r.Header.Del("Signature")
		r.Header.Del("Digest")

		// Perform request signing
		if err := signer(r); err != nil {
			return nil, err
		}

		l.Infof("performing request")

		// Attempt to perform request
		rsp, err := t.controller.client.Do(r)
		if err == nil { //nolint:gocritic
			// TooManyRequest means we need to slow
			// down and retry our request. Codes over
			// 500 generally indicate temp. outages.
			if code := rsp.StatusCode; code < 500 &&
				code != http.StatusTooManyRequests {
				return rsp, nil
			}

			// Generate error from status code for logging
			err = errors.New(`http response "` + rsp.Status + `"`)

			// Search for a provided "Retry-After" header value.
			if after := rsp.Header.Get("Retry-After"); after != "" {

				if u, _ := strconv.ParseUint(after, 10, 32); u != 0 {
					// An integer number of backoff seconds was provided.
					backoff = time.Duration(u) * time.Second
				} else if at, _ := http.ParseTime(after); !at.Before(now) {
					// An HTTP formatted future date-time was provided.
					backoff = at.Sub(now)
				}

				// Don't let their provided backoff exceed our max.
				if max := baseBackoff * maxRetries; backoff > max {
					backoff = max
				}
			}

		} else if errorsv2.Is(err,
			context.DeadlineExceeded,
			context.Canceled,
			httpclient.ErrInvalidRequest,
			httpclient.ErrBodyTooLarge,
			httpclient.ErrReservedAddr,
		) {
			// Return on non-retryable errors
			return nil, err
		} else if strings.Contains(err.Error(), "stopped after 10 redirects") {
			// Don't bother if net/http returned after too many redirects
			return nil, err
		} else if errors.As(err, &x509.UnknownAuthorityError{}) {
			// Unknown authority errors we do NOT recover from
			return nil, err
		}

		if fastFail {
			// on fast-fail, don't bother backoff/retry
			return nil, fmt.Errorf("%w (fast fail)", err)
		}

		if backoff == 0 {
			// No retry-after found, set our predefined backoff.
			backoff = time.Duration(i) * baseBackoff
		}

		l.Errorf("backing off for %s after http request error: %v", backoff.String(), err)

		select {
		// Request ctx cancelled
		case <-r.Context().Done():
			return nil, r.Context().Err()

		// Backoff for some time
		case <-time.After(backoff):
			backoff *= 2
		}
	}

	// Add "bad" entry for this host
	t.controller.badHosts.Set(host, struct{}{})

	return nil, errors.New("transport reached max retries")
}

// signGET will safely sign an HTTP GET request.
func (t *transport) signGET(r *http.Request) (err error) {
	t.safesign(func() {
		err = t.getSigner.SignRequest(t.privkey, t.pubKeyID, r, nil)
	})
	return
}

// signPOST will safely sign an HTTP POST request for given body.
func (t *transport) signPOST(r *http.Request, body []byte) (err error) {
	t.safesign(func() {
		err = t.postSigner.SignRequest(t.privkey, t.pubKeyID, r, body)
	})
	return
}

// safesign will perform sign function within mutex protection,
// and ensured that httpsig.Signers are up-to-date.
func (t *transport) safesign(sign func()) {
	// Perform within mu safety
	t.signerMu.Lock()
	defer t.signerMu.Unlock()

	if now := time.Now(); now.After(t.signerExp) {
		const expiry = 120

		// Signers have expired and require renewal
		t.getSigner, _ = NewGETSigner(expiry)
		t.postSigner, _ = NewPOSTSigner(expiry)
		t.signerExp = now.Add(time.Second * expiry)
	}

	// Perform signing
	sign()
}