// Copyright Istio Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package wasm

import (
	"archive/tar"
	"bytes"
	"compress/gzip"
	"context"
	"crypto/tls"
	"fmt"
	"io"
	"net/http"
	"time"

	"istio.io/istio/pkg/backoff"
)

var (
	// Referred to https://en.wikipedia.org/wiki/Tar_(computing)#UStar_format
	tarMagicNumber = []byte{0x75, 0x73, 0x74, 0x61, 0x72}
	// Referred to https://en.wikipedia.org/wiki/Gzip#File_format
	gzMagicNumber = []byte{0x1f, 0x8b}
)

// HTTPFetcher fetches remote wasm module with HTTP get.
type HTTPFetcher struct {
	client          *http.Client
	insecureClient  *http.Client
	initialBackoff  time.Duration
	requestMaxRetry int
}

// NewHTTPFetcher create a new HTTP remote wasm module fetcher.
// requestTimeout is a timeout for each HTTP/HTTPS request.
// requestMaxRetry is the number of maximum retries of HTTP/HTTPS requests.
func NewHTTPFetcher(requestTimeout time.Duration, requestMaxRetry int) *HTTPFetcher {
	if requestTimeout == 0 {
		requestTimeout = 5 * time.Second
	}
	transport := http.DefaultTransport.(*http.Transport).Clone()
	// nolint: gosec
	// This is only when a user explicitly sets a flag to enable insecure mode
	transport.TLSClientConfig = &tls.Config{InsecureSkipVerify: true}
	return &HTTPFetcher{
		client: &http.Client{
			Timeout: requestTimeout,
		},
		insecureClient: &http.Client{
			Timeout:   requestTimeout,
			Transport: transport,
		},
		initialBackoff:  time.Millisecond * 500,
		requestMaxRetry: requestMaxRetry,
	}
}

// Fetch downloads a wasm module with HTTP get.
func (f *HTTPFetcher) Fetch(ctx context.Context, url string, allowInsecure bool) ([]byte, error) {
	c := f.client
	if allowInsecure {
		c = f.insecureClient
	}
	attempts := 0
	o := backoff.DefaultOption()
	o.InitialInterval = f.initialBackoff
	b := backoff.NewExponentialBackOff(o)
	var lastError error
	for attempts < f.requestMaxRetry {
		attempts++
		req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
		if err != nil {
			wasmLog.Debugf("wasm module download request failed: %v", err)
			return nil, err
		}
		resp, err := c.Do(req)
		if err != nil {
			lastError = err
			wasmLog.Debugf("wasm module download request failed: %v", err)
			if ctx.Err() != nil {
				// If there is context timeout, exit this loop.
				return nil, fmt.Errorf("wasm module download failed after %v attempts, last error: %v", attempts, lastError)
			}
			time.Sleep(b.NextBackOff())
			continue
		}
		if resp.StatusCode == http.StatusOK {
			// Limit wasm module to 256MB; in reality it must be much smaller
			body, err := io.ReadAll(io.LimitReader(resp.Body, 1024*1024*256))
			if err != nil {
				return nil, err
			}
			err = resp.Body.Close()
			if err != nil {
				wasmLog.Infof("wasm server connection is not closed: %v", err)
			}
			return unboxIfPossible(body), err
		}
		lastError = fmt.Errorf("wasm module download request failed: status code %v", resp.StatusCode)
		if retryable(resp.StatusCode) {
			// Limit wasm module to 256MB; in reality it must be much smaller
			body, err := io.ReadAll(io.LimitReader(resp.Body, 1024*1024*256))
			if err != nil {
				return nil, err
			}
			wasmLog.Debugf("wasm module download failed: status code %v, body %v", resp.StatusCode, string(body))
			err = resp.Body.Close()
			if err != nil {
				wasmLog.Infof("wasm server connection is not closed: %v", err)
			}
			time.Sleep(b.NextBackOff())
			continue
		}
		err = resp.Body.Close()
		if err != nil {
			wasmLog.Infof("wasm server connection is not closed: %v", err)
		}
		break
	}
	return nil, fmt.Errorf("wasm module download failed after %v attempts, last error: %v", attempts, lastError)
}

func retryable(code int) bool {
	return code >= 500 &&
		!(code == http.StatusNotImplemented ||
			code == http.StatusHTTPVersionNotSupported ||
			code == http.StatusNetworkAuthenticationRequired)
}

func isPosixTar(b []byte) bool {
	return len(b) > 262 && bytes.Equal(b[257:262], tarMagicNumber)
}

// wasm plugin should be the only file in the tarball.
func getFirstFileFromTar(b []byte) []byte {
	buf := bytes.NewBuffer(b)

	// Limit wasm module to 256MB; in reality it must be much smaller
	tr := tar.NewReader(io.LimitReader(buf, 1024*1024*256))

	h, err := tr.Next()
	if err != nil {
		return nil
	}

	ret := make([]byte, h.Size)
	_, err = io.ReadFull(tr, ret)
	if err != nil {
		return nil
	}
	return ret
}

func isGZ(b []byte) bool {
	return len(b) > 2 && bytes.Equal(b[:2], gzMagicNumber)
}

func getFileFromGZ(b []byte) []byte {
	buf := bytes.NewBuffer(b)

	zr, err := gzip.NewReader(buf)
	if err != nil {
		return nil
	}

	// Limit wasm module to 256MB; in reality it must be much smaller
	ret, err := io.ReadAll(io.LimitReader(zr, 1024*1024*256))
	if err != nil {
		return nil
	}
	return ret
}

// Just do the best effort.
// If an error is encountered, just return the original bytes.
// Errors will be handled upper layers.
func unboxIfPossible(origin []byte) []byte {
	b := origin
	for {
		if isValidWasmBinary(b) {
			return b
		} else if isGZ(b) {
			if b = getFileFromGZ(b); b == nil {
				return origin
			}
		} else if isPosixTar(b) {
			if b = getFirstFileFromTar(b); b == nil {
				return origin
			}
		} else {
			return origin
		}
	}
}
