// Copyright 2018 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 modfetch

import (
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"io/fs"
	"net/url"
	"path"
	pathpkg "path"
	"path/filepath"
	"strings"
	"sync"
	"time"

	"cmd/go/internal/base"
	"cmd/go/internal/cfg"
	"cmd/go/internal/modfetch/codehost"
	"cmd/go/internal/web"

	"golang.org/x/mod/module"
	"golang.org/x/mod/semver"
)

var HelpGoproxy = &base.Command{
	UsageLine: "goproxy",
	Short:     "module proxy protocol",
	Long: `
A Go module proxy is any web server that can respond to GET requests for
URLs of a specified form. The requests have no query parameters, so even
a site serving from a fixed file system (including a file:/// URL)
can be a module proxy.

The GET requests sent to a Go module proxy are:

GET $GOPROXY/<module>/@v/list returns a list of known versions of the given
module, one per line.

GET $GOPROXY/<module>/@v/<version>.info returns JSON-formatted metadata
about that version of the given module.

GET $GOPROXY/<module>/@v/<version>.mod returns the go.mod file
for that version of the given module.

GET $GOPROXY/<module>/@v/<version>.zip returns the zip archive
for that version of the given module.

GET $GOPROXY/<module>/@latest returns JSON-formatted metadata about the
latest known version of the given module in the same format as
<module>/@v/<version>.info. The latest version should be the version of
the module the go command may use if <module>/@v/list is empty or no
listed version is suitable. <module>/@latest is optional and may not
be implemented by a module proxy.

When resolving the latest version of a module, the go command will request
<module>/@v/list, then, if no suitable versions are found, <module>/@latest.
The go command prefers, in order: the semantically highest release version,
the semantically highest pre-release version, and the chronologically
most recent pseudo-version. In Go 1.12 and earlier, the go command considered
pseudo-versions in <module>/@v/list to be pre-release versions, but this is
no longer true since Go 1.13.

To avoid problems when serving from case-sensitive file systems,
the <module> and <version> elements are case-encoded, replacing every
uppercase letter with an exclamation mark followed by the corresponding
lower-case letter: github.com/Azure encodes as github.com/!azure.

The JSON-formatted metadata about a given module corresponds to
this Go data structure, which may be expanded in the future:

    type Info struct {
        Version string    // version string
        Time    time.Time // commit time
    }

The zip archive for a specific version of a given module is a
standard zip file that contains the file tree corresponding
to the module's source code and related files. The archive uses
slash-separated paths, and every file path in the archive must
begin with <module>@<version>/, where the module and version are
substituted directly, not case-encoded. The root of the module
file tree corresponds to the <module>@<version>/ prefix in the
archive.

Even when downloading directly from version control systems,
the go command synthesizes explicit info, mod, and zip files
and stores them in its local cache, $GOPATH/pkg/mod/cache/download,
the same as if it had downloaded them directly from a proxy.
The cache layout is the same as the proxy URL space, so
serving $GOPATH/pkg/mod/cache/download at (or copying it to)
https://example.com/proxy would let other users access those
cached module versions with GOPROXY=https://example.com/proxy.
`,
}

var proxyOnce struct {
	sync.Once
	list []proxySpec
	err  error
}

type proxySpec struct {
	// url is the proxy URL or one of "off", "direct", "noproxy".
	url string

	// fallBackOnError is true if a request should be attempted on the next proxy
	// in the list after any error from this proxy. If fallBackOnError is false,
	// the request will only be attempted on the next proxy if the error is
	// equivalent to os.ErrNotFound, which is true for 404 and 410 responses.
	fallBackOnError bool
}

func proxyList() ([]proxySpec, error) {
	proxyOnce.Do(func() {
		if cfg.GONOPROXY != "" && cfg.GOPROXY != "direct" {
			proxyOnce.list = append(proxyOnce.list, proxySpec{url: "noproxy"})
		}

		goproxy := cfg.GOPROXY
		for goproxy != "" {
			var url string
			fallBackOnError := false
			if i := strings.IndexAny(goproxy, ",|"); i >= 0 {
				url = goproxy[:i]
				fallBackOnError = goproxy[i] == '|'
				goproxy = goproxy[i+1:]
			} else {
				url = goproxy
				goproxy = ""
			}

			url = strings.TrimSpace(url)
			if url == "" {
				continue
			}
			if url == "off" {
				// "off" always fails hard, so can stop walking list.
				proxyOnce.list = append(proxyOnce.list, proxySpec{url: "off"})
				break
			}
			if url == "direct" {
				proxyOnce.list = append(proxyOnce.list, proxySpec{url: "direct"})
				// For now, "direct" is the end of the line. We may decide to add some
				// sort of fallback behavior for them in the future, so ignore
				// subsequent entries for forward-compatibility.
				break
			}

			// Single-word tokens are reserved for built-in behaviors, and anything
			// containing the string ":/" or matching an absolute file path must be a
			// complete URL. For all other paths, implicitly add "https://".
			if strings.ContainsAny(url, ".:/") && !strings.Contains(url, ":/") && !filepath.IsAbs(url) && !path.IsAbs(url) {
				url = "https://" + url
			}

			// Check that newProxyRepo accepts the URL.
			// It won't do anything with the path.
			if _, err := newProxyRepo(url, "golang.org/x/text"); err != nil {
				proxyOnce.err = err
				return
			}

			proxyOnce.list = append(proxyOnce.list, proxySpec{
				url:             url,
				fallBackOnError: fallBackOnError,
			})
		}

		if len(proxyOnce.list) == 0 ||
			len(proxyOnce.list) == 1 && proxyOnce.list[0].url == "noproxy" {
			// There were no proxies, other than the implicit "noproxy" added when
			// GONOPROXY is set. This can happen if GOPROXY is a non-empty string
			// like "," or " ".
			proxyOnce.err = fmt.Errorf("GOPROXY list is not the empty string, but contains no entries")
		}
	})

	return proxyOnce.list, proxyOnce.err
}

// TryProxies iterates f over each configured proxy (including "noproxy" and
// "direct" if applicable) until f returns no error or until f returns an
// error that is not equivalent to fs.ErrNotExist on a proxy configured
// not to fall back on errors.
//
// TryProxies then returns that final error.
//
// If GOPROXY is set to "off", TryProxies invokes f once with the argument
// "off".
func TryProxies(f func(proxy string) error) error {
	proxies, err := proxyList()
	if err != nil {
		return err
	}
	if len(proxies) == 0 {
		panic("GOPROXY list is empty")
	}

	// We try to report the most helpful error to the user. "direct" and "noproxy"
	// errors are best, followed by proxy errors other than ErrNotExist, followed
	// by ErrNotExist.
	//
	// Note that errProxyOff, errNoproxy, and errUseProxy are equivalent to
	// ErrNotExist. errUseProxy should only be returned if "noproxy" is the only
	// proxy. errNoproxy should never be returned, since there should always be a
	// more useful error from "noproxy" first.
	const (
		notExistRank = iota
		proxyRank
		directRank
	)
	var bestErr error
	bestErrRank := notExistRank
	for _, proxy := range proxies {
		err := f(proxy.url)
		if err == nil {
			return nil
		}
		isNotExistErr := errors.Is(err, fs.ErrNotExist)

		if proxy.url == "direct" || (proxy.url == "noproxy" && err != errUseProxy) {
			bestErr = err
			bestErrRank = directRank
		} else if bestErrRank <= proxyRank && !isNotExistErr {
			bestErr = err
			bestErrRank = proxyRank
		} else if bestErrRank == notExistRank {
			bestErr = err
		}

		if !proxy.fallBackOnError && !isNotExistErr {
			break
		}
	}
	return bestErr
}

type proxyRepo struct {
	url         *url.URL
	path        string
	redactedURL string
}

func newProxyRepo(baseURL, path string) (Repo, error) {
	base, err := url.Parse(baseURL)
	if err != nil {
		return nil, err
	}
	switch base.Scheme {
	case "http", "https":
		// ok
	case "file":
		if *base != (url.URL{Scheme: base.Scheme, Path: base.Path, RawPath: base.RawPath}) {
			return nil, fmt.Errorf("invalid file:// proxy URL with non-path elements: %s", base.Redacted())
		}
	case "":
		return nil, fmt.Errorf("invalid proxy URL missing scheme: %s", base.Redacted())
	default:
		return nil, fmt.Errorf("invalid proxy URL scheme (must be https, http, file): %s", base.Redacted())
	}

	enc, err := module.EscapePath(path)
	if err != nil {
		return nil, err
	}
	redactedURL := base.Redacted()
	base.Path = strings.TrimSuffix(base.Path, "/") + "/" + enc
	base.RawPath = strings.TrimSuffix(base.RawPath, "/") + "/" + pathEscape(enc)
	return &proxyRepo{base, path, redactedURL}, nil
}

func (p *proxyRepo) ModulePath() string {
	return p.path
}

// versionError returns err wrapped in a ModuleError for p.path.
func (p *proxyRepo) versionError(version string, err error) error {
	if version != "" && version != module.CanonicalVersion(version) {
		return &module.ModuleError{
			Path: p.path,
			Err: &module.InvalidVersionError{
				Version: version,
				Pseudo:  IsPseudoVersion(version),
				Err:     err,
			},
		}
	}

	return &module.ModuleError{
		Path:    p.path,
		Version: version,
		Err:     err,
	}
}

func (p *proxyRepo) getBytes(path string) ([]byte, error) {
	body, err := p.getBody(path)
	if err != nil {
		return nil, err
	}
	defer body.Close()
	return io.ReadAll(body)
}

func (p *proxyRepo) getBody(path string) (io.ReadCloser, error) {
	fullPath := pathpkg.Join(p.url.Path, path)

	target := *p.url
	target.Path = fullPath
	target.RawPath = pathpkg.Join(target.RawPath, pathEscape(path))

	resp, err := web.Get(web.DefaultSecurity, &target)
	if err != nil {
		return nil, err
	}
	if err := resp.Err(); err != nil {
		resp.Body.Close()
		return nil, err
	}
	return resp.Body, nil
}

func (p *proxyRepo) Versions(prefix string) ([]string, error) {
	data, err := p.getBytes("@v/list")
	if err != nil {
		return nil, p.versionError("", err)
	}
	var list []string
	for _, line := range strings.Split(string(data), "\n") {
		f := strings.Fields(line)
		if len(f) >= 1 && semver.IsValid(f[0]) && strings.HasPrefix(f[0], prefix) && !IsPseudoVersion(f[0]) {
			list = append(list, f[0])
		}
	}
	SortVersions(list)
	return list, nil
}

func (p *proxyRepo) latest() (*RevInfo, error) {
	data, err := p.getBytes("@v/list")
	if err != nil {
		return nil, p.versionError("", err)
	}

	var (
		bestTime             time.Time
		bestTimeIsFromPseudo bool
		bestVersion          string
	)

	for _, line := range strings.Split(string(data), "\n") {
		f := strings.Fields(line)
		if len(f) >= 1 && semver.IsValid(f[0]) {
			// If the proxy includes timestamps, prefer the timestamp it reports.
			// Otherwise, derive the timestamp from the pseudo-version.
			var (
				ft             time.Time
				ftIsFromPseudo = false
			)
			if len(f) >= 2 {
				ft, _ = time.Parse(time.RFC3339, f[1])
			} else if IsPseudoVersion(f[0]) {
				ft, _ = PseudoVersionTime(f[0])
				ftIsFromPseudo = true
			} else {
				// Repo.Latest promises that this method is only called where there are
				// no tagged versions. Ignore any tagged versions that were added in the
				// meantime.
				continue
			}
			if bestTime.Before(ft) {
				bestTime = ft
				bestTimeIsFromPseudo = ftIsFromPseudo
				bestVersion = f[0]
			}
		}
	}
	if bestVersion == "" {
		return nil, p.versionError("", codehost.ErrNoCommits)
	}

	if bestTimeIsFromPseudo {
		// We parsed bestTime from the pseudo-version, but that's in UTC and we're
		// supposed to report the timestamp as reported by the VCS.
		// Stat the selected version to canonicalize the timestamp.
		//
		// TODO(bcmills): Should we also stat other versions to ensure that we
		// report the correct Name and Short for the revision?
		return p.Stat(bestVersion)
	}

	return &RevInfo{
		Version: bestVersion,
		Name:    bestVersion,
		Short:   bestVersion,
		Time:    bestTime,
	}, nil
}

func (p *proxyRepo) Stat(rev string) (*RevInfo, error) {
	encRev, err := module.EscapeVersion(rev)
	if err != nil {
		return nil, p.versionError(rev, err)
	}
	data, err := p.getBytes("@v/" + encRev + ".info")
	if err != nil {
		return nil, p.versionError(rev, err)
	}
	info := new(RevInfo)
	if err := json.Unmarshal(data, info); err != nil {
		return nil, p.versionError(rev, fmt.Errorf("invalid response from proxy %q: %w", p.redactedURL, err))
	}
	if info.Version != rev && rev == module.CanonicalVersion(rev) && module.Check(p.path, rev) == nil {
		// If we request a correct, appropriate version for the module path, the
		// proxy must return either exactly that version or an error — not some
		// arbitrary other version.
		return nil, p.versionError(rev, fmt.Errorf("proxy returned info for version %s instead of requested version", info.Version))
	}
	return info, nil
}

func (p *proxyRepo) Latest() (*RevInfo, error) {
	data, err := p.getBytes("@latest")
	if err != nil {
		if !errors.Is(err, fs.ErrNotExist) {
			return nil, p.versionError("", err)
		}
		return p.latest()
	}
	info := new(RevInfo)
	if err := json.Unmarshal(data, info); err != nil {
		return nil, p.versionError("", fmt.Errorf("invalid response from proxy %q: %w", p.redactedURL, err))
	}
	return info, nil
}

func (p *proxyRepo) GoMod(version string) ([]byte, error) {
	if version != module.CanonicalVersion(version) {
		return nil, p.versionError(version, fmt.Errorf("internal error: version passed to GoMod is not canonical"))
	}

	encVer, err := module.EscapeVersion(version)
	if err != nil {
		return nil, p.versionError(version, err)
	}
	data, err := p.getBytes("@v/" + encVer + ".mod")
	if err != nil {
		return nil, p.versionError(version, err)
	}
	return data, nil
}

func (p *proxyRepo) Zip(dst io.Writer, version string) error {
	if version != module.CanonicalVersion(version) {
		return p.versionError(version, fmt.Errorf("internal error: version passed to Zip is not canonical"))
	}

	encVer, err := module.EscapeVersion(version)
	if err != nil {
		return p.versionError(version, err)
	}
	body, err := p.getBody("@v/" + encVer + ".zip")
	if err != nil {
		return p.versionError(version, err)
	}
	defer body.Close()

	lr := &io.LimitedReader{R: body, N: codehost.MaxZipFile + 1}
	if _, err := io.Copy(dst, lr); err != nil {
		return p.versionError(version, err)
	}
	if lr.N <= 0 {
		return p.versionError(version, fmt.Errorf("downloaded zip file too large"))
	}
	return nil
}

// pathEscape escapes s so it can be used in a path.
// That is, it escapes things like ? and # (which really shouldn't appear anyway).
// It does not escape / to %2F: our REST API is designed so that / can be left as is.
func pathEscape(s string) string {
	return strings.ReplaceAll(url.PathEscape(s), "%2F", "/")
}