internal/frontend/details.go - pkgsite - Git at Google

 // 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 frontend

 import (
 	"context"
 	"errors"
 	"fmt"
 	"net/http"
 	"strings"
 	"time"

 	"github.com/google/safehtml/template"
 	"go.opencensus.io/stats"
 	"go.opencensus.io/stats/view"
 	"go.opencensus.io/tag"
 	"golang.org/x/mod/module"
 	"golang.org/x/mod/semver"
 	"golang.org/x/pkgsite/internal"
 	"golang.org/x/pkgsite/internal/derrors"
 	"golang.org/x/pkgsite/internal/experiment"
 	"golang.org/x/pkgsite/internal/log"
 	"golang.org/x/pkgsite/internal/postgres"
 	"golang.org/x/pkgsite/internal/stdlib"
 )

 // DetailsPage contains data for a package of module details template.
 type DetailsPage struct {
 	basePage

 	// Name is the name of the package or command name, or the full
 	// directory or module path.
 	Name string

 	// PageType is the type of page (pkg, cmd, dir, etc.).
 	PageType string

 	// CanShowDetails indicates whether details can be shown or must be
 	// hidden due to issues like license restrictions.
 	CanShowDetails bool

 	// Settings contains tab-specific metadata.
 	Settings TabSettings

 	// Details contains data specific to the type of page being rendered.
 	Details interface{}

 	// Header contains data to be rendered in the heading of all details pages.
 	Header interface{}

 	// Breadcrumb contains data used to render breadcrumb UI elements.
 	Breadcrumb breadcrumb

 	// Tabs contains data to render the varioius tabs on each details page.
 	Tabs []TabSettings

 	// CanonicalURLPath is the representation of the URL path for the details
 	// page, after the requested version and module path have been resolved.
 	// For example, if the latest version of /my.module/pkg is version v1.5.2,
 	// the canonical url for that path would be /my.module@v1.5.2/pkg
 	CanonicalURLPath string
 }

 const (
 	pageTypeModule    = "mod"
 	pageTypeDirectory = "dir"
 	pageTypePackage   = "pkg"
 	pageTypeCommand   = "cmd"
 	pageTypeStdLib    = stdlib.ModulePath
 )

 var (
 	keyVersionType     = tag.MustNewKey("frontend.version_type")
 	versionTypeResults = stats.Int64(
 		"go-discovery/frontend_version_type_count",
 		"The version type of a request to package, module, or directory page.",
 		stats.UnitDimensionless,
 	)
 	VersionTypeCount = &view.View{
 		Name:        "go-discovery/frontend_version_type/result_count",
 		Measure:     versionTypeResults,
 		Aggregation: view.Count(),
 		Description: "version type results, by latest, master, or semver",
 		TagKeys:     []tag.Key{keyVersionType},
 	}
 )

 // serveDetails handles requests for package/directory/module details pages. It
 // expects paths of the form "[/mod]/<module-path>[@<version>?tab=<tab>]".
 // stdlib module pages are handled at "/std", and requests to "/mod/std" will
 // be redirected to that path.
 func (s *Server) serveDetails(w http.ResponseWriter, r *http.Request, ds internal.DataSource) (err error) {
 	if r.Method != http.MethodGet {
 		return &serverError{status: http.StatusMethodNotAllowed}
 	}

 	switch r.URL.Path {
 	case "/":
 		s.staticPageHandler("index.tmpl", "")(w, r)
 		return nil
 	case "/C":
 		// Package "C" is a special case: redirect to the Go Blog article on cgo.
 		// (This is what godoc.org does.)
 		http.Redirect(w, r, "https://golang.org/doc/articles/c_go_cgo.html", http.StatusMovedPermanently)
 		return nil
 	case "/mod/std":
 		// The stdlib module page is hosted at "/std".
 		http.Redirect(w, r, "/std", http.StatusMovedPermanently)
 		return nil
 	}

 	urlInfo, err := extractURLPathInfo(r.URL.Path)
 	if err != nil {
 		return &serverError{
 			status: http.StatusBadRequest,
 			err:    err,
 		}
 	}
 	ctx := r.Context()
 	// If page statistics are enabled, use the "exp" query param to adjust
 	// the active experiments.
 	if s.serveStats {
 		ctx = setExperimentsFromQueryParam(ctx, r)
 	}
 	// Validate the fullPath and requestedVersion that were parsed.
 	if err := validatePathAndVersion(ctx, ds, urlInfo.fullPath, urlInfo.requestedVersion); err != nil {
 		return err
 	}
 	recordVersionTypeMetric(ctx, urlInfo.requestedVersion)

 	urlInfo.resolvedVersion = urlInfo.requestedVersion
 	um, err := ds.GetUnitMeta(ctx, urlInfo.fullPath, urlInfo.modulePath, urlInfo.requestedVersion)
 	if err != nil {
 		if !errors.Is(err, derrors.NotFound) {
 			return err
 		}
 		return s.servePathNotFoundPage(w, r, ds, urlInfo.fullPath, urlInfo.requestedVersion)
 	}
 	if urlInfo.isModule && um.ModulePath != urlInfo.fullPath {
 		return s.servePathNotFoundPage(w, r, ds, urlInfo.fullPath, urlInfo.requestedVersion)
 	}

 	urlInfo.modulePath = um.ModulePath
 	urlInfo.resolvedVersion = um.Version
 	if urlInfo.requestedVersion == internal.MasterVersion {
 		// Since path@master is a moving target, we don't want it to be stale.
 		// As a result, we enqueue every request of path@master to the frontend
 		// task queue, which will initiate a fetch request depending on the
 		// last time we tried to fetch this module version.
 		//
 		// Use a separate context here to prevent the context from being canceled
 		// elsewhere before a task is enqueued.
 		go func() {
 			ctx, cancel := context.WithTimeout(context.Background(), 1*time.Minute)
 			defer cancel()
 			if _, err := s.queue.ScheduleFetch(ctx, urlInfo.modulePath, internal.MasterVersion, "", s.taskIDChangeInterval); err != nil {
 				log.Errorf(ctx, "serveDetails(%q): %v", r.URL.Path, err)
 			}
 		}()
 	}
 	if experiment.IsActive(ctx, internal.ExperimentUnitPage) {
 		return s.serveUnitPage(ctx, w, r, ds, um, urlInfo.requestedVersion)
 	}
 	return s.serveDetailsPage(w, r, ds, um, urlInfo)
 }

 // serveDetailsPage serves a details page for a path using the paths,
 // modules, documentation, readmes, licenses, and package_imports tables.
 func (s *Server) serveDetailsPage(w http.ResponseWriter, r *http.Request, ds internal.DataSource, um *internal.UnitMeta, info *urlPathInfo) (err error) {
 	defer derrors.Wrap(&err, "serveDetailsPage(w, r, %v)", info)
 	ctx := r.Context()

 	if info.isModule {
 		return s.serveModulePage(ctx, w, r, ds, um, info.requestedVersion)
 	}
 	if um.IsPackage() {
 		return s.servePackagePage(ctx, w, r, ds, um, info.requestedVersion)
 	}
 	return s.serveDirectoryPage(ctx, w, r, ds, um, info.requestedVersion)
 }

 type urlPathInfo struct {
 	// fullPath is the full import path corresponding to the requested
 	// package/module/directory page.
 	fullPath string
 	// isModule indicates whether the /mod page should be shown.
 	isModule bool
 	// modulePath is the path of the module corresponding to the fullPath and
 	// resolvedVersion. If unknown, it is set to internal.UnknownModulePath.
 	modulePath string
 	// requestedVersion is the version requested by the user, which will be one
 	// of the following: "latest", "master", a Go version tag, or a semantic
 	// version.
 	requestedVersion string
 	// resolvedVersion is the semantic version stored in the database.
 	resolvedVersion string
 }

 func extractURLPathInfo(urlPath string) (_ *urlPathInfo, err error) {
 	defer derrors.Wrap(&err, "extractURLPathInfo(%q)", urlPath)

 	info := &urlPathInfo{}
 	if strings.HasPrefix(urlPath, "/mod/") {
 		urlPath = strings.TrimPrefix(urlPath, "/mod")
 		info.isModule = true
 	}
 	// Parse the fullPath, modulePath and requestedVersion, based on whether
 	// the path is in the stdlib. If unable to parse these elements, return
 	// http.StatusBadRequest.
 	if parts := strings.SplitN(strings.TrimPrefix(urlPath, "/"), "@", 2); stdlib.Contains(parts[0]) {
 		info.fullPath, info.requestedVersion, err = parseStdLibURLPath(urlPath)
 		info.modulePath = stdlib.ModulePath
 		if info.fullPath == stdlib.ModulePath {
 			info.isModule = true
 		}
 	} else {
 		info.fullPath, info.modulePath, info.requestedVersion, err = parseDetailsURLPath(urlPath)
 	}
 	if err != nil {
 		return nil, err
 	}
 	return info, nil
 }

 // parseDetailsURLPath parses a URL path that refers (or may refer) to something
 // in the Go ecosystem.
 //
 // After trimming leading and trailing slashes, the path is expected to have one
 // of three forms, and we divide it into three parts: a full path, a module
 // path, and a version.
 //
 // 1. The path has no '@', like github.com/hashicorp/vault/api.
 //    This is the full path. The module path is unknown. So is the version, so we
 //    treat it as the latest version for whatever the path denotes.
 //
 // 2. The path has "@version" at the end, like github.com/hashicorp/vault/api@v1.2.3.
 //    We split this at the '@' into a full path (github.com/hashicorp/vault/api)
 //    and version (v1.2.3); the module path is still unknown.
 //
 // 3. The path has "@version" in the middle, like github.com/hashicorp/vault@v1.2.3/api.
 //    (We call this the "canonical" form of a path.)
 //    We remove the version to get the full path, which is again
 //    github.com/hashicorp/vault/api. The version is v1.2.3, and the module path is
 //    the part before the '@', github.com/hashicorp/vault.
 //
 // In one case, we do a little more than parse the urlPath into parts: if the full path
 // could be a part of the standard library (because it has no '.'), we assume it
 // is and set the modulePath to indicate the standard library.
 func parseDetailsURLPath(urlPath string) (fullPath, modulePath, requestedVersion string, err error) {
 	defer derrors.Wrap(&err, "parseDetailsURLPath(%q)", urlPath)

 	// This splits urlPath into either:
 	//   /<module-path>[/<suffix>]
 	// or
 	//   /<module-path>, @<version>/<suffix>
 	// or
 	//  /<module-path>/<suffix>, @<version>
 	parts := strings.SplitN(urlPath, "@", 2)
 	basePath := strings.TrimSuffix(strings.TrimPrefix(parts[0], "/"), "/")
 	if len(parts) == 1 { // no '@'
 		modulePath = internal.UnknownModulePath
 		requestedVersion = internal.LatestVersion
 		fullPath = basePath
 	} else {
 		// Parse the version and suffix from parts[1], the string after the '@'.
 		endParts := strings.Split(parts[1], "/")
 		suffix := strings.Join(endParts[1:], "/")
 		// The first path component after the '@' is the version.
 		requestedVersion = endParts[0]
 		// You cannot explicitly write "latest" for the version.
 		if requestedVersion == internal.LatestVersion {
 			return "", "", "", fmt.Errorf("invalid version: %q", requestedVersion)
 		}
 		if suffix == "" {
 			// "@version" occurred at the end of the path; we don't know the module path.
 			modulePath = internal.UnknownModulePath
 			fullPath = basePath
 		} else {
 			// "@version" occurred in the middle of the path; the part before it
 			// is the module path.
 			modulePath = basePath
 			fullPath = basePath + "/" + suffix
 		}
 	}
 	// The full path must be a valid import path (that is, package path), even if it denotes
 	// a module, directory or collection.
 	if err := module.CheckImportPath(fullPath); err != nil {
 		return "", "", "", fmt.Errorf("malformed path %q: %v", fullPath, err)
 	}

 	// If the full path is (or could be) in the standard library, change the
 	// module path to say so. But in that case, disallow versions in the middle,
 	// like "net@go1.14/http". That says that the module is "net", and it isn't.
 	if stdlib.Contains(fullPath) {
 		if modulePath != internal.UnknownModulePath {
 			return "", "", "", fmt.Errorf("non-final version in standard library path %q", urlPath)
 		}
 		modulePath = stdlib.ModulePath
 	}
 	return fullPath, modulePath, requestedVersion, nil
 }

 // validatePathAndVersion verifies that the requested path and version are
 // acceptable. The given path may be a module or package path.
 func validatePathAndVersion(ctx context.Context, ds internal.DataSource, fullPath, requestedVersion string) error {
 	if !isSupportedVersion(fullPath, requestedVersion) {
 		return &serverError{
 			status: http.StatusBadRequest,
 			epage: &errorPage{
 				messageTemplate: template.MakeTrustedTemplate(`
 					<h3 class="Error-message">{{.Version}} is not a valid semantic version.</h3>
 					<p class="Error-message">
 					  To search for packages like {{.Path}}, <a href="/search?q={{.Path}}">click here</a>.
 					</p>`),
 				MessageData: struct{ Path, Version string }{fullPath, requestedVersion},
 			},
 		}
 	}
 	db, ok := ds.(*postgres.DB)
 	if !ok {
 		return nil
 	}
 	excluded, err := db.IsExcluded(ctx, fullPath)
 	if err != nil {
 		return err
 	}
 	if excluded {
 		// Return NotFound; don't let the user know that the package was excluded.
 		return &serverError{status: http.StatusNotFound}
 	}
 	return nil
 }

 // isSupportedVersion reports whether the version is supported by the frontend.
 func isSupportedVersion(fullPath, requestedVersion string) bool {
 	if stdlib.Contains(fullPath) && requestedVersion == internal.MasterVersion {
 		return false
 	}
 	if requestedVersion == internal.LatestVersion || semver.IsValid(requestedVersion) {
 		return true
 	}
 	return requestedVersion == internal.MasterVersion
 }

 // pathNotFoundError returns a page with an option on how to
 // add a package or module to the site.
 func pathNotFoundError(fullPath, requestedVersion string) error {
 	if stdlib.Contains(fullPath) {
 		return &serverError{status: http.StatusNotFound}
 	}
 	path := fullPath
 	if requestedVersion != internal.LatestVersion {
 		path = fmt.Sprintf("%s@%s", fullPath, requestedVersion)
 	}
 	return &serverError{
 		status: http.StatusNotFound,
 		epage: &errorPage{
 			templateName: "fetch.tmpl",
 			MessageData:  path,
 		},
 	}
 }

 func proxydatasourceNotSupportedErr() error {
 	return &serverError{
 		status: http.StatusFailedDependency,
 		epage: &errorPage{
 			messageTemplate: template.MakeTrustedTemplate(
 				`<h3 class="Error-message">This page is not supported by the proxydatasource.</h3>`),
 		},
 	}
 }

 func parseStdLibURLPath(urlPath string) (path, requestedVersion string, err error) {
 	defer derrors.Wrap(&err, "parseStdLibURLPath(%q)", urlPath)

 	// This splits urlPath into either:
 	//   /<path>@<tag> or /<path>
 	parts := strings.SplitN(urlPath, "@", 2)
 	path = strings.TrimSuffix(strings.TrimPrefix(parts[0], "/"), "/")
 	if err := module.CheckImportPath(path); err != nil {
 		return "", "", err
 	}

 	if len(parts) == 1 {
 		return path, internal.LatestVersion, nil
 	}
 	requestedVersion = stdlib.VersionForTag(strings.TrimSuffix(parts[1], "/"))
 	if requestedVersion == "" {
 		return "", "", fmt.Errorf("invalid Go tag for url: %q", urlPath)
 	}
 	return path, requestedVersion, nil
 }

 func (s *Server) servePathNotFoundPage(w http.ResponseWriter, r *http.Request, ds internal.DataSource, fullPath, requestedVersion string) (err error) {
 	defer derrors.Wrap(&err, "servePathNotFoundPage(w, r, %q, %q)", fullPath, requestedVersion)

 	ctx := r.Context()
 	path, err := stdlibPathForShortcut(ctx, ds, fullPath)
 	if err != nil {
 		// Log the error, but prefer a "path not found" error for a
 		// better user experience.
 		log.Error(ctx, err)
 	}
 	if path != "" {
 		http.Redirect(w, r, fmt.Sprintf("/%s", path), http.StatusFound)
 		return
 	}
 	if stdlib.Contains(fullPath) {
 		return &serverError{status: http.StatusNotFound}
 	}
 	db, ok := ds.(*postgres.DB)
 	if !ok {
 		return pathNotFoundError(fullPath, requestedVersion)
 	}
 	modulePaths, err := candidateModulePaths(fullPath)
 	if err != nil {
 		return err
 	}
 	results := s.checkPossibleModulePaths(ctx, db, fullPath, requestedVersion, modulePaths, false)
 	for _, fr := range results {
 		if fr.status == statusNotFoundInVersionMap {
 			// If the result is statusNotFoundInVersionMap, it means that
 			// we haven't attempted to fetch this path before. Return an
 			// error page giving the user the option to fetch the path.
 			return pathNotFoundError(fullPath, requestedVersion)
 		}
 	}
 	status, responseText := fetchRequestStatusAndResponseText(results, fullPath, requestedVersion)
 	return &serverError{
 		status: status,
 		epage: &errorPage{
 			messageTemplate: template.MakeTrustedTemplate(`
 					<h3 class="Error-message">{{.StatusText}}</h3>
 					<p class="Error-message">{{.Response}}</p>`),
 			MessageData: struct{ StatusText, Response string }{http.StatusText(status), responseText},
 		},
 	}
 }

 func recordVersionTypeMetric(ctx context.Context, requestedVersion string) {
 	// Tag versions based on latest, master and semver.
 	v := requestedVersion
 	if semver.IsValid(v) {
 		v = "semver"
 	}
 	stats.RecordWithTags(ctx, []tag.Mutator{
 		tag.Upsert(keyVersionType, v),
 	}, versionTypeResults.M(1))
 }

 func setExperimentsFromQueryParam(ctx context.Context, r *http.Request) context.Context {
 	if err := r.ParseForm(); err != nil {
 		log.Errorf(ctx, "ParseForm: %v", err)
 		return ctx
 	}
 	return newContextFromExps(ctx, r.Form["exp"])
 }

 // newContextFromExps adds and removes experiments from the context's experiment
 // set, creates a new set with the changes, and returns a context with the new
 // set. Each string in expMods can be either an experiment name, which means
 // that the experiment should be added, or "!" followed by an experiment name,
 // meaning that it should be removed.
 func newContextFromExps(ctx context.Context, expMods []string) context.Context {
 	var (
 		exps   []string
 		remove = map[string]bool{}
 	)
 	set := experiment.FromContext(ctx)
 	for _, exp := range expMods {
 		if strings.HasPrefix(exp, "!") {
 			exp = exp[1:]
 			if set.IsActive(exp) {
 				remove[exp] = true
 			}
 		} else if !set.IsActive(exp) {
 			exps = append(exps, exp)
 		}
 	}
 	if len(exps) == 0 && len(remove) == 0 {
 		return ctx
 	}
 	for _, a := range set.Active() {
 		if !remove[a] {
 			exps = append(exps, a)
 		}
 	}
 	return experiment.NewContext(ctx, exps...)
 }
	// 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 frontend

	import (
	"context"
	"errors"
	"fmt"
	"net/http"
	"strings"
	"time"

	"github.com/google/safehtml/template"
	"go.opencensus.io/stats"
	"go.opencensus.io/stats/view"
	"go.opencensus.io/tag"
	"golang.org/x/mod/module"
	"golang.org/x/mod/semver"
	"golang.org/x/pkgsite/internal"
	"golang.org/x/pkgsite/internal/derrors"
	"golang.org/x/pkgsite/internal/experiment"
	"golang.org/x/pkgsite/internal/log"
	"golang.org/x/pkgsite/internal/postgres"
	"golang.org/x/pkgsite/internal/stdlib"
	)

	// DetailsPage contains data for a package of module details template.
	type DetailsPage struct {
	basePage

	// Name is the name of the package or command name, or the full
	// directory or module path.
	Name string

	// PageType is the type of page (pkg, cmd, dir, etc.).
	PageType string

	// CanShowDetails indicates whether details can be shown or must be
	// hidden due to issues like license restrictions.
	CanShowDetails bool

	// Settings contains tab-specific metadata.
	Settings TabSettings

	// Details contains data specific to the type of page being rendered.
	Details interface{}

	// Header contains data to be rendered in the heading of all details pages.
	Header interface{}

	// Breadcrumb contains data used to render breadcrumb UI elements.
	Breadcrumb breadcrumb

	// Tabs contains data to render the varioius tabs on each details page.
	Tabs []TabSettings

	// CanonicalURLPath is the representation of the URL path for the details
	// page, after the requested version and module path have been resolved.
	// For example, if the latest version of /my.module/pkg is version v1.5.2,
	// the canonical url for that path would be /my.module@v1.5.2/pkg
	CanonicalURLPath string
	}

	const (
	pageTypeModule = "mod"
	pageTypeDirectory = "dir"
	pageTypePackage = "pkg"
	pageTypeCommand = "cmd"
	pageTypeStdLib = stdlib.ModulePath
	)

	var (
	keyVersionType = tag.MustNewKey("frontend.version_type")
	versionTypeResults = stats.Int64(
	"go-discovery/frontend_version_type_count",
	"The version type of a request to package, module, or directory page.",
	stats.UnitDimensionless,
	)
	VersionTypeCount = &view.View{
	Name: "go-discovery/frontend_version_type/result_count",
	Measure: versionTypeResults,
	Aggregation: view.Count(),
	Description: "version type results, by latest, master, or semver",
	TagKeys: []tag.Key{keyVersionType},
	}
	)

	// serveDetails handles requests for package/directory/module details pages. It
	// expects paths of the form "[/mod]/<module-path>[@<version>?tab=<tab>]".
	// stdlib module pages are handled at "/std", and requests to "/mod/std" will
	// be redirected to that path.
	func (s Server) serveDetails(w http.ResponseWriter, r http.Request, ds internal.DataSource) (err error) {
	if r.Method != http.MethodGet {
	return &serverError{status: http.StatusMethodNotAllowed}
	}

	switch r.URL.Path {
	case "/":
	s.staticPageHandler("index.tmpl", "")(w, r)
	return nil
	case "/C":
	// Package "C" is a special case: redirect to the Go Blog article on cgo.
	// (This is what godoc.org does.)
	http.Redirect(w, r, "https://golang.org/doc/articles/c_go_cgo.html", http.StatusMovedPermanently)
	return nil
	case "/mod/std":
	// The stdlib module page is hosted at "/std".
	http.Redirect(w, r, "/std", http.StatusMovedPermanently)
	return nil
	}

	urlInfo, err := extractURLPathInfo(r.URL.Path)
	if err != nil {
	return &serverError{
	status: http.StatusBadRequest,
	err: err,
	}
	}
	ctx := r.Context()
	// If page statistics are enabled, use the "exp" query param to adjust
	// the active experiments.
	if s.serveStats {
	ctx = setExperimentsFromQueryParam(ctx, r)
	}
	// Validate the fullPath and requestedVersion that were parsed.
	if err := validatePathAndVersion(ctx, ds, urlInfo.fullPath, urlInfo.requestedVersion); err != nil {
	return err
	}
	recordVersionTypeMetric(ctx, urlInfo.requestedVersion)

	urlInfo.resolvedVersion = urlInfo.requestedVersion
	um, err := ds.GetUnitMeta(ctx, urlInfo.fullPath, urlInfo.modulePath, urlInfo.requestedVersion)
	if err != nil {
	if !errors.Is(err, derrors.NotFound) {
	return err
	}
	return s.servePathNotFoundPage(w, r, ds, urlInfo.fullPath, urlInfo.requestedVersion)
	}
	if urlInfo.isModule && um.ModulePath != urlInfo.fullPath {
	return s.servePathNotFoundPage(w, r, ds, urlInfo.fullPath, urlInfo.requestedVersion)
	}

	urlInfo.modulePath = um.ModulePath
	urlInfo.resolvedVersion = um.Version
	if urlInfo.requestedVersion == internal.MasterVersion {
	// Since path@master is a moving target, we don't want it to be stale.
	// As a result, we enqueue every request of path@master to the frontend
	// task queue, which will initiate a fetch request depending on the
	// last time we tried to fetch this module version.
	//
	// Use a separate context here to prevent the context from being canceled
	// elsewhere before a task is enqueued.
	go func() {
	ctx, cancel := context.WithTimeout(context.Background(), 1*time.Minute)
	defer cancel()
	if _, err := s.queue.ScheduleFetch(ctx, urlInfo.modulePath, internal.MasterVersion, "", s.taskIDChangeInterval); err != nil {
	log.Errorf(ctx, "serveDetails(%q): %v", r.URL.Path, err)
	}
	}()
	}
	if experiment.IsActive(ctx, internal.ExperimentUnitPage) {
	return s.serveUnitPage(ctx, w, r, ds, um, urlInfo.requestedVersion)
	}
	return s.serveDetailsPage(w, r, ds, um, urlInfo)
	}

	// serveDetailsPage serves a details page for a path using the paths,
	// modules, documentation, readmes, licenses, and package_imports tables.
	func (s Server) serveDetailsPage(w http.ResponseWriter, r http.Request, ds internal.DataSource, um internal.UnitMeta, info urlPathInfo) (err error) {
	defer derrors.Wrap(&err, "serveDetailsPage(w, r, %v)", info)
	ctx := r.Context()

	if info.isModule {
	return s.serveModulePage(ctx, w, r, ds, um, info.requestedVersion)
	}
	if um.IsPackage() {
	return s.servePackagePage(ctx, w, r, ds, um, info.requestedVersion)
	}
	return s.serveDirectoryPage(ctx, w, r, ds, um, info.requestedVersion)
	}

	type urlPathInfo struct {
	// fullPath is the full import path corresponding to the requested
	// package/module/directory page.
	fullPath string
	// isModule indicates whether the /mod page should be shown.
	isModule bool
	// modulePath is the path of the module corresponding to the fullPath and
	// resolvedVersion. If unknown, it is set to internal.UnknownModulePath.
	modulePath string
	// requestedVersion is the version requested by the user, which will be one
	// of the following: "latest", "master", a Go version tag, or a semantic
	// version.
	requestedVersion string
	// resolvedVersion is the semantic version stored in the database.
	resolvedVersion string
	}

	func extractURLPathInfo(urlPath string) (_ *urlPathInfo, err error) {
	defer derrors.Wrap(&err, "extractURLPathInfo(%q)", urlPath)

	info := &urlPathInfo{}
	if strings.HasPrefix(urlPath, "/mod/") {
	urlPath = strings.TrimPrefix(urlPath, "/mod")
	info.isModule = true
	}
	// Parse the fullPath, modulePath and requestedVersion, based on whether
	// the path is in the stdlib. If unable to parse these elements, return
	// http.StatusBadRequest.
	if parts := strings.SplitN(strings.TrimPrefix(urlPath, "/"), "@", 2); stdlib.Contains(parts[0]) {
	info.fullPath, info.requestedVersion, err = parseStdLibURLPath(urlPath)
	info.modulePath = stdlib.ModulePath
	if info.fullPath == stdlib.ModulePath {
	info.isModule = true
	}
	} else {
	info.fullPath, info.modulePath, info.requestedVersion, err = parseDetailsURLPath(urlPath)
	}
	if err != nil {
	return nil, err
	}
	return info, nil
	}

	// parseDetailsURLPath parses a URL path that refers (or may refer) to something
	// in the Go ecosystem.
	//
	// After trimming leading and trailing slashes, the path is expected to have one
	// of three forms, and we divide it into three parts: a full path, a module
	// path, and a version.
	//
	// 1. The path has no '@', like github.com/hashicorp/vault/api.
	// This is the full path. The module path is unknown. So is the version, so we
	// treat it as the latest version for whatever the path denotes.
	//
	// 2. The path has "@version" at the end, like github.com/hashicorp/vault/api@v1.2.3.
	// We split this at the '@' into a full path (github.com/hashicorp/vault/api)
	// and version (v1.2.3); the module path is still unknown.
	//
	// 3. The path has "@version" in the middle, like github.com/hashicorp/vault@v1.2.3/api.
	// (We call this the "canonical" form of a path.)
	// We remove the version to get the full path, which is again
	// github.com/hashicorp/vault/api. The version is v1.2.3, and the module path is
	// the part before the '@', github.com/hashicorp/vault.
	//
	// In one case, we do a little more than parse the urlPath into parts: if the full path
	// could be a part of the standard library (because it has no '.'), we assume it
	// is and set the modulePath to indicate the standard library.
	func parseDetailsURLPath(urlPath string) (fullPath, modulePath, requestedVersion string, err error) {
	defer derrors.Wrap(&err, "parseDetailsURLPath(%q)", urlPath)

	// This splits urlPath into either:
	// /<module-path>[/<suffix>]
	// or
	// /<module-path>, @<version>/<suffix>
	// or
	// /<module-path>/<suffix>, @<version>
	parts := strings.SplitN(urlPath, "@", 2)
	basePath := strings.TrimSuffix(strings.TrimPrefix(parts[0], "/"), "/")
	if len(parts) == 1 { // no '@'
	modulePath = internal.UnknownModulePath
	requestedVersion = internal.LatestVersion
	fullPath = basePath
	} else {
	// Parse the version and suffix from parts[1], the string after the '@'.
	endParts := strings.Split(parts[1], "/")
	suffix := strings.Join(endParts[1:], "/")
	// The first path component after the '@' is the version.
	requestedVersion = endParts[0]
	// You cannot explicitly write "latest" for the version.
	if requestedVersion == internal.LatestVersion {
	return "", "", "", fmt.Errorf("invalid version: %q", requestedVersion)
	}
	if suffix == "" {
	// "@version" occurred at the end of the path; we don't know the module path.
	modulePath = internal.UnknownModulePath
	fullPath = basePath
	} else {
	// "@version" occurred in the middle of the path; the part before it
	// is the module path.
	modulePath = basePath
	fullPath = basePath + "/" + suffix
	}
	}
	// The full path must be a valid import path (that is, package path), even if it denotes
	// a module, directory or collection.
	if err := module.CheckImportPath(fullPath); err != nil {
	return "", "", "", fmt.Errorf("malformed path %q: %v", fullPath, err)
	}

	// If the full path is (or could be) in the standard library, change the
	// module path to say so. But in that case, disallow versions in the middle,
	// like "net@go1.14/http". That says that the module is "net", and it isn't.
	if stdlib.Contains(fullPath) {
	if modulePath != internal.UnknownModulePath {
	return "", "", "", fmt.Errorf("non-final version in standard library path %q", urlPath)
	}
	modulePath = stdlib.ModulePath
	}
	return fullPath, modulePath, requestedVersion, nil
	}

	// validatePathAndVersion verifies that the requested path and version are
	// acceptable. The given path may be a module or package path.
	func validatePathAndVersion(ctx context.Context, ds internal.DataSource, fullPath, requestedVersion string) error {
	if !isSupportedVersion(fullPath, requestedVersion) {
	return &serverError{
	status: http.StatusBadRequest,
	epage: &errorPage{
	messageTemplate: template.MakeTrustedTemplate(`
	<h3 class="Error-message">{{.Version}} is not a valid semantic version.</h3>
	<p class="Error-message">
	To search for packages like {{.Path}}, <a href="/search?q={{.Path}}">click here</a>.
	</p>`),
	MessageData: struct{ Path, Version string }{fullPath, requestedVersion},
	},
	}
	}
	db, ok := ds.(*postgres.DB)
	if !ok {
	return nil
	}
	excluded, err := db.IsExcluded(ctx, fullPath)
	if err != nil {
	return err
	}
	if excluded {
	// Return NotFound; don't let the user know that the package was excluded.
	return &serverError{status: http.StatusNotFound}
	}
	return nil
	}

	// isSupportedVersion reports whether the version is supported by the frontend.
	func isSupportedVersion(fullPath, requestedVersion string) bool {
	if stdlib.Contains(fullPath) && requestedVersion == internal.MasterVersion {
	return false
	}
	if requestedVersion == internal.LatestVersion \|\| semver.IsValid(requestedVersion) {
	return true
	}
	return requestedVersion == internal.MasterVersion
	}

	// pathNotFoundError returns a page with an option on how to
	// add a package or module to the site.
	func pathNotFoundError(fullPath, requestedVersion string) error {
	if stdlib.Contains(fullPath) {
	return &serverError{status: http.StatusNotFound}
	}
	path := fullPath
	if requestedVersion != internal.LatestVersion {
	path = fmt.Sprintf("%s@%s", fullPath, requestedVersion)
	}
	return &serverError{
	status: http.StatusNotFound,
	epage: &errorPage{
	templateName: "fetch.tmpl",
	MessageData: path,
	},
	}
	}

	func proxydatasourceNotSupportedErr() error {
	return &serverError{
	status: http.StatusFailedDependency,
	epage: &errorPage{
	messageTemplate: template.MakeTrustedTemplate(
	`<h3 class="Error-message">This page is not supported by the proxydatasource.</h3>`),
	},
	}
	}

	func parseStdLibURLPath(urlPath string) (path, requestedVersion string, err error) {
	defer derrors.Wrap(&err, "parseStdLibURLPath(%q)", urlPath)

	// This splits urlPath into either:
	// /<path>@<tag> or /<path>
	parts := strings.SplitN(urlPath, "@", 2)
	path = strings.TrimSuffix(strings.TrimPrefix(parts[0], "/"), "/")
	if err := module.CheckImportPath(path); err != nil {
	return "", "", err
	}

	if len(parts) == 1 {
	return path, internal.LatestVersion, nil
	}
	requestedVersion = stdlib.VersionForTag(strings.TrimSuffix(parts[1], "/"))
	if requestedVersion == "" {
	return "", "", fmt.Errorf("invalid Go tag for url: %q", urlPath)
	}
	return path, requestedVersion, nil
	}

	func (s Server) servePathNotFoundPage(w http.ResponseWriter, r http.Request, ds internal.DataSource, fullPath, requestedVersion string) (err error) {
	defer derrors.Wrap(&err, "servePathNotFoundPage(w, r, %q, %q)", fullPath, requestedVersion)

	ctx := r.Context()
	path, err := stdlibPathForShortcut(ctx, ds, fullPath)
	if err != nil {
	// Log the error, but prefer a "path not found" error for a
	// better user experience.
	log.Error(ctx, err)
	}
	if path != "" {
	http.Redirect(w, r, fmt.Sprintf("/%s", path), http.StatusFound)
	return
	}
	if stdlib.Contains(fullPath) {
	return &serverError{status: http.StatusNotFound}
	}
	db, ok := ds.(*postgres.DB)
	if !ok {
	return pathNotFoundError(fullPath, requestedVersion)
	}
	modulePaths, err := candidateModulePaths(fullPath)
	if err != nil {
	return err
	}
	results := s.checkPossibleModulePaths(ctx, db, fullPath, requestedVersion, modulePaths, false)
	for _, fr := range results {
	if fr.status == statusNotFoundInVersionMap {
	// If the result is statusNotFoundInVersionMap, it means that
	// we haven't attempted to fetch this path before. Return an
	// error page giving the user the option to fetch the path.
	return pathNotFoundError(fullPath, requestedVersion)
	}
	}
	status, responseText := fetchRequestStatusAndResponseText(results, fullPath, requestedVersion)
	return &serverError{
	status: status,
	epage: &errorPage{
	messageTemplate: template.MakeTrustedTemplate(`
	<h3 class="Error-message">{{.StatusText}}</h3>
	<p class="Error-message">{{.Response}}</p>`),
	MessageData: struct{ StatusText, Response string }{http.StatusText(status), responseText},
	},
	}
	}

	func recordVersionTypeMetric(ctx context.Context, requestedVersion string) {
	// Tag versions based on latest, master and semver.
	v := requestedVersion
	if semver.IsValid(v) {
	v = "semver"
	}
	stats.RecordWithTags(ctx, []tag.Mutator{
	tag.Upsert(keyVersionType, v),
	}, versionTypeResults.M(1))
	}

	func setExperimentsFromQueryParam(ctx context.Context, r *http.Request) context.Context {
	if err := r.ParseForm(); err != nil {
	log.Errorf(ctx, "ParseForm: %v", err)
	return ctx
	}
	return newContextFromExps(ctx, r.Form["exp"])
	}

	// newContextFromExps adds and removes experiments from the context's experiment
	// set, creates a new set with the changes, and returns a context with the new
	// set. Each string in expMods can be either an experiment name, which means
	// that the experiment should be added, or "!" followed by an experiment name,
	// meaning that it should be removed.
	func newContextFromExps(ctx context.Context, expMods []string) context.Context {
	var (
	exps []string
	remove = map[string]bool{}
	)
	set := experiment.FromContext(ctx)
	for _, exp := range expMods {
	if strings.HasPrefix(exp, "!") {
	exp = exp[1:]
	if set.IsActive(exp) {
	remove[exp] = true
	}
	} else if !set.IsActive(exp) {
	exps = append(exps, exp)
	}
	}
	if len(exps) == 0 && len(remove) == 0 {
	return ctx
	}
	for _, a := range set.Active() {
	if !remove[a] {
	exps = append(exps, a)
	}
	}
	return experiment.NewContext(ctx, exps...)
	}