internal/frontend: add isSupportedVersion

isSupportedVersion is added. master is now a supported version if the
feature flag ExperimentFrontendPackageAtMaster is on.

Updates golang/go#36811

Change-Id: Ib1c885a5e75cb697b8e99fd8e8c9a347e2b344ed
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/753605
CI-Result: Cloud Build <devtools-proctor-result-processor@system.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

internal/experiment.go

@@ -10,6 +10,7 @@ import (
 
 const (
 	ExperimentFrontendFetch               = "frontend-fetch"
+	ExperimentFrontendPackageAtMaster     = "frontend-package-at-master"
 	ExperimentInsertDirectories           = "insert-directories"
 	ExperimentInsertPlaygroundLinks       = "insert-playground-links"
 	ExperimentInsertSerializable          = "insert-serializable-txn"

internal/frontend/details.go

@@ -15,6 +15,7 @@ import (
 	"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/stdlib"
 )
 
@@ -128,7 +129,7 @@ func parseDetailsURLPath(urlPath string) (fullPath, modulePath, version string,
 // checkPathAndVersion verifies that the requested path and version are
 // acceptable. The given path may be a module or package path.
 func checkPathAndVersion(ctx context.Context, ds internal.DataSource, path, version string) error {
-	if version != internal.LatestVersion && !semver.IsValid(version) {
+	if !isSupportedVersion(ctx, version) {
 		return &serverError{
 			status: http.StatusBadRequest,
 			epage: &errorPage{
@@ -148,6 +149,24 @@ func checkPathAndVersion(ctx context.Context, ds internal.DataSource, path, vers
 	return nil
 }
 
+// isSupportedVersion reports whether the version is supported by the frontend.
+func isSupportedVersion(ctx context.Context, version string) bool {
+	if version == internal.LatestVersion || semver.IsValid(version) {
+		return true
+	}
+	if isActivePathAtMaster(ctx) {
+		return version == internal.MasterVersion
+	}
+	return false
+}
+
+func isActivePathAtMaster(ctx context.Context) bool {
+	return experiment.IsActive(ctx, internal.ExperimentFrontendPackageAtMaster) &&
+		experiment.IsActive(ctx, internal.ExperimentFrontendFetch) &&
+		experiment.IsActive(ctx, internal.ExperimentInsertDirectories) &&
+		experiment.IsActive(ctx, internal.ExperimentUseDirectories)
+}
+
 // pathNotFoundError returns an error page with instructions on how to
 // add a package or module to the site. pathType is always either the string
 // "package" or "module".