internal/frontend: fix redirects for shortcuts with trailing slashes

Opening /http/ on pkg.go.dev is considered a shortcut to
pkg.go.dev/net/http and should redirect there. However, http.Redirect()
creates a URL relative to the current URL (/http/) and redirects to
pkg.go.dev/http/net/http instead.

In this CL, relative redirect paths are replaced by absolute paths so
that /http/ redirects to /net/http.

Fixes golang/go#40166.

Change-Id: Ic3b9e6da59bfa2358b7f75e565b8a52662f17eb8
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/242138
Reviewed-by: Julie Qiu <julie@golang.org>

internal/frontend/details.go

@@ -410,7 +410,7 @@ func parseStdLibURLPath(urlPath string) (path, requestedVersion string, err erro
 	if len(parts) == 1 {
 		return path, internal.LatestVersion, nil
 	}
-	requestedVersion = stdlib.VersionForTag(parts[1])
+	requestedVersion = stdlib.VersionForTag(strings.TrimSuffix(parts[1], "/"))
 	if requestedVersion == "" {
 		return "", "", fmt.Errorf("invalid Go tag for url: %q", urlPath)
 	}
@@ -428,7 +428,7 @@ func (s *Server) servePathNotFoundPage(w http.ResponseWriter, r *http.Request, f
 		log.Error(ctx, err)
 	}
 	if path != "" {
-		http.Redirect(w, r, path, http.StatusFound)
+		http.Redirect(w, r, fmt.Sprintf("/%s", path), http.StatusFound)
 		return
 	}
 

internal/frontend/server_test.go

@@ -851,6 +851,18 @@ func serverTestCases() ([]serverTestCase, []serverTestCase, []serverTestCase, []
 			wantStatusCode: http.StatusFound,
 			wantLocation:   "/net/http",
 		},
+		{
+			name:           "stdlib shortcut with trailing slash",
+			urlPath:        "/http/",
+			wantStatusCode: http.StatusFound,
+			wantLocation:   "/net/http",
+		},
+		{
+			name:           "stdlib shortcut with args and trailing slash",
+			urlPath:        "/http@go1.13/?tab=doc",
+			wantStatusCode: http.StatusFound,
+			wantLocation:   "/net/http",
+		},
 	}
 	return testCases, noExpUserDirTCs, frontendFetchTCs, withPathTCs
 }