gddo-server: redirect invalid import paths to homepage

When a URL path for a package page fails the check for
module.CheckImportPath, redirect to the homepage. Otherwise, the page
will 400 on pkg.go.dev.

Change-Id: Ic337b968a3919b2e4fcd0ab4dd6b76c631f5edf9
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/279035
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

gddo-server/pkgsite.go

@@ -16,6 +16,7 @@ import (
 	"strings"
 	"time"
 
+	"golang.org/x/mod/module"
 	"golang.org/x/net/context/ctxhttp"
 )
 
@@ -247,16 +248,20 @@ func pkgGoDevURL(godocURL *url.URL) *url.URL {
 		} else {
 			u.Path = "/"
 		}
+	case "":
+		u.Path = ""
 	case "/-/subrepo":
 		u.Path = "/search"
 		q.Set("q", "golang.org/x")
 	default:
 		{
-			_, isSVG := godocURL.Query()["status.svg"]
-			_, isPNG := godocURL.Query()["status.png"]
-			if isSVG || isPNG {
-				u.Path = "/badge" + godocURL.Path
-				break
+			// If the import path is invalid, redirect to
+			// https://golang.org/issue/43036, so that the users has more context
+			// on why this path does not work on pkg.go.dev.
+			if err := module.CheckImportPath(strings.TrimPrefix(godocURL.Path, "/")); err != nil {
+				u.Host = "golang.org"
+				u.Path = "/issue/43036"
+				return u
 			}
 
 			u.Path = godocURL.Path

gddo-server/pkgsite_test.go

@@ -167,6 +167,10 @@ func TestPkgGoDevURL(t *testing.T) {
 			from: "https://godoc.org/golang.org/x/vgo/vendor",
 			to:   "https://pkg.go.dev/?utm_source=godoc",
 		},
+		{
+			from: "https://godoc.org/cryptoscope.co/go/specialκ",
+			to:   "https://golang.org/issue/43036",
+		},
 	}
 
 	for _, tc := range testCases {