internal/frontend: return bad request errors

Errors from candidateModulePaths are now returned in
servePathNotFoundPage, which will all be bad request errors.

Previously, if a request was made to an predetermined invalid path, the
user would only get a BadRequest error after clicking "Request <path>".

The error is now returned on page load.

Change-Id: I98b25122c678ab2a59b70939cab8b74e2b627a2a
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/245902
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

internal/frontend/details.go

@@ -488,7 +488,7 @@ func (s *Server) servePathNotFoundPage(w http.ResponseWriter, r *http.Request, d
 		}
 		modulePaths, err := candidateModulePaths(fullPath)
 		if err != nil {
-			return pathNotFoundError(ctx, pathType, fullPath, requestedVersion)
+			return err
 		}
 		results := s.checkPossibleModulePaths(ctx, db, fullPath, requestedVersion, modulePaths, false)
 		for _, fr := range results {

internal/frontend/fetch.go

@@ -469,6 +469,12 @@ func candidateModulePaths(fullPath string) (_ []string, err error) {
 		modulePaths = append([]string{path}, modulePaths...)
 		path += "/"
 	}
+	if len(modulePaths) == 0 {
+		return nil, &serverError{
+			status: http.StatusBadRequest,
+			err:    fmt.Errorf("invalid path: %q", fullPath),
+		}
+	}
 	return modulePaths, nil
 }
 

internal/frontend/server_test.go

@@ -881,6 +881,12 @@ func frontendFetchTestCases() []serverTestCase {
 				in("h3.Fetch-message.js-fetchMessage", text("example.com/unknown"))),
 			requiredExperiments: experiment.NewSet(internal.ExperimentFrontendFetch),
 		},
+		{
+			name:                "bad request, invalid github module path, frontend experiment",
+			urlPath:             "/github.com/foo",
+			wantStatusCode:      http.StatusBadRequest,
+			requiredExperiments: experiment.NewSet(internal.ExperimentFrontendFetch),
+		},
 	}
 }