Check validity of URL rewrite rules for non-index projects.

In commit 600c8a7848, we have introduced
an unintentionally breaking change to how redirects are handled, thus
breaking every single-page application deployed to a non-index project.

This cannot be easily fixed due to Codeberg Pages already having a mix
of broken and fixed redirects in manifests without any way to tell them
apart, but we can at least emit a diagnostic for redirects that are
statically known to be broken at deploy time.

See also #273.

src/manifest.go

@@ -338,9 +338,11 @@ func CompressFiles(ctx context.Context, manifest *Manifest) {
 // Apply post-processing steps to the manifest.
 // At the moment, there isn't a good way to report errors except to log them on the terminal.
 // (Perhaps in the future they could be exposed at `.git-pages/status.txt`?)
-func PrepareManifest(ctx context.Context, manifest *Manifest) error {
+func PrepareManifest(ctx context.Context, webRoot string, manifest *Manifest) error {
+	_, project, _ := strings.Cut(webRoot, "/")
+
 	// Parse Netlify-style `_redirects`.
-	if err := ProcessRedirectsFile(ctx, manifest); err != nil {
+	if err := ProcessRedirectsFile(ctx, project, manifest); err != nil {
 		logc.Printf(ctx, "redirects err: %s\n", err)
 	} else if len(manifest.Redirects) > 0 {
 		logc.Printf(ctx, "redirects ok: %d rules\n", len(manifest.Redirects))

src/redirects.go

@@ -43,7 +43,7 @@ func Is3xxHTTPStatus(status int) bool {
 	return status >= 300 && status <= 399
 }
 
-func validateRedirectRule(rule *redirects.Rule) error {
+func validateRedirectRule(project string, rule *redirects.Rule) error {
 	if len(rule.Params) > 0 {
 		return fmt.Errorf("rules with parameters are not supported")
 	}
@@ -76,11 +76,20 @@ func validateRedirectRule(rule *redirects.Rule) error {
 			return fmt.Errorf("'to' URL may only include a hostname for 3xx status rules")
 		}
 	}
+	// After v0.9.1, we've changed the redirect handling so that the 'to' URL for a rewrite rule
+	// must be a valid URL pointing to this site, rather than a valid path within the manifest
+	// (which was the old, incorrect interpretation). This broke every SPA on git-pages that was
+	// deployed to a non-index project. Unfortunately, we can't do much more than diagnosing
+	// the problematic pattern at deploy time.
+	if rule.Status == http.StatusOK && project != ".index" && toURL.Host == "" &&
+		!strings.HasPrefix(toURL.Path, fmt.Sprintf("/%s/", project)) {
+		return fmt.Errorf("'to' URL must start with '/%s/' for a 200 status rule", project)
+	}
 	return nil
 }
 
 // Parses redirects file and injects rules into the manifest.
-func ProcessRedirectsFile(ctx context.Context, manifest *Manifest) error {
+func ProcessRedirectsFile(ctx context.Context, project string, manifest *Manifest) error {
 	redirectsEntry := manifest.Contents[RedirectsFileName]
 	delete(manifest.Contents, RedirectsFileName)
 	if redirectsEntry == nil {
@@ -103,7 +112,7 @@ func ProcessRedirectsFile(ctx context.Context, manifest *Manifest) error {
 	}
 
 	for index, rule := range rules {
-		if err := validateRedirectRule(&rule); err != nil {
+		if err := validateRedirectRule(project, &rule); err != nil {
 			ruleString := redirects.Must(redirects.UnparseString([]redirects.Rule{rule}))
 			AddProblem(manifest, RedirectsFileName,
 				"rule #%d %q: %s", index+1, strings.TrimSpace(ruleString), err)

src/update.go

@@ -70,7 +70,7 @@ func Update(
 				outcome = UpdateDeleted
 			}
 		}
-	} else if err = PrepareManifest(ctx, newManifest); err == nil {
+	} else if err = PrepareManifest(ctx, webRoot, newManifest); err == nil {
 		storedManifest, err = StoreManifest(ctx, webRoot, newManifest, opts)
 		if err == nil {
 			domain, _, _ := strings.Cut(webRoot, "/")