Expose the binary serialization of the manifest.

This action implies a commitment to maintain the binary serialization
in a forward-compatible way (which is really the status quo).

README.md

@@ -70,7 +70,8 @@ Features
         - If the URL matches `https://<hostname>/...` and the previous rule did not apply, the index site is selected.
     - Site URLs that have a path starting with `.git-pages/...` are reserved for _git-pages_ itself.
         - The `.git-pages/health` URL returns `ok` with the `Last-Modified:` header set to the manifest modification time.
-        - The `.git-pages/manifest.json` URL returns a [ProtoJSON](https://protobuf.dev/programming-guides/json/) representation of the deployed site manifest with the `Last-Modified:` header set to the manifest modification time. It enumerates site structure, redirect rules, and errors that were not severe enough to abort publishing. Note that **the manifest JSON format is not stable and will change without notice**.
+        - The `.git-pages/manifest.json` URL returns a [ProtoJSON](https://protobuf.dev/programming-guides/json/) representation of the deployed site manifest with the `Last-Modified:` header set to the manifest modification time. It enumerates site structure, redirect rules, and errors that were not severe enough to abort publishing. Note that **the JSON manifest format is not stable and will change without notice**.
+        - The `.git-pages/manifest.pb` URL returns a binary representation of the deployed site manifest with the `Last-Modified:` header set to the manifest modification time. It contains the same information as what's exposed by the `.git-pages/manifest.json` endpoint. The binary manifest format is stable and backward-compatible with a [defined schema](src/schema.proto). Currently we do not publish a formal behavioral specification for this format; in case of doubt, [open an issue][new-issue] for clarification.
         - The `.git-pages/archive.tar` URL returns a tar archive of all site contents, including `_redirects` and `_headers` files (reconstructed from the manifest), with the `Last-Modified:` header set to the manifest modification time. Compression can be enabled using the `Accept-Encoding:` HTTP header (only).
 * In response to a `PUT` or `POST` request, the server updates a site with new content. The URL of the request must be the root URL of the site that is being published.
     - If the `PUT` method receives an `application/x-www-form-urlencoded` body, it contains a repository URL to be shallowly cloned. The `Branch` header contains the branch to be checked out; the `pages` branch is used if the header is absent.
@@ -106,6 +107,7 @@ Features
 [isolation]: https://web.dev/articles/cross-origin-isolation-guide
 [go-git-sha256]: https://github.com/go-git/go-git/issues/706
 [whiteout]: https://docs.kernel.org/filesystems/overlayfs.html#whiteouts-and-opaque-directories
+[new-issue]: https://codeberg.org/git-pages/git-pages/issues/new
 
 
 Authorization

src/pages.go

@@ -214,7 +214,7 @@ func getPage(w http.ResponseWriter, r *http.Request) error {
 			fmt.Fprintf(w, "ok\n")
 			return nil
 
-		case metadataPath == "manifest.json":
+		case metadataPath == "manifest.json" || metadataPath == "manifest.pb":
 			// metadata requests require authorization to avoid making pushes from private
 			// repositories enumerable or exposing basic-auth protected sections
 			_, err := AuthorizeMetadataRetrieval(r, ManifestHasBasicAuth(manifest))
@@ -222,11 +222,22 @@ func getPage(w http.ResponseWriter, r *http.Request) error {
 				return err
 			}
 
-			w.Header().Add("Content-Type", "application/json; charset=utf-8")
+			var contentType string
+			var content []byte
+			switch metadataPath {
+			case "manifest.json":
+				contentType = "application/protobuf+json; charset=utf-8"
+				content = ManifestJSON(manifest)
+			case "manifest.pb":
+				contentType = "application/protobuf"
+				content = EncodeManifest(manifest)
+			}
+
+			w.Header().Add("Content-Type", contentType)
 			w.Header().Add("Last-Modified", lastModified)
 			w.Header().Add("ETag", fmt.Sprintf("\"%s-manifest\"", metadata.ETag))
 			w.WriteHeader(http.StatusOK)
-			w.Write(ManifestJSON(manifest))
+			w.Write(content)
 			return nil
 
 		case metadataPath == "archive.tar":