Stream reader

desktop/src/main/stream.ts

@@ -1,15 +1,16 @@
 /**
  * @file stream data to-from renderer using a custom protocol handler.
  */
-import { protocol } from "electron/main";
+import { net, protocol } from "electron/main";
 import { createWriteStream, existsSync } from "node:fs";
 import fs from "node:fs/promises";
 import { Readable } from "node:stream";
+import { pathToFileURL } from "node:url";
 import log from "./log";
 
 /**
  * Register a protocol handler that we use for streaming large files between the
- * main process (node) and the renderer process (browser) layer.
+ * main (Node.js) and renderer (Chromium) processes.
  *
  * [Note: IPC streams]
  *
@@ -17,11 +18,14 @@ import log from "./log";
  * across IPC. And passing the entire contents of the file is not feasible for
  * large video files because of the memory pressure the copying would entail.
  *
- * As an alternative, we register a custom protocol handler that can provided a
+ * As an alternative, we register a custom protocol handler that can provides a
  * bi-directional stream. The renderer can stream data to the node side by
  * streaming the request. The node side can stream to the renderer side by
  * streaming the response.
  *
+ * The stream is not full duplex - while both reads and writes can be streamed,
+ * they need to be streamed separately.
+ *
  * See also: [Note: Transferring large amount of data over IPC]
  *
  * Depends on {@link registerPrivilegedSchemes}.
@@ -29,12 +33,16 @@ import log from "./log";
 export const registerStreamProtocol = () => {
     protocol.handle("stream", async (request: Request) => {
         const url = request.url;
+        // The request URL contains the command to run as the host, and the
+        // pathname of the file as the path. For example,
+        //
+        //     stream://write/path/to/file
+        //              host-pathname-----
+        //
         const { host, pathname } = new URL(url);
         // Convert e.g. "%20" to spaces.
         const path = decodeURIComponent(pathname);
         switch (host) {
-            /* stream://write/path/to/file */
-            /*          host-pathname----- */
             case "write":
                 try {
                     await writeStream(path, request.body);
@@ -46,6 +54,17 @@ export const registerStreamProtocol = () => {
                         { status: 500 },
                     );
                 }
+
+            case "read":
+                try {
+                    return net.fetch(pathToFileURL(path).toString());
+                } catch (e) {
+                    log.error(`Failed to read stream for ${url}`, e);
+                    return new Response(`Failed to read stream: ${e.message}`, {
+                        status: 500,
+                    });
+                }
+
             default:
                 return new Response("", { status: 404 });
         }