rebroadcast: support recording truncation

plugins/prebuffer-mixin/src/file-rtsp-server.ts

@@ -1,5 +1,7 @@
+import { Deferred } from "@scrypted/common/src/deferred";
 import { Headers, RtspServer } from "@scrypted/common/src/rtsp-server";
 import fs from 'fs';
+import { format } from "path";
 import { Duplex } from "stream";
 
 // non standard extension that dumps the rtp payload to a file.
@@ -28,17 +30,48 @@ export class FileRtspServer extends RtspServer {
         ws?.end(() => ws?.destroy());
     }
 
-    write(url: string, requestHeaders: Headers) {
-        this.cleanup();
-        this.segmentBytesWritten = 0;
-
+    async write(url: string, requestHeaders: Headers) {
         const file = requestHeaders['x-scrypted-rtsp-file'];
 
         if (!file)
             return this.respond(400, 'Bad Request', requestHeaders, {});
 
+        const truncate = requestHeaders['x-scrypted-rtsp-file-truncate'];
+
         // this.writeConsole?.log('RTSP WRITE file', file);
-        this.writeStream = fs.createWriteStream(file);
+
+        // truncation preparation must happen before cleanup.
+        let truncateWriteStream: fs.WriteStream;
+        if (truncate) {
+            try {
+                const d = new Deferred<number>();
+                fs.open(truncate, 'w+', (e, fd) => {
+                    if (e)
+                        d.reject(e);
+                    else
+                        d.resolve(fd);
+                });
+                const fd = await d.promise;
+                try {
+                    await fs.promises.rename(truncate, file);
+                    truncateWriteStream = fs.createWriteStream(undefined, {
+                        fd,
+                    })
+                }
+                catch (e) {
+                    throw e;
+                }
+            }
+            catch (e) {
+                this.writeConsole?.error('RTSP WRITE error renaming truncate file', truncate);
+            }
+        }
+
+        // everything after this point must be sync due to cleanup potentially causing dangling state.
+        this.cleanup();
+        this.segmentBytesWritten = 0;
+
+        this.writeStream = truncateWriteStream || fs.createWriteStream(truncate || file);
         this.writeStream.on('error', e => {
             this.writeConsole?.error('RTSP WRITE error', e);
         });