web: very early implementation of a fetch worker

web/src/lib/workers/fetch.ts

@@ -0,0 +1,74 @@
+const error = (code: string) => {
+    // TODO: return proper errors and code here
+    self.postMessage({
+        cobaltFetchWorker: {
+            error: code,
+        }
+    })
+};
+
+const fetchFile = async (url: string) => {
+    try {
+        const response = await fetch(url);
+
+        if (!response.ok) {
+            error("file response wasn't ok");
+            return self.close();
+        }
+
+        const contentType = response.headers.get('Content-Type') || 'application/octet-stream';
+        const contentLength = response.headers.get('Content-Length');
+
+        const totalBytes = contentLength ? parseInt(contentLength, 10) : null;
+        const reader = response.body?.getReader();
+
+        if (!reader) {
+            error("no reader");
+            return self.close();
+        }
+
+        let receivedBytes = 0;
+        const chunks = [];
+
+        while (true) {
+            const { done, value } = await reader.read();
+            if (done) break;
+
+            receivedBytes += value.length;
+            chunks.push(value);
+
+            if (totalBytes) {
+                self.postMessage({
+                    cobaltFetchWorker: {
+                        progress: Math.round((receivedBytes / totalBytes) * 100),
+                        size: receivedBytes,
+                    }
+                });
+            }
+        }
+
+        if (receivedBytes === 0) {
+            error("tunnel is broken");
+            return self.close();
+        }
+
+        const file = new File(chunks, "file", { type: contentType });
+
+        self.postMessage({
+            cobaltFetchWorker: {
+                file
+            }
+        });
+    } catch (e) {
+        console.log(e)
+        error("error when downloading the file");
+        return self.close();
+    }
+}
+
+self.onmessage = async (event: MessageEvent) => {
+    if (event.data.cobaltFetchWorker) {
+        await fetchFile(event.data.cobaltFetchWorker.url);
+        self.close();
+    }
+}