perf(mattermost): decode inbound websocket frames once (#127870)

Co-authored-by: Amp <amp@ampcode.com>

extensions/mattermost/src/mattermost/monitor-websocket.test.ts

@@ -377,7 +377,7 @@ describe("mattermost websocket monitor", () => {
     });
   });
 
-  it("hands posted envelopes to ingress raw and keeps post_edited out", async () => {
+  it("hands posted envelopes to ingress raw without duplicate decoding", async () => {
     const socket = new FakeWebSocket();
     const onPosted = vi.fn(async () => {});
     const connectOnce = createMattermostConnectOnce({
@@ -398,6 +398,9 @@ describe("mattermost websocket monitor", () => {
         }),
       },
     };
+    const raw = JSON.stringify(posted);
+    const frame = Buffer.from(raw);
+    const decode = vi.spyOn(frame, "toString");
 
     const connected = connectOnce();
     socket.emitOpen();
@@ -413,14 +416,15 @@ describe("mattermost websocket monitor", () => {
       queueMicrotask(resolve);
     });
     expect(onPosted).not.toHaveBeenCalled();
-    socket.emitMessage(Buffer.from(JSON.stringify(posted)));
+    socket.emitMessage(frame);
     await vi.waitFor(() => {
       expect(onPosted).toHaveBeenCalledTimes(1);
     });
     socket.emitClose(1000);
     await connected;
 
-    expect(onPosted).toHaveBeenCalledWith(JSON.stringify(posted));
+    expect(onPosted).toHaveBeenCalledWith(raw);
+    expect(decode.mock.calls.length).toBeLessThanOrEqual(1);
   });
 
   it("terminates when bot update_at changes (disable/enable cycle)", async () => {

extensions/mattermost/src/mattermost/monitor-websocket.ts

@@ -336,15 +336,15 @@ export function createMattermostConnectOnce(
         });
 
         ws.on("message", async (data) => {
+          const raw = rawDataToString(data);
           captureWsEvent({
             url: opts.wsUrl,
             direction: "inbound",
             kind: "ws-frame",
             flowId,
-            payload: Buffer.from(rawDataToString(data)),
+            payload: Buffer.from(raw),
             meta: { subsystem: "mattermost-websocket" },
           });
-          const raw = rawDataToString(data);
           const payload = parseMattermostEventPayload(raw);
           if (!payload) {
             return;