core: refactor finding of trace start

[connorjclark]

Split from #6366.

Refactor how we get the pid/tid/frameId of the current target. The tracing processor was returning a specific event, and even synthesizing it. The callers of this function only grabbed the pid/tid/frameId, so instead of returning an event object I return just the pertinent data.

[patrickhulce]

very excited about the explicitness this introduces on why startedInPage was even a thing!!

LGTM other than the ts point, happy to approve if no one else thinks ts change is worth it :)

[connorjclark]

I'll remove the ts stuff now.

Sidenote, check out this cool VSCode Jest plugin (orta.vscode-jest)!

[patrickhulce]

LGTM!

[brendankenny]

Style bikeshedding: what about some early returns? That makes it a little easier to see it's either the first conditional matches and it's done, or it's the next conditional and done, or it throws.

Something like

static findMainFrameIds(events) {
  // Prefer the newer TracingStartedInBrowser event first, if it exists
  const startedInBrowserEvt = events.find(e => e.name === 'TracingStartedInBrowser');
  if (startedInBrowserEvt && startedInBrowserEvt.args.data &&
      startedInBrowserEvt.args.data.frames) {
    const mainFrame = startedInBrowserEvt.args.data.frames.find(frame => !frame.parent);
    const frameId = mainFrame && mainFrame.frame;
    const pid = mainFrame && mainFrame.processId;

    const threadNameEvt = events.find(e => e.pid === pid && e.ph === 'M' &&
      e.cat === '__metadata' && e.name === 'thread_name' && e.args.name === 'CrRendererMain');
    const tid = threadNameEvt && threadNameEvt.tid;

    if (pid && tid && frameId) {
      return {
        pid,
        tid,
        frameId,
      };
    }
  }

  // Support legacy browser versions that do not emit TracingStartedInBrowser event.
  // The first TracingStartedInPage in the trace is definitely our renderer thread of interest
  // Beware: the tracingStartedInPage event can appear slightly after a navigationStart
  const startedInPageEvt = events.find(e => e.name === 'TracingStartedInPage');
  if (startedInPageEvt && startedInPageEvt.args && startedInPageEvt.args.data) {
    const frameId = startedInPageEvt.args.data.page;

    if (frameId) {
      return {
        pid: startedInPageEvt.pid,
        tid: startedInPageEvt.tid,
        frameId,
      };
    }
  }

  throw new LHError(LHError.errors.NO_TRACING_STARTED);
}

[brendankenny]

maybe something like /** IDs for the trace's main frame, process, and thread. */?

[brendankenny]

LGTM2!