Dart DevTools caching issues

[elliette]

Spinning this out from flutter/flutter#100247, for investigation into #2 as described in flutter/flutter#100247 (comment). See also flutter/flutter#100247 (comment).

[elliette]

I haven’t been able to reproduce this issue since the first time (described here), so I’m not sure exactly what happened. But I do have a few theories about things that could be causing issues with caching.

• The index.html file has the response header cache-control: max-age=900. This means it is cached in the browser cache for 900 seconds/15 minutes (this is set here). As mentioned above, it is the version number in index.html that determines whether or not the service worker should update. This means that a user could switch to a new Flutter/DevTools version, but if they recently had DevTools open the index.html would still be cached and therefore the service worker would not update as expected. We should set this to 0 to make sure we are always using the newest index.html.

• Other assets (for example, main.dart.js) have a last-modified header, which is set by the StaticHandler that the DevTools server uses to serve web assets (see here). The browser uses this to determine whether or not to use a copy cached in its cache, or request a new one. This means that the service worker might update, but the cache will still contain older assets because the browser doesn’t think they have been modified (this is probably often true, but I think it might make more sense to just request everything once the service worker updates to make sure we aren’t caching anything from the previous DevTools version). I think we should remove the last-modified header altogether.

• Whenever a new service worker is activated, we have it claim all open DevTools tabs/windows (see here). This was added so that if there is a new DevTools available, it is used everywhere (and replaces older DevTools). Thinking more about this, I don’t think this is what we want, because it causes the Service Worker to take over DevTools that are connected to apps running older versions of Flutter. Conversely, if a user downgrades their Flutter, you can have older versions of DevTools now connected to apps running on a newer version of Flutter.

[elliette]

I think the changes #1 and #3 (checked in #3896 (comment)) should resolve the caching issues, but will keep this open for a while and if we are still having issues will look into #2. Since the last modified header is not added by the devtools server but instead by the shelf package's static_handler, we will likely need to add middleware to remove it.

[kenzieschmoll]

I have not seen this issue in a while. We'll assume it is fixed and can re-open this issue if it comes up again.

[elliette]

Re-opening because I just noticed this while doing the DevTools roll into google3. It seems index.html is still being cached longer than it should be. I'm guessing this caching is happening on the server-side (might be the staticHandler logic mentioned above). I need to do more investigation into this.

[kenzieschmoll]

related #1674