Course images can't be cached by the browser

[leonstr]

For the My courses (my/courses.php) page, there's a request to load an image, for example, GET pluginfile.php/7565598/course/overviewfiles/Clipboard01.jpg, which gets a 303 response redirecting to:

https://[site].s3.eu-west-2.amazonaws.com/2f/11/2f11a9...?response-content-disposition=inline%3B%20filename%3D%22Clipboard01.jpg%22&response-content-type=image%2Fjpeg&X-Amz-Content-Sha256=UNSIGNED-PAYLOAD&X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=...&X-Amz-Date=20250327T152617Z&X-Amz-SignedHeaders=host&X-Amz-Expires=3583&X-Amz-Signature=9d0...159

If I then refresh the page the 303 redirects to:

https://[site].s3.eu-west-2.amazonaws.com/2f/11/2f11a9...?response-content-disposition=inline%3B%20filename%3D%22Clipboard01.jpg%22&response-content-type=image%2Fjpeg&X-Amz-Content-Sha256=UNSIGNED-PAYLOAD&X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=...&X-Amz-Date=20250327T153442Z&X-Amz-SignedHeaders=host&X-Amz-Expires=3558&X-Amz-Signature=9d3...95d

a different URL. The browser doesn't have the payload for this new URL in it's cache so it fetches the image again despite it being identical.

Environment

Moodle 4.5.3+ (Build: 20250326)
tool_objectfs 2025031800
PHP version 8.2.27

[leonstr]

This might be a Firefox-specific issue, it doesn't appear to cache the 303 redirects, whereas Chrome does. (I posted to Reddit post asking about this behaviour but haven't got any further forward).

[cwarwicker]

Hi @leonstr,

Do you experience the same issue with other files loaded from s3? Or is it just in the my courses area?

Conn

[leonstr]

@cwarwicker I've tried a mod_resource instance and the user profile image on user/profile.php, both have the same issue, i.e. the 303 redirect is not cached, so there's a new request to pluginfile.php, and a new URL (i.e. same image with different signature) is returned and so needs to be fetched again.

I was wondering whether Safari or Edge cache the redirects (our site has more of these than Firefox), but I don't have a way to test with them.

[cwarwicker]

I'm actually getting the same behaviour in Chrome in my testing, it's generating a new signature each time so it's enver cached. I'll have a look for another hour or so, but I don't have much time I can work on this.

Have you tried disabling Pre-Signed Urls? When I tested that, I was able to confirm that the image was loaded from the memory cache, since it's not redirecting to anything. But I don't know if that is the right solution for your site, as it could effect performance a bit.

[leonstr]

As far as I can see it's not the caching of images that's the problem, it's the caching of redirects. For example, for pluginfile.php/…/image01.jpg there's a redirect to amazonaws.com/…image01.jpg/…123. If you then refresh the page in Chrome the dev tools show the redirect as "from disk cache":

The cached redirect is to the image with signature ...123 which is in the cache. But with Firefox the redirect is not cached so pluginfile.php/…/image01.jpg results in a real request to the web server which this time redirects to amazonaws.com/…image01.jpg/…abc. That's not in the cache so Firefox has to get the image again.

I haven't tried disabling Pre-Signed URLs (it's a bit time consuming to change this on our test site), in the light of the above do you think it's worth it?

[aspark21]

Hi Conn, Leon works with us at UCL, so you can appreciate rolling back pre signed urls isn’t really an option as this addressed other scaling challenges. We do have a WR logged for this.

If this is really just a Firefox issue maybe we can get Firefox to address this?

[cwarwicker]

As far as I can see it's not the caching of images that's the problem, it's the caching of redirects. For example, for pluginfile.php/…/image01.jpg there's a redirect to amazonaws.com/…image01.jpg/…123. If you then refresh the page in Chrome the dev tools show the redirect as "from disk cache":

The cached redirect is to the image with signature ...123 which is in the cache. But with Firefox the redirect is not cached so pluginfile.php/…/image01.jpg results in a real request to the web server which this time redirects to amazonaws.com/…image01.jpg/…abc. That's not in the cache so Firefox has to get the image again.

I haven't tried disabling Pre-Signed URLs (it's a bit time consuming to change this on our test site), in the light of the above do you think it's worth it?

Ah ok, I understand now (i think).

Strange that I'm getting that same behaviour in Chrome on a vanilla moodle with objectfs. But on your production site I get the same as you. I will see if I can see any config differences which might be the cause, to try and reproduce on vanilla.

[cwarwicker]

Okay so after some testing this looks like just some firefox specific behaviour. If i change the code to a temporary redirect it works fine, it's just 303 it won't cache.

I will speak internally and see what the consensus is, if we can change this code for firefox clients, or maybe all browsers. Need to understand the effects it might have.

[leonstr]

I've reported #1958245 Firefox not caching 303 redirects.