Testing GitHub for images

test_image.md

The goal is to test what GitHub is really doing with the images.

first experiment.

Link an image

![amanite one](http://la-grange.net/tmp/amanite01.webp)

And check the markup and the server logs

<p>
    <a target="_blank"
       rel="noopener noreferrer"
       href="https://camo.githubusercontent.com/706e439b559eefecc94899ebd7d07c6f6168499f/687474703a2f2f6c612d6772616e67652e6e65742f746d702f616d616e69746530312e77656270">
        <img src="https://camo.githubusercontent.com/706e439b559eefecc94899ebd7d07c6f6168499f/687474703a2f2f6c612d6772616e67652e6e65742f746d702f616d616e69746530312e77656270"
             alt="amanite one"
             data-canonical-src="http://la-grange.net/tmp/amanite01.webp"
             style="max-width:100%;"></a></p>

Then I can see one HTTP request.

192.30.252.98 - - [21/Nov/2019:00:45:57 +0000] "GET /tmp/amanite01.webp HTTP/1.1" 200 8810 "-" "github-camo (876de43e)"

A reload of the page didn't create any HTTP requests on the origin server, but github replied with a 304 Not Modified.

HTTP/1.1 304 Not Modified
Date: Thu, 21 Nov 2019 00:52:28 GMT
Via: 1.1 varnish
Cache-Control: public, max-age=31536000
Expires: Thu, 28 Nov 2019 00:45:57 GMT
Age: 349
Connection: keep-alive
X-Served-By: cache-hnd18734-HND
X-Cache: HIT
X-Cache-Hits: 1
X-Timer: S1574297548.398821,VS0,VE1
Vary: Accept-Encoding
X-Fastly-Request-ID: 0527f2da6b384f3993ea5c48b9dfad00bd2c5f46
Timing-Allow-Origin: https://github.com

A shift reload created a second request on the origin server.

192.30.252.98 - - [21/Nov/2019:00:55:22 +0000] "GET /tmp/amanite01.webp HTTP/1.1" 200 8810 "-" "github-camo (876de43e)"

HTTP/1.1 200 OK
Content-Type: image/webp
Cache-Control: public, max-age=31536000
Content-Security-Policy: default-src 'none'; img-src data:; style-src 'unsafe-inline'
Expires: Thu, 28 Nov 2019 00:55:22 GMT
Last-Modified: Thu, 21 Nov 2019 00:16:55 GMT
Server: github-camo (876de43e)
Strict-Transport-Security: max-age=31536000; includeSubDomains
X-Content-Type-Options: nosniff
X-Frame-Options: deny
X-Xss-Protection: 1; mode=block
X-GitHub-Request-Id: 5338:49F8:B5E0B:C4313:5DD5E0A3
Content-Length: 8810
Accept-Ranges: bytes
Date: Thu, 21 Nov 2019 00:56:04 GMT
Via: 1.1 varnish
Age: 0
Connection: keep-alive
X-Served-By: cache-tyo19926-TYO
X-Cache: MISS
X-Cache-Hits: 0
X-Timer: S1574297764.109975,VS0,VE345
Vary: Accept-Encoding
X-Fastly-Request-ID: 7d2cdc504617bf3274b02db5b35b5b318ff857be
Timing-Allow-Origin: https://github.com

second experiment

On the server, let's change the image.

cp amanite02.webp amanite01.webp

A reload doesn't change the image. There is no request to the HTTP server.

HTTP/1.1 304 Not Modified
Date: Thu, 21 Nov 2019 01:01:00 GMT
Via: 1.1 varnish
Cache-Control: public, max-age=31536000
Expires: Thu, 28 Nov 2019 00:55:22 GMT
Age: 296
Connection: keep-alive
X-Served-By: cache-tyo19942-TYO
X-Cache: HIT
X-Cache-Hits: 2
X-Timer: S1574298060.158992,VS0,VE0
Vary: Accept-Encoding
X-Fastly-Request-ID: 2f361d717cbf4bdb51dfcc2f0f92045f66e357fa
Timing-Allow-Origin: https://github.com

Even a shift reload doesn't refresh the image in the comment. There is a 200, but no http request to the server.

First test

Posting here amanite 2 as a reference

The first image should be identical to the second image.

Here is what I see:

Ah interesting!
Thanks @miketaylr

on my side I still see them as different. Browser caching maybe.
And I have seen your request on my server.

In a different browser (no caching for this page), I entered the URL and still different images.

I deleted the image and it is still being served by github even after shift reload.
So the image really lives in the github caching systems.

Expires: Thu, 28 Nov 2019 01:12:42 GMT

to check if the image has disappeared from their caching system.

Also the

             data-canonical-src="http://la-grange.net/tmp/amanite01.webp"

is not used by any JavaScript in the page. This is "only cosmetics" informational.