preload output overrides normal output later

[iamareebjamal]

Let's say you wrote this to create a link preload tag:

{% compress js preload %}
  <script ...a.js" />
  <script ...b.js" />
  <script ...c.js" />
{% endcompress %}

It will produce this:

<link rel="preload" href="..." />

Now, when you want the actual script and you write it without preload attribute,

{% compress js %}
  <script ...a.js" />
  <script ...b.js" />
  <script ...c.js" />
{% endcompress %}

It still emits link preload tag.

So,

Expected:
<script src="shamalamadingdong.js"></script>

Actual:
<link rel="preload" href="shamalamadingdong.js" as="script" />

[karyon]

thanks for the report. the preload feature was implemented in #951. it's not immediately obvious to me what the problem might be. The PR doesn't even add code that's specific to the preload feature, so you might be able to repro this even with the file or inline output modes... (it's just that you wouldn't have any reason to do that with those modes, whereas the preload tag is more or less designed to be used twice...)

anyway, we take PRs! :)

[VivienGiraud]

I have the same behaviour here. Though I was doing it wrong. preload system is a bit young, but could be an interesting feature for sure !

[jsumnerPhD]

Hi @VivienGiraud and @iamareebjamal - I just tested wrapping multiple scripts in preload and then the normal compress and it seems to work fine.

But... it unfortunately all breaks down if COMPRESS_OFFLINE is True. The cache manifest is updated when the first tag is encountered (in this case with preload) and the file hash is then always mapped to the same HTML output. @karyon's intuition seems correct: any set of tags would have this behaviour (but the behaviour would be correct in any other context).

It looks like this can be fixed by updating how the cache key is generated by e.g. prepending the mode. I will put together a PR to get the discussion rolling.

[jsumnerPhD]

@karyon - I added an extra commit to #1033 which fixes this problem. I don't love the solution as the node object passed by the django engine and the node object passed by the jinja2 engine seem to be different and I have to treat each separately. It feels a bit brittle.