bug#26201: No notification of cache misses when downloading substitutes

[Tobias Geerinckx-Rice]

Hullo!

On 21/03/17 07:49, address@hidden wrote:
> I'm not sure how any of this matters. If you are running a local 
> Hydra instance or whatever, then I'd assume you'd be aware of what, 
> if any, problems that could arise.

It matters for the reasons mentioned. It's not a ‘local Hydra’ & I have
no idea what problems you're talking about.

My problem is that every invocation of Guix already fills several
screens with Guile cache misses. Adding another warning (‘warning! the
system is working exactly as designed!’) will only serve to make those
other warnings look less silly, and I think that would be a shame.

To clarify:

- Warnings should be scary because warnings should be actionable.
  There's nothing the user can or needs to do about a cache miss.
- It would be randomly shown to everyone, since this happens constantly.
- The behaviour warned about is not incorrect or abnormal.
- As already noted, it's how caching works.

> I don't see how this would have to be "an ugly hack". It's simply a 
> query and response. The simplest way I can see for this to work would
> be for mirror.hydra to either just send the requested file, or a
> response that the file isn't cached then start to trickle the file on
> to the client.

Well, yeah... That's the ugly hack. :-)

It's not that your suggestion's hard to implement. In fact, it's
just one line for nginx (which it turns out I already had):

  add_header X-Cache-Status $upstream_cache_status;

and 6 lines of lightly-tested Guile (attached)¹. And presto. This thing.

Doesn't mean we should.

Kind regards,

T G-R

¹: Why? Practice. Irony. Light masochism.

0001-http-client-Warn-on-proxy-cache-misses.patch
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