From:  Honza Bambas <>
Date:  06 Jan 2016 21:52:23 Hong Kong Time

Re: devtools re-fetching problem


On 12/20/2015 0:36, Jason Duell wrote:
> On Fri, Dec 18, 2015 at 6:15 PM, Ehsan Akhgari 
> wrote:
>> On 2015-12-18 4:56 PM, Tom Tromey wrote:
>>> I mentioned this on #necko the other day.
>>> We've had a "re-fetching" problem in devtools for a while now.
>>> The basic issue is that the platform drops the original text of some
>>> things, like style sheets.  (Instead just the parsed form is preserved.)
>>> So, in order to make some tools work with the source text, devtools
>>> re-fetches the sources.  However, this is sub-optimal -- maybe the
>>> sources have changed on the server, leading to confusing results.
>>> We were wondering if there was some way we could solve this problem.
>>> One idea we had is to always cache these things, in a way that would let
>>> the devtools retrieve them.  Is this possible?
>> We can't cache the resources if the web server tells us "don't cache
>> this", so even if we use things such as LOAD_ONLY_FROM_CACHE, we still need
>> a solution for that case.
>> Or ... some other idea here.
>> How about forcing Gecko retain the original source text when devtools are
>> being used?
> Sounds like we need something like a CACHE_GECKO_COPY flag that keeps a
> "secret" cache only for internal gecko use?

If that would be enforced only for active devtools, we could go with that.

> 1) if the resource would normally be cached, does nothing (the resource
> gets cached normally)
> 2) If the resource wouldn't be cached, cache it (perhaps only in RAM?

Easy to evict on devtools close but easy to waste memory...

>    Or
> maybe we'd need that only if INHIBIT_PERSISTENT_CACHING is set), with some
> sort of flag that indicates it should be invisible to cache reads unless
> CACHE_HIDDEN_COPY is again present.

Such a devtools cache is very hard to build on the Necko level.  We need 
new flags or somehow expose the info on the loadgroup.  If we use HTTP 
force caching, we need to ensure it goes away with closing the devtools 
window.  Being in-memory only might be the way here. Use the 
defer-caching capability from bug 1203113 could be other way to achieve 
it (we are save with eviction on devtools close and also don't waste 

Lot of work.  Don't we have more important stuff to do? ;)

Also, this caching enforce on the HTTP level would actually break the 
web - the behavior would simply be different.  I think this kind of 
caching should do devtools, not necko.


> Jason
> P.S. Honza/Michal: right now nsIRequest.idl says that "For HTTPS,
> [INHIBIT_PERSISTENT_CACHING] is set automatically."  That's out of date now
> IIRC--we should change the comment.
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