Closed Bug 1224892 Opened 6 years ago Closed 6 years ago

2 .override JS properties are dynamic (not cached), whereas the other 3 are not (cached & not reflecting changes until page (re)loading)


(Core :: DOM: Core & HTML, defect)

42 Branch
Not set



Tracking Status
firefox47 --- fixed


(Reporter: haqer, Assigned: aidin, Mentored)




(1 file)

User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:42.0) Gecko/20100101 Firefox/42.0
Build ID: 20151029151421

Steps to reproduce:

1. Ran the JS from step 2. of description of bug 1224786 on an http(s): tab, & marked down the results.
2. Set overrides distinct from defaults for the following in about:config:
3. Re-ran the JS from step 1. w/o reloading the page or loading a different one.

Actual results:

show the latest override values,
whereas the other 3 show the values that were in effect during step 1. (before changes made in step 2.).

Expected results:

IMHO, they should all be consistent: all 5 should either be the latest override values, or all 5 should show the cached values that were current when the page was first loaded. 

P.S. I've seen this on tabs using http(s):, resource:, & ftp: URLs (as well as on about:home). It'd probably only make sense to deal w/ the rest of the protocols (such as a.-through-c. from bug 1224786), when that bug 1224786 is addressed. 
P.P.S. I've used (your humble servant's) Blend In[1] extension to change values back and forth by toggling  
preference in about:config on Linux. 
P.P.P.S. If there's some consensus that caching of some & not others manifesting before page (re)loading is a fringe case that can just be WONTFIX'ed, i'm not gonna argue much with that, but it's still worth documenting this behavior (which this bug gets done in either case).
Depends on: 1224786
The cached values are because some of the properties are marked as [Constant, Cached] in the IDL.  See bug 1069401 -- making them cached simplifies things considerably for workers.  But the [Cached] stuff was added only to the properties exposed in workers, since it was only needed there.

If someone wants to make the other ones [Cached] as well, go for it.  It's a good way to get some familiarity with parts of the DOM code.
Mentor: bzbarsky
Ever confirmed: true
I am working on this.
Assignee: nobody → aidin
Bug 1224892 - Caching `oscpu' and `buildID' like other attributes related to User Agent. r?smaug
Attachment #8698504 - Flags: review?(bugs)
Because Boris is on vaccation, I asked Olli to kindly review this.

Here's the try server build:

I ran Mochitests to check if anything broke. Seems those failures are unrelated to my changes.
Comment on attachment 8698504 [details]
MozReview Request: Bug 1224892 - Caching `oscpu' and `buildID' like other attributes related to User Agent. r?smaug

It is tiny bit odd to have Constant, Cached only in those two properies, but looks like it should be fine.
Many of the properties just return some constant value, but this isn't performance critical, so Constant, Cached isn't really needed with those.
Attachment #8698504 - Flags: review?(bugs) → review+
Thanks for the review (:
Keywords: checkin-needed
sorry backed out for test failures like
Flags: needinfo?(aidin)
Depends on: 1247346
The failure was due to bug 1247346.  Once that's fixed, this can reland.
Closed: 6 years ago
Resolution: --- → FIXED
Target Milestone: --- → mozilla47
Flags: needinfo?(aidin)
Component: DOM → DOM: Core & HTML
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