Shallow Thoughts : : Jun

Akkana's Musings on Open Source Computing and Technology, Science, and Nature.

Sat, 23 Jun 2018

Modifying Firefox Files Inside Omni.ja

My article on Fixing key bindings in Firefox Quantum by modifying the source tree got attention from several people who offered helpful suggestions via Twitter and email on how to accomplish the same thing using just files in omni.ja, so it could be done without rebuilding the Firefox source. That would be vastly better, especially for people who need to change something like key bindings or browser messages but don't have a souped-up development machine to build the whole browser.

Brian Carpenter had several suggestions and eventually pointed me to an old post by Mike Kaply, Don’t Unpack and Repack omni.ja[r] that said there were better ways to override specific files.

Unfortunately, Mike Kaply responded that that article was written for XUL extensions, which are now obsolete, so the article ought to be removed. That's too bad, because it did sound like a much nicer solution. I looked into trying it anyway, but the instructions it points to for Overriding specific files is woefully short on detail on how to map a path inside omni.ja like chrome://package/type/original-uri.whatever, to a URL, and the single example I could find was so old that the file it referenced didn't exist at the same location any more. After a fruitless half hour or so, I took Mike's warning to heart and decided it wasn't worth wasting more time chasing something that wasn't expected to work anyway. (If someone knows otherwise, please let me know!)

But then Paul Wise offered a solution that actually worked, as an easy to follow sequence of shell commands. (I've changed some of them very slightly.)

$ tar xf ~/Tarballs/firefox-60.0.2.tar.bz2
  # (This creates a "firefox" directory inside the current one.)

$ mkdir omni
$ cd omni

$ unzip -q ../firefox/browser/omni.ja
warning [../firefox-60.0.2/browser/omni.ja]:  34187320 extra bytes at beginning or within zipfile
  (attempting to process anyway)
error [../firefox-60.0.2/browser/omni.ja]:  reported length of central directory is
  -34187320 bytes too long (Atari STZip zipfile?  J.H.Holm ZIPSPLIT 1.1
  zipfile?).  Compensating...
zsh: exit 2     unzip -q ../firefox-60.0.2/browser/omni.ja

$ sed -i 's/or enter address/or just twiddle your thumbs/' chrome/en-US/locale/browser/browser.dtd chrome/en-US/locale/browser/browser.properties

I was a little put off by all the warnings unzip gave, but kept going.

Of course, you can just edit those two files rather than using sed; but the sed command was Paul's way of being very specific about the changes he was suggesting, which I appreciated.

Use these flags to repackage omni.ja:

$ zip -qr9XD ../omni.ja *

I had tried that before (without the q since I like to see what zip and tar commands are doing) and hadn't succeeded. And indeed, when I listed the two files, the new omni.ja I'd just packaged was about a third the size of the original:

$ ls -l ../omni.ja ../firefox-60.0.2/browser/omni.ja
-rw-r--r-- 1 akkana akkana 34469045 Jun  5 12:14 ../firefox/browser/omni.ja
-rw-r--r-- 1 akkana akkana 11828315 Jun 17 10:37 ../omni.ja

But still, it's worth a try:

$ cp ../omni.ja ../firefox/browser/omni.ja

Then run the new Firefox. I have a spare profile I keep around for testing, but Paul's instructions included a nifty way of running with a brand new profile and it's definitely worth knowing:

$ cd ../firefox

$ MOZILLA_DISABLE_PLUGINS=1 ./firefox -safe-mode -no-remote -profile $(mktemp -d tmp-firefox-profile-XXXXXXXXXX) -offline about:blank

Also note the flags like safe-mode and no-remote, plus disabling plugins -- all good ideas when testing something new.

And it worked! When I started up, I got the new message, "Search or just twiddle your thumbs", in the URL bar.

Fixing Ctrl-W

Of course, now I had to test it with my real change. Since I like Paul's way of using sed to specify exactly what changes to make, here's a sed version of my Ctrl-W fix:

$ sed -i '/key_close/s/ reserved="true"//' chrome/browser/content/browser/browser.xul

Then run it. To test Ctrl-W, you need a website that includes a text field you can type in, so -offline isn't an option unless you happen to have a local web page that includes some text fields. Google is an easy way to test ... and you might as well re-use that firefox profile you just made rather than making another one:

$ MOZILLA_DISABLE_PLUGINS=1 ./firefox -safe-mode -no-remote -profile tmp-firefox-profile-* https://google.com

I typed a few words in the google search field that came up, deleted them with Ctrl-W -- all was good! Thanks, Paul! And Brian, and everybody else who sent suggestions.

Why are the sizes so different?

I was still puzzled by that threefold difference in size between the omni.ja I repacked and the original that comes with Firefox. Was something missing? Paul had the key to that too: use zipinfo on both versions of the file to see what differed. Turned out Mozilla's version, after a long file listing, ends with

2650 files, 33947999 bytes uncompressed, 33947999 bytes compressed:  0.0%
while my re-packaged version ends with
2650 files, 33947969 bytes uncompressed, 11307294 bytes compressed:  66.7%

So apparently Mozilla's omni.ja is using no compression at all. It may be that that makes it start up a little faster; but Quantum takes so long to start up that any slight difference in uncompressing omni.ja isn't noticable to me.

I was able to run through this whole procedure on my poor slow netbook, the one where building Firefox took something like 15 hours ... and in a few minutes I had a working modified Firefox. And with the sed command, this is all scriptable, so it'll be easy to re-do whenever Firefox has a security update. Win!

Update: I have a simple shell script to do this: Script to modify omni.ja for a custom Firefox.

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[ 20:37 Jun 23, 2018    More tech/web | permalink to this entry | comments ]

Thu, 14 Jun 2018

Firefox Quantum: Fixing Ctrl W (or other key bindings)

When I first tried switching to Firefox Quantum, the regression that bothered me most was Ctrl-W, which I use everywhere as word erase (try it -- you'll get addicted, like I am). Ctrl-W deletes words in the URL bar; but if you type Ctrl-W in a text field on a website, like when editing a bug report or a "Contact" form, it closes the current tab, losing everything you've just typed. It's always worked in Firefox in the past; this is a new problem with Quantum, and after losing a page of typing for about the 20th time, I was ready to give up and find another browser.

A web search found plenty of people online asking about key bindings like Ctrl-W, but apparently since the deprecation of XUL and XBL extensions, Quantum no longer offers any way to change or even just to disable its built-in key bindings.

I wasted a few days chasing a solution inspired by this clever way of remapping keys only for certain windows using xdotool getactivewindow; I even went so far as to write a Python script that intercepts keystrokes, determines the application for the window where the key was typed, and remaps it if the application and keystroke match a list of keys to be remapped. So if Ctrl-W is typed in a Firefox window, Firefox will instead receive Alt-Backspace. (Why not just type Alt-Backspace, you ask? Because it's much harder to type, can't be typed from the home position, and isn't in the same place on every keyboard the way W is.)

But sadly, that approach didn't work because it turned out my window manager, Openbox, acts on programmatically-generated key bindings as well as ones that are actually typed. If I type a Ctrl-W and it's in Firefox, that's fine: my Python program sees it, generates an Alt-Backspace and everything is groovy. But if I type a Ctrl-W in any other application, the program doesn't need to change it, so it generates a Ctrl-W, which Openbox sees and calls the program again, and you have an infinite loop. I couldn't find any way around this. And admittedly, it's a horrible hack having a program intercept every keystroke. So I needed to fix Firefox somehow.

But after spending days searching for a way to customize Firefox's keys, to no avail, I came to the conclusion that the only way was to modify the source code and rebuild Firefox from source.

Ironically, one of the snags I hit in building it was that I'd named my key remapper "pykey.py", and it was still in my PYTHONPATH; it turns out the Firefox build also has a module called pykey.py and mine was interfering. But eventually I got the build working.

Firefox Key Bindings

I was lucky: building was the only hard part, because a very helpful person on Mozilla's #introduction IRC channel pointed me toward the solution, saving me hours of debugging. Edit browser/base/content/browser-sets.inc around line 240 and remove reserved="true" from key_closeWindow. It turned out I needed to remove reserved="true" from the adjacent key_close line as well.

Another file that's related, but more general, is nsXBLWindowKeyHandler.cpp around line 832; but I didn't need that since the simpler fix worked.

Transferring omni.ja -- or Not

In theory, since browser-sets.inc isn't compiled C++, it seems like you should be able to make this fix without building the whole source tree. In an actual Firefox release, browser-sets.inc is part of omni.ja, and indeed if you unpack omni.ja you'll see the key_closeWindow and key_close lines. So it seems like you ought to be able to regenerate omni.ja without rebuilding all the C++ code.

Unfortunately, in practice omni.ja is more complicated than that. Although you can unzip it and edit the files, if you zip it back up, Firefox doesn't see it as valid. I guess that's why they renamed it .ja: long ago it used to be omni.jar and, like other .jar files, was a standard zip archive that you could edit. But the new .ja file isn't documented anywhere I could find, and all the web discussions I found on how to re-create it amounted to "it's complicated, you probably don't want to try".

And you'd think that I could take the omni.ja file from my desktop machine, where I built Firefox, and copy it to my laptop, replacing the omni.ja file from a released copy of Firefox. But no -- somehow, it isn't seen, and the old key bindings are still active. They must be duplicated somewhere else, and I haven't figured out where.

It sure would be nice to have a way to transfer an omni.ja. Building Firefox on my laptop takes nearly a full day (though hopefully rebuilding after pulling minor security updates won't be quite so bad). If anyone knows of a way, please let me know!

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[ 16:45 Jun 14, 2018    More tech/web | permalink to this entry | comments ]

Sat, 09 Jun 2018

Building Firefox for ALSA (non PulseAudio) Sound

I did the work to built my own Firefox primarily to fix a couple of serious regressions that couldn't be fixed any other way. I'll start with the one that's probably more common (at least, there are many people complaining about it in many different web forums): the fact that Firefox won't play sound on Linux machines that don't use PulseAudio.

There's a bug with a long discussion of the problem, Bug 1345661 - PulseAudio requirement breaks Firefox on ALSA-only systems; and the discussion in the bug links to another discussion of the Firefox/PulseAudio problem). Some comments in those discussions suggest that some near-future version of Firefox may restore ALSA sound for non-Pulse systems; but most of those comments are six months old, yet it's still not fixed in the version Mozilla is distributing now.

In theory, ALSA sound is easy to enable. Build pptions in Firefox are controlled through a file called mozconfig. Create that file at the top level of your build directory, then add to it:

ac_add_options --enable-alsa
ac_add_options --disable-pulseaudio

You can see other options with ./configure --help

Of course, like everything else in the computer world, there were complications. When I typed mach build, I got:

Assertion failed in _parse_loader_output:
Traceback (most recent call last):
  File "/home/akkana/outsrc/gecko-dev/python/mozbuild/mozbuild/mozconfig.py", line 260, in read_mozconfig
    parsed = self._parse_loader_output(output)
  File "/home/akkana/outsrc/gecko-dev/python/mozbuild/mozbuild/mozconfig.py", line 375, in _parse_loader_output
    assert not in_variable
AssertionError
Error loading mozconfig: /home/akkana/outsrc/gecko-dev/mozconfig

Evaluation of your mozconfig produced unexpected output.  This could be
triggered by a command inside your mozconfig failing or producing some warnings
or error messages. Please change your mozconfig to not error and/or to catch
errors in executed commands.

mozconfig output:

------BEGIN_ENV_BEFORE_SOURCE
... followed by a many-page dump of all my environment variables, twice.

It turned out that was coming from line 449 of python/mozbuild/mozbuild/mozconfig.py:

   # Lines with a quote not ending in a quote are multi-line.
    if has_quote and not value.endswith("'"):
        in_variable = name
        current.append(value)
        continue
    else:
        value = value[:-1] if has_quote else value

I'm guessing this was added because some Mozilla developer sets a multi-line environment variable that has a quote in it but doesn't end with a quote. Or something. Anyway, some fairly specific case. I, on the other hand, have a different specific case: a short environment variable that includes one or more single quotes, and the test for their specific case breaks my build.

(In case you're curious why I have quotes in an environment variable: The prompt-setting code in my .zshrc includes a variable called PRIMES. In a login shell, this is set to the empty string, but in subshells, I add ' for each level of shell under the login shell. So my regular prompt might be (hostname)-, but if I run a subshell to test something, the prompt will be (hostname')-, a subshell inside that will be (hostname'')-, and so on. It's a reminder that I'm still in a subshell and need to exit when I'm done testing. In theory, I could do that with SHLVL, but SHLVL doesn't care about login shells, so my normal shells inside X are all SHLVL=2 while shells on a console or from an ssh are SHLVL=1, so if I used SHLVL I'd have to have some special case code to deal with that.

Also, of course I could use a character other than a single-quote. But in the thirty or so years I've used this, Firefox is the first program that's ever had a problem with it. And apparently I'm not the first one to have a problem with this: bug 1455065 was apparently someone else with the same problem. Maybe that will show up in the release branch eventually.)

Anyway, disabling that line fixed the problem:

   # Lines with a quote not ending in a quote are multi-line.
    if False and has_quote and not value.endswith("'"):
and after that, mach build succeeded, I built a new Firefox, and lo and behond! I can play sound in YouTube videos and on Xeno-Canto again, without needing an additional browser.

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[ 16:49 Jun 09, 2018    More tech/web | permalink to this entry | comments ]

Sun, 03 Jun 2018

Building Firefox Quantum

With Firefox Quantum, Mozilla has moved away from letting users configure the browser they way they like. If I was going to switch to Quantum as my everyday browser, there were several problems I needed to fix first -- and they all now require modifying the source code, then building the whole browser from scratch.

I'll write separately about fixing the specific problems; but first I had to build Firefox. Although I was a Firefox developer way back in the day, the development environment has changed completely since then, so I might as well have been starting from scratch.

Setting up a Firefox build

I started with Mozilla's Linux build preparation page. There's a script called bootstrap.py that's amazingly comprehensive. It will check what's installed on your machine and install what's needed for a Firefox build -- and believe me, there are a lot of dependencies. Don't take the "quick" part of the "quick and easy" comment at the beginning of the script too seriously; I think on my machine, which already has a fairly full set of build tools, the script was downloading additional dependencies for 45 minutes or so. But it was indeed fairly easy: the script asks lots of questions about optional dependencies, and usually has suggestions, which I mostly followed.

Eventually bootstrap.py finishes loading the dependencies and gets to the point of wanting to check out the mozilla-unified repository, and that's where I got into trouble.

The script wants to check out the bleeding edge tip of Mozilla development. That's what you want if you're a developer checking in to the project. What I wanted was a copy of the currently released Firefox, but with a chance to make my own customizations. And that turns out to be difficult.

Getting a copy of the release tree

In theory, once you've checked out mozilla-unified with Mercurial, assuming you let bootstrap.py enable the recommended "firefoxtree" hg extension (which I did), you can switch to the release branch with:

hg pull release
hg up -c release

That didn't work for me: I tried it numerous times over the course of the day, and every time it died with "abort: HTTP request error (incomplete response; expected 5328 bytes got 2672)" after "adding manifests" when it started "adding file changes".

That sent me on a long quest aided by someone in Mozilla's helpful #introduction channel, where they help people with build issues. You might think it would be a common thing to want to build a copy of the released version of Firefox, and update it when a new release comes out. But apparently not; although #introduction is a friendly and helpful channel, everyone seemed baffled as to why hg up didn't work and what the possible alternatives might be.

Bundles and Artifacts

Eventually someone pointed me to the long list of "bundle" tarballs and advised me on how to get a release tarball there. I actually did that, and (skipping ahead briefly) it built and ran; but I later discovered that "bundles" aren't actually hg repositories and can't be updated. So once you've downloaded your 3 gigabytes or so of Mozilla stuff and built it, it's only good for a week or two until the next Mozilla release, when you're now hopelessly out of date and have to download a whole nuther bundle. Bundles definitely aren't the answer, and they aren't well supported or documented either. I recommend staying away from them.

I should also mention "artifact builds". These sound like a great idea: a lot of the source is already built for you, so you just build a little bit of it. However, artifact builds are only available for a few platforms and branches. If your OS differs in any way from whoever made the artifact build, or if you're requesting a branch, you're likely to waste a lot of time (like I did) downloading stuff only to get mysterious error messages. And even if it works, you won't be able to update it to keep on top of security fixes. Doesn't seem like a good bet.

GitHub to the rescue

Okay, so Mercurial's branch switching doesn't work. But it turns out you don't have to use Mercurial. There's a GitHub mirror for Firefox called gecko-dev, and after cloning it you can use normal git commands to switch branches:

git clone https://github.com/mozilla/gecko-dev.git
cd gecko-dev/
git checkout -t origin/release

You can verify you're on the right branch with git branch -vv, or if you want to list all branches and their remotes, git branch -avv.

Finally: a Firefox release branch that you can actually update!

Building Firefox

Once you have a source tree, you can use the all-powerful mach script to build the current release of Firefox:

./mach build

Of course that takes forever -- hours and hours, depending on how fast your machine is.

Running your New Firefox

The build, after it finishes, helpfully tells you to test it with ./mach run, which runs your newly-built firefox with a special profile, so it doesn't interfere with your running build. It also prints:

For more information on what to do now, see https://developer.mozilla.org/docs/Developer_Guide/So_You_Just_Built_Firefox

Great! Except there's no information there on how to package or run your build -- it's just a placeholder page asking people to contribute to the page.

It turns out that obj-whatever/dist/bin is the directory that corresponds to the tarball you download from Mozilla, and you can run /path/to/mozilla-release/obj-whatever/dist/bin/firefox from anywhere.

I tried filing a bug request to have a sub-page created explaining how to run a newly built Firefox, but it hasn't gotten any response. Maybe I'll just edit the "So You Just Built" page.

Incidentally, my gecko-dev build takes 16G of disk space, of which 9.3G is things it built, which are helpfully segregated in obj-x86_64-pc-linux-gnu.

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[ 15:55 Jun 03, 2018    More tech/web | permalink to this entry | comments ]