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<p>I am pleased to announce that, unless something crazy happens,
the next Daily will use more than one process. This builds on
the work to make Firefox multi-process capable (AKA the
Electrolysis project) and is the result of my work over the last
few months.</p>
<p>Don't get too excited… this is more about enabling potential
improvements in the future than noticeable improvements now.<br>
</p>
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<h2>What has actually changed?</h2>
<ul>
<li>When Thunderbird loads a web page in the message pane (such
as the home page), or in a tab, or in a window, that will be
done by a secondary "child" process, not the main Thunderbird
process as was previously the case.</li>
<li>Transitioning from a web page to a message, or vice versa,
has become more complicated, but this is handled
automatically.<br>
</li>
<li>Extensions are now loaded in child processes.</li>
</ul>
<h2>What does this mean for me?</h2>
<h3>End-users:</h3>
<p>Users <i>should not</i> notice any changes. This change is
entirely in the background and shouldn't make any difference to
what the user interface does.<br>
</p>
<h3>Developers:</h3>
<p>This won't affect most of what developers are working on.
However if you need to do something with the contents of a web
page, your life just became more complicated. You'll need to
reach across the process boundary, probably using actors. This
may seem complicated compared to using <font face="monospace">contentDocument</font>
etc. but is pretty easy once you get the hang of it. The Firefox
docs have <a
href="https://firefox-source-docs.mozilla.org/dom/ipc/jsactors.html">some
good information</a> or you can ask me for help.</p>
<p>The same applies to tests looking at the content of web pages.
In most cases, <font face="monospace"><a
href="https://searchfox.org/mozilla-central/rev/c03e8de87cdb0ce0378c0886d3c0ce8bbf9dc44e/testing/specialpowers/content/SpecialPowersChild.jsm#1509-1545">SpecialPowers.spawn</a></font>
will be all you need.</p>
<p>When loading or changing URLs in a <font face="monospace"><browser></font>
element, please use <font face="monospace">MailE10SUtils.loadURI</font>.
This will handle the transition between parent and child
processes, if necessary.<br>
</p>
<p>One other thing to note is that the "content" tab type has
changed. Where once you would open a content tab and specify how
clicks on links in that tab should behave, this now happens
automatically. By default links to URLs on the same website will
open in Thunderbird and all others in an external browser, but
you can make this more restrictive (only URLs on the same page)
or less restrictive (any URLs). See the JSDoc on <font
face="monospace">specialTabs.contentTabType.openTab</font> for
details.<br>
</p>
<h3>Extension developers:</h3>
<p>You might notice some changes, depending on what your extension
does.<br>
</p>
<ul>
<li>If it's wholly a WebExtension (no experiments) it should
just work as before.</li>
<li>Experiments run in the main "parent" process, just like they
did before. Most things should continue working, unless you're
reaching into web pages (which you shouldn't need to because
the <font face="monospace"><a
href="https://webextension-api.thunderbird.net/en/latest/tabs.html#executescript-tabid-details">tabs.executeScript</a></font>
function and friends exist).</li>
<li>If you open tabs using an experiment you should read about
the changes above. And probably think about moving to <font
face="monospace"><a moz-do-not-send="true"
href="https://webextension-api.thunderbird.net/en/latest/tabs.html#create-createproperties">tabs.create</a></font>.<br>
</li>
</ul>
<h2>When will this happen?</h2>
<p>I've <i>just</i> landed the changes. They'll be in the next
Daily build. Unless something serious happens, I expect they
will be in 86 beta 1 next week, and then in 2021's ESR.<br>
</p>
<h2>Can I turn it off?</h2>
<p>If you must, yes, technically it will be possible to return to
using a single process. However, it's not possible to run our
automated testing on both single-process and multi-process
Thunderbird at the same time, so the default will be
multi-process.<br>
</p>
<p>The preferences to change are <font face="monospace">browser.tabs.remote.autostart</font>
and <font face="monospace">extensions.webextensions.remote</font>.
Set them to false to go back to single-process.<br>
</p>
<h2>What will happen in the future?</h2>
<p>While the changes I've made today don't do a lot, it does bring
us a lot closer to what Firefox does so we can take advantage of
what they have been working on for the last few years. (It also
means there should be fewer problems caused by the differences
between our code and theirs.)</p>
<p>I haven't really looked into what could now be possible, but
theoretically it should allow us to move things out of the main
process for performance and security advantages. For example,
loading network calendars or CardDAV address books should be
possible in a child process. Potentially we could move mail
operations to another process (don't hold your breath).<br>
</p>
<h2>What should I do if I find a bug?<br>
</h2>
<p>Report bugs as per usual, and if you're convinced today's
changes are the cause, mark as regressed by <a
href="https://bugzilla.mozilla.org/show_bug.cgi?id=1646648">bug
1646648</a>. If you're not convinced of the cause but
suspicious, CC/NI me and ask.</p>
<p>GL<br>
</p>
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