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Signal ships the Apple emoji set without a licence via an npm package
and upstream does not seem terribly interested in fixing this; see:
* <https://github.com/signalapp/Signal-Android/issues/5862>
* <https://whispersystems.discoursehosting.net/t/signal-is-likely-violating-apple-license-terms-by-using-apple-emoji-in-the-sticker-creator-and-android-and-desktop-apps/52883>
I don’t want to mark Signal as `lib.licenses.unfree`, so this change
instead replaces the bundled Apple emoji PNGs with ones generated
from our freely‐licensed Noto Color Emoji font.
I chose Noto Color Emoji because it is the best‐maintained FOSS
emoji font, and because Signal Android will also use the Noto emoji
if the “Chats → Keyboard → Use system emoji” setting is
turned on. Therefore, Noto Color Emoji is both within the bounds
of the Signal user experience on other platforms, and more likely
to match the emoji font installed on a NixOS system to boot. I have
verified that Noto Color Emoji covers all the standalone emoji that
the bundled Apple set does, and could not find any emoji sequence
that reliably displayed correctly in Signal before these changes but
did not afterwards. (Though I sure did find a good number of emoji
that displayed weirdly in Signal both before and after.)
Signal will also download and cache large versions of the Apple
emoji from their own update server at runtime. This does not pose
a copyright concern for the Nixpkgs cache, but would result in
inconsistent presentation between small and large emoji. Therefore,
we also point these to our Noto Color Emoji PNGs, and gain a little
privacy in the process.
**No invasive patches are made to the Signal code;** the only
changes are to replace the unlicensed Apple emoji files with our own,
and replace the URL that large versions are fetched from to point
to them. There is no functional change to the application other
than showing different images on the client and not requesting the
jumbomoji pack files from the Signal update server. Ideally we’d
build this package from source and simply omit the problematic files
in the first place, but apparently that’s a little tricky and we
should solve the compliance problem now.
The best solution, of course, would be for Signal to replace their
unlicensed copy of Apple’s emoji with a freely‐licensed set
compatible with their AGPLv3 licence. I may try and raise this
situation again with Signal, although given the past response I am
not optimistic, but I wanted to first address the potential copyright
violation in Nixpkgs as swiftly as possible.
Although the Python script used to copy and rename the Noto PNGs
is very simple, I have extensively documented it to help increase
confidence in it and ease further maintenance. To reflect my
willingness to keep this change maintained and take responsibility
for it, I have added myself to the package maintainer list.
These changes actually result in the uncompressed size of the resulting
package decreasing from 450 MiB to 435 MiB; as Signal would ordinarily
download and cache up to 27 MiB of jumbomoji sheets from their servers
during use, the effective disk space savings are likely to be higher.
Thanks to @mjm for helping test this.
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