signal-desktop: Document the build "expiration"

This adds a comment why it's important to always backport any updates as
suggested in #68232. I've als included a few more details to document
the behaviour.

Sources:
- Default build lifetime [0]
- asar - Electron Archive [1]

[0]: https://github.com/signalapp/Signal-Desktop/blob/d3d2b0ec529ffd5ad74deb17831005a1acacc949/Gruntfile.js#L222
[1]: https://github.com/electron/asar

1 files changed, 7 insertions, 1 deletions

diff --git a/pkgs/applications/networking/instant-messengers/signal-desktop/default.nix b/pkgs/applications/networking/instant-messengers/signal-desktop/default.nix
index c5f4468df5aa..f217084585a2 100644
--- a/pkgs/applications/networking/instant-messengers/signal-desktop/default.nix
+++ b/pkgs/applications/networking/instant-messengers/signal-desktop/default.nix
@@ -58,7 +58,13 @@ let
 
 in stdenv.mkDerivation rec {
   pname = "signal-desktop";
-  version = "1.27.2";
+  version = "1.27.2"; # Please backport all updates to the stable channel.
+  # All releases have a limited lifetime and "expire" 90 days after the release.
+  # When releases "expire" the application becomes unusable until an update is
+  # applied. The expiration date for the current release can be extracted with:
+  # $ grep -a "^{\"buildExpiration" "${signal-desktop}/libexec/resources/app.asar"
+  # (Alternatively we could try to patch the asar archive, but that requires a
+  # few additional steps and might not be the best idea.)
 
   src = fetchurl {
     url = "https://updates.signal.org/desktop/apt/pool/main/s/signal-desktop/signal-desktop_${version}_amd64.deb";