[Experiment] Generate .AppImage.zsync files for automatic updates

Author: pol-rivero

.github/workflows/ci-linux.yml

@@ -30,12 +30,15 @@ jobs:
           submodules: recursive
       - name: Package and test application in container
         uses: pol-rivero/desktop-ubuntu-arm64-packaging@268f378f0fcea447cbb773ccf5bdc82f7389dcce
+      - name: Generate AppImage zsync
+        run: script/generate-appimage-zsync.sh dist/*.AppImage arm64
       - name: Upload output artifacts
         uses: actions/upload-artifact@v4
         with:
           name: ubuntu-arm64-artifacts
           path: |
             dist/*.AppImage
+            dist/*.AppImage.zsync
             dist/*.deb
             dist/*.rpm
             dist/*.sha256
@@ -54,12 +57,15 @@ jobs:
           submodules: recursive
       - name: Package and test application in container
         uses: pol-rivero/desktop-ubuntu-arm-packaging@2d4df8604295eb7bf767101c365435e7e029e3df
+      - name: Generate AppImage zsync
+        run: script/generate-appimage-zsync.sh dist/*.AppImage armv7l
       - name: Upload output artifacts
         uses: actions/upload-artifact@v4
         with:
           name: ubuntu-arm-artifacts
           path: |
             dist/*.AppImage
+            dist/*.AppImage.zsync
             dist/*.deb
             dist/*.rpm
             dist/*.sha256
@@ -78,12 +84,15 @@ jobs:
           submodules: recursive
       - name: Package and test application in container
         uses: pol-rivero/desktop-ubuntu-amd64-packaging@fd44033e38ccc986e39ed8e7a6d38640506e8bde
+      - name: Generate AppImage zsync
+        run: script/generate-appimage-zsync.sh dist/*.AppImage x86_64
       - name: Upload output artifacts
         uses: actions/upload-artifact@v4
         with:
           name: ubuntu-amd64-artifacts
           path: |
             dist/*.AppImage
+            dist/*.AppImage.zsync
             dist/*.deb
             dist/*.rpm
             dist/*.sha256
@@ -142,6 +151,7 @@ jobs:
             Upstream: [GitHub Desktop ${{ env.VERSION_WITHOUT_V }} release notes](https://github.com/desktop/desktop/releases/tag/release-${{ env.VERSION_WITHOUT_V }})
           files: |
             artifacts/**/*.AppImage
+            artifacts/**/*.AppImage.zsync
             artifacts/**/*.deb
             artifacts/**/*.rpm
           draft: false

script/generate-appimage-zsync.sh

@@ -0,0 +1,31 @@
+#!/bin/bash
+
+set -e
+
+APPIMAGE_FILE="$1"
+ARCH="$2"
+if [ -z "$APPIMAGE_FILE" ] || [ -z "$ARCH" ]; then
+  echo "Usage: $0 <AppImage file> <architecture>"
+  exit 1
+fi
+
+REPO_OWNER="pol-rivero"
+REPO_NAME="github-desktop-plus"
+RELEASES_ZSYNC_PATTERN="GitHubDesktopPlus-*-linux-$ARCH.AppImage.zsync"
+
+# Extract AppImage contents
+cd "$(dirname "$APPIMAGE_FILE")"
+chmod +x "$APPIMAGE_FILE"
+"./$APPIMAGE_FILE" --appimage-extract
+
+TAG="latest"  # https://github.com/AppImage/AppImageSpec/blob/master/draft.md#release-name-values
+UPDATE_INFO="gh-releases-zsync|$REPO_OWNER|$REPO_NAME|$TAG|$RELEASES_ZSYNC_PATTERN"
+
+curl -L -o appimagetool https://github.com/AppImage/appimagetool/releases/download/continuous/appimagetool-x86_64.AppImage
+chmod +x appimagetool
+
+# Embed update info and re-pack (this will generate the .zsync file)
+./appimagetool -u "$UPDATE_INFO" squashfs-root "$APPIMAGE_FILE"
+
+# Cleanup
+rm -rf squashfs-root appimagetool