Key the system-image cache on the image revision

.github/actions/preview-emulator/action.yml

@@ -47,26 +47,26 @@ runs:
       shell: bash
       run: |
         echo "path=${ANDROID_HOME:-${ANDROID_SDK_ROOT:-/usr/local/lib/android/sdk}}" >> "$GITHUB_OUTPUT"
-        # actions/cache never re-saves on an exact key hit, so a fixed key would
-        # freeze the image at whatever revision was current when it was first
-        # cached: sdkmanager would re-fetch each new revision on every run and
-        # the cache would never catch up, while CI quietly tested a stale image.
-        # Rotating weekly bounds that. The restore-keys prefix means a rotation
-        # restores last week's copy and lets sdkmanager fetch just the delta,
-        # rather than paying for a full download.
-        echo "week=$(date -u +%G-W%V)" >> "$GITHUB_OUTPUT"
 
     # The system image is the one large download here (system.img alone is
     # ~4.1 GiB, ~2 GiB compressed). The preview emulator package is
     # deliberately NOT cached: it moves often and pinning a stale build would
-    # defeat the purpose of these jobs. sdkmanager still runs after a cache
-    # hit, so a newer image revision is still picked up.
+    # defeat the purpose of these jobs.
+    #
+    # Restore and save are split so the entry can be keyed on the image
+    # revision, which is only knowable after sdkmanager has run. A single
+    # actions/cache with a fixed key never re-saves on a hit, so it would pin
+    # whatever revision was cached first and re-download every subsequent
+    # revision on every run, forever. Here: restore whatever copy exists,
+    # let sdkmanager bring it up to date, then save under the revision that
+    # actually ended up on disk. A new revision lands in the cache on the
+    # first run that sees it.
     - name: Restore cached system image
-      id: sysimg-cache
-      uses: actions/cache@v4
+      id: sysimg-restore
+      uses: actions/cache/restore@v4
       with:
         path: ${{ steps.sdk.outputs.path }}/system-images/android-${{ inputs.api-level }}/${{ inputs.target }}/${{ inputs.abi }}
-        key: sysimg-v1-android-${{ inputs.api-level }}-${{ inputs.target }}-${{ inputs.abi }}-${{ steps.sdk.outputs.week }}
+        key: sysimg-v1-android-${{ inputs.api-level }}-${{ inputs.target }}-${{ inputs.abi }}-rev
         restore-keys: |
           sysimg-v1-android-${{ inputs.api-level }}-${{ inputs.target }}-${{ inputs.abi }}
 
@@ -100,6 +100,7 @@ runs:
     # The AVD must be created BEFORE emulators;latest is installed: with the
     # preview package present, avdmanager create silently no-ops.
     - name: Install system image and create AVD
+      id: sysimg
       shell: bash
       run: |
         set +o pipefail  # see "Update SDK cmdline-tools" -- `yes |` vs pipefail
@@ -108,11 +109,17 @@ runs:
         PKG='system-images;android-${{ inputs.api-level }};${{ inputs.target }};${{ inputs.abi }}'
         # Timed so the value of caching the image can be read straight off the
         # job log rather than inferred.
-        echo "SYSIMG-CACHE hit=${{ steps.sysimg-cache.outputs.cache-hit || 'false' }}"
+        echo "SYSIMG-CACHE restored-from='${{ steps.sysimg-restore.outputs.cache-matched-key }}'"
         T0=$(date +%s)
         yes | "$SDK/cmdline-tools/latest/bin/sdkmanager" --channel=3 --install "$PKG" platform-tools > /dev/null
         echo "SYSIMG-INSTALL seconds=$(( $(date +%s) - T0 ))"
-        du -sh "$SDK/system-images/android-${{ inputs.api-level }}/${{ inputs.target }}/${{ inputs.abi }}" 2>/dev/null | sed 's/^/SYSIMG-SIZE /' || true
+        IMGDIR="$SDK/system-images/android-${{ inputs.api-level }}/${{ inputs.target }}/${{ inputs.abi }}"
+        du -sh "$IMGDIR" 2>/dev/null | sed 's/^/SYSIMG-SIZE /' || true
+        # Key the cache on the revision that actually ended up on disk.
+        REV="$(sed -n 's/^Pkg\.Revision=//p' "$IMGDIR/source.properties" 2>/dev/null | head -1 | tr -d ' \r')"
+        [ -n "$REV" ] || REV=unknown
+        echo "SYSIMG-REVISION $REV"
+        echo "revision=$REV" >> "$GITHUB_OUTPUT"
         # Mimic the emulator-runner action's invocation (--abi and --device
         # skip the auto-select/prompt path where bare creates silently no-op).
         echo no | "$SDK/cmdline-tools/latest/bin/avdmanager" create avd --force -n "$AVD" \
@@ -153,6 +160,15 @@ runs:
             >> "$HOME/.android/avd/$AVD.avd/config.ini"
         echo "===== AVD root ini ====="; cat "$HOME/.android/avd/$AVD.ini"
 
+    # Only writes when the restored entry is not already this revision, so a
+    # steady state costs nothing and a new revision is picked up once.
+    - name: Save system image to cache
+      if: steps.sysimg-restore.outputs.cache-matched-key != format('sysimg-v1-android-{0}-{1}-{2}-rev{3}', inputs.api-level, inputs.target, inputs.abi, steps.sysimg.outputs.revision)
+      uses: actions/cache/save@v4
+      with:
+        path: ${{ steps.sdk.outputs.path }}/system-images/android-${{ inputs.api-level }}/${{ inputs.target }}/${{ inputs.abi }}
+        key: sysimg-v1-android-${{ inputs.api-level }}-${{ inputs.target }}-${{ inputs.abi }}-rev${{ steps.sysimg.outputs.revision }}
+
     - name: Install preview emulator
       shell: bash
       run: |