ref(actions): use a better logic for disk image selection

This supersedes #8936 using a different approach with `${VAR:-value}`

.github/workflows/scripts/gcp-get-cached-disks.sh

@@ -34,31 +34,29 @@ find_cached_disk_image() {
         echo "Found ${git_source} Disk: ${disk_name}" >&2
         disk_description=$(gcloud compute images describe "${disk_name}" --format="value(DESCRIPTION)")
         echo "Description: ${disk_description}" >&2
-        echo "${disk_name}"  # This is the actual return value when a disk is found
+        echo "${disk_name}" # This is the actual return value when a disk is found
     else
         echo "No ${git_source} disk found with '${disk_search_pattern}' pattern." >&2
     fi
 }
 
-# Check if both $DISK_PREFIX and $DISK_SUFFIX are set, as they are required to find a cached disk image
+# Check if both $DISK_PREFIX and $DISK_SUFFIX are set, as they are required to
+# find a cached disk image.
 if [[ -n "${DISK_PREFIX}" && -n "${DISK_SUFFIX}" ]]; then
     # Find the most suitable cached disk image
     echo "Finding a ${DISK_PREFIX}-${DISK_SUFFIX} disk image for ${NETWORK}..."
     CACHED_DISK_NAME=""
 
-    # Check if main branch images are preferred
+    # Try to find an image based on the `main` branch if that branch is preferred.
     if [[ "${PREFER_MAIN_CACHED_STATE}" == "true" ]]; then
         CACHED_DISK_NAME=$(find_cached_disk_image "main-[0-9a-f]+" "main branch")
-    # Else, try to find a cached disk image from the current branch (or PR)
-    else
-        CACHED_DISK_NAME=$(find_cached_disk_image ".+-${GITHUB_REF}" "branch")
-        # If no cached disk image is found, try to find one from any branch
-        if [[ -z "${CACHED_DISK_NAME}" ]]; then
-            CACHED_DISK_NAME=$(find_cached_disk_image ".+-[0-9a-f]+" "any branch")
-        fi
     fi
+    # If no image was found, try to find one from the current branch (or PR).
+    CACHED_DISK_NAME=${CACHED_DISK_NAME:-$(find_cached_disk_image ".+-${GITHUB_REF}" "branch")}
+    # If we still have no image, try to find one from any branch.
+    CACHED_DISK_NAME=${CACHED_DISK_NAME:-$(find_cached_disk_image ".+-[0-9a-f]+" "any branch")}
 
-    # Handle case where no suitable disk image is found
+    # Handle the case where no suitable disk image is found
     if [[ -z "${CACHED_DISK_NAME}" ]]; then
         echo "No suitable cached state disk available. Try running the cached state rebuild job."
         exit 1
@@ -84,10 +82,10 @@ find_available_disk_type() {
         echo "Found ${disk_type^^} disk: ${disk_name} for ${base_name^^} on network: ${NETWORK}" >&2
         disk_description=$(gcloud compute images describe "${disk_name}" --format="value(DESCRIPTION)")
         echo "Description: ${disk_description}" >&2
-        echo "true"  # This is the actual return value when a disk is found
+        echo "true" # This is the actual return value when a disk is found
     else
         echo "No ${disk_type^^} disk found for ${base_name^^} on network: ${NETWORK}" >&2
-        echo "false"  # This is the actual return value when no disk is found
+        echo "false" # This is the actual return value when no disk is found
     fi
 }
 if [[ -n "${NETWORK}" ]]; then