[gnome-shell/gnome-42] ci: Try falling back to stable branch before HEAD

[Florian Müllner <fmuellner src gnome org>]

commit 246affa24cc024004d618fc9ff42cb409b556479
Author: Florian Müllner <fmuellner gnome org>
Date:   Fri May 6 16:45:20 2022 +0200

    ci: Try falling back to stable branch before HEAD
    
    For non-MR pipelines, we check for a matching branch or tag name
    in mutter. As we use the same naming policy for stable branches
    and branch at about the same time, this works generally fine for
    branches.
    
    However for tags, it is less unlikely that there is no matching
    tag in mutter, in particularly late in a stable cycle when most
    changes are translation updates (which are much rare in mutter
    than gnome-shell).
    
    Before falling back to main (which is doomed to fail in that case),
    try to guess the correct stable branch based on the tag name.
    
    Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2293>
    (cherry picked from commit 9e30afe678323cd26e06f91c5e4a5a4e691b7bb6)

 .gitlab-ci/checkout-mutter.sh | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)
---
diff --git a/.gitlab-ci/checkout-mutter.sh b/.gitlab-ci/checkout-mutter.sh
index 449a34e5d9..765e39dccd 100755
--- a/.gitlab-ci/checkout-mutter.sh
+++ b/.gitlab-ci/checkout-mutter.sh
@@ -47,9 +47,23 @@ if [ -z "$mutter_target" ]; then
     mutter_target=FETCH_HEAD
   else
     echo \ not found
-    mutter_target=HEAD
-    echo Using $mutter_target instead
   fi
 fi
 
+fallback_branch=${CI_COMMIT_TAG:+gnome-}${CI_COMMIT_TAG%%.*}
+if [ -z "$mutter_target" -a "$fallback_branch" ]; then
+  echo -n Looking for $fallback_branch instead ...
+  if fetch origin $fallback_branch; then
+    echo \ found
+    mutter_target=FETCH_HEAD
+  else
+    echo \ not found
+  fi
+fi
+
+if [ -z "$mutter_target" ]; then
+  mutter_target=HEAD
+  echo Using $mutter_target instead
+fi
+
 git checkout -q $mutter_target