chore(ci): determine if it is a backport by listing branches

ci/which-channel.sh

@@ -13,36 +13,34 @@ set -euo pipefail
 # so hope `HEAD~` to find the previous commit on master branch.
 base_sha=$(git rev-parse "${BASE_SHA:-HEAD~1}")
 
-# Default to nightly channel
-CHANNEL="nightly"
-
 # Get symbolic names for the base_sha.
-# We only care about `rust-1.y.z` branches
-ref=$(git name-rev --name-only --refs="origin/rust-1.*" $base_sha)
+# Assumption: Cargo branches are always in the format of `rust-1.*.0`,
+#             otherwise `git name-rev` will return "undefined".
+ref=$(git name-rev --name-only --refs='origin/rust-1.*.0' $base_sha)
 
-# Remove the patch version `.0` and keep only `1.y`,
-# so that rustup can install the correct patched toolchain
-if [[ "$ref" =~ ^origin/rust-(.*).0$ ]]
-then
-    rustup update
-    rustup toolchain install stable beta
-    version="${BASH_REMATCH[1]}"
-    beta="$(rustc +beta -V)"
-    stable="$(rustc +stable -V)"
+# Get the latest `rust-1.*.0` branch from remote origin.
+# Assumption: The latest branch is always beta branch.
+beta=$(git branch --remotes --list 'origin/rust-1.*.0' | sort | tail -n1 | tr -d "[:space:]")
 
-    if [[ "$beta" == "rustc ${version}"* ]]
-    then
-        CHANNEL="beta"
-    fi
+master=$(git rev-parse origin/master)
 
-    if [[ "$stable" = "rustc ${version}"* ]]
+# Backport pull requests always target at a `rust-1.*.0` branch.
+if [[ "$ref" = "undefined" ]] || [[ "$base_sha" = "$master" ]]
+then
+    channel="nightly"
+else
+    if [[ "$ref" = "$beta" ]]
     then
-        CHANNEL="stable"
+        channel="beta"
+    else
+        channel="stable"
     fi
 fi
 
-echo "Base sha:     $base_sha"
-echo "Possible ref: $ref"
-echo "Channel:      $CHANNEL"
+echo "Base sha: $base_sha"
+echo "Git Ref:  $ref"
+echo "master:   $master"
+echo "beta:     $beta"
+echo "Channel:  $channel"
 
-echo "CHANNEL=$CHANNEL" >> "$GITHUB_OUTPUT"
+echo "CHANNEL=$channel" >> "$GITHUB_OUTPUT"