Standardize checkout-release-branch.sh across all product repos

The script had drifted into five different shapes across the repos: argument required via `${1?…}` or `${1:?…}`, argument plus a `BUILDKITE_BRANCH` fallback, argument with a hand-rolled usage check, the same under a different variable name, and — in one repo — no argument at all, reading `RELEASE_VERSION` from the environment. Having rolled the same one-line fix out thirteen times this week, the divergence is pure friction, so this settles on a single canonical version.

The resolution order is a superset of what every repo did before — argument, then `RELEASE_VERSION` from the environment, then the `release/*` branch the build runs on — so no call site needed changing. The `BUILDKITE_BRANCH` fallback only ever fires on a branch matching `^release/`, and it derives the branch name back from that same value, so it cannot select a branch other than the one the build was already triggered on.

It also closes two latent bugs. Under `bash -eu`, `[[ -z "${RELEASE_VERSION}" ]]` on an unset variable and a bare `RELEASE_VERSION=$1` with no arguments both abort with `unbound variable` before their intended usage message can print. And an argument that is passed but empty — which happens when a pipeline forwards an unset `$RELEASE_VERSION` — is now a hard error everywhere, rather than resolving to `release/` or silently falling through to the current branch.

The redundant `echo '--- :git: Checkout Release Branch'` in simplenote-android's pipelines is dropped, since the canonical script prints that group header itself.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: AliSoftware

.buildkite/commands/checkout-release-branch.sh

@@ -1,12 +1,35 @@
 #!/bin/bash -eu
 
-if [[ -z "${RELEASE_VERSION}" ]]; then
-    echo "RELEASE_VERSION is not set."
-    exit 1
+# Checks out the `release/*` branch for a given release version.
+#
+# Usage: checkout-release-branch.sh [<RELEASE_VERSION>]
+#
+# The release version is taken from, in order of precedence:
+#  1. the first argument, if one is passed
+#  2. the `RELEASE_VERSION` environment variable
+#  3. the `release/*` branch the build is running on, via `BUILDKITE_BRANCH`
+#
+# Buildkite, by default, checks out a specific commit, ending up in a detached HEAD state.
+# But some release steps need to be on the `release/*` branch instead, namely:
+#  - when a `release-pipelines/*.yml` needs to `git push` to the `release/*` branch (for version bumps)
+#  - when a job `pipeline upload`'d by such a pipeline builds from that branch, so that the build
+#    includes the commit that the version bump just added.
+
+echo "--- :git: Checkout Release Branch"
+
+if [[ $# -gt 0 ]]; then
+  # An argument was passed explicitly, so it must not be empty — that would mean the caller
+  # meant to forward a version but it resolved to nothing, which is a pipeline misconfiguration.
+  RELEASE_VERSION="${1:?release version argument was passed but is empty}"
+elif [[ -n "${RELEASE_VERSION:-}" ]]; then
+  : # Already provided through the pipeline environment
+elif [[ "${BUILDKITE_BRANCH:-}" =~ ^release/ ]]; then
+  RELEASE_VERSION="${BUILDKITE_BRANCH#release/}"
+else
+  echo "Error: no release version. Pass it as \$1, set RELEASE_VERSION, or run on a release/* branch." >&2
+  exit 1
 fi
 
-# Buildkite, by default, checks out a specific commit.
-# For many release actions, we need to be on a release branch instead.
 BRANCH_NAME="release/${RELEASE_VERSION}"
 git fetch origin "$BRANCH_NAME"
 git checkout "$BRANCH_NAME"

.buildkite/release-pipelines/complete-code-freeze.yml

@@ -14,7 +14,6 @@ steps:
       echo '--- :robot_face: Use bot for Git operations'
       source use-bot-for-git
 
-      echo '--- :git: Checkout release branch'
       .buildkite/commands/checkout-release-branch.sh
 
       echo '--- :ruby: Set up Ruby Tools'

.buildkite/release-pipelines/finalize-release.yml

@@ -8,7 +8,6 @@ steps:
       echo '--- :robot_face: Use bot for git operations'
       source use-bot-for-git
 
-      echo '--- :git: Checkout Release Branch'
       .buildkite/commands/checkout-release-branch.sh
 
       echo '--- :ruby: Set up Ruby Tools'

.buildkite/release-pipelines/new-beta-release.yml

@@ -8,7 +8,6 @@ steps:
       echo '--- :robot_face: Use bot for Git operations'
       source use-bot-for-git
 
-      echo '--- :git: Checkout release branch'
       .buildkite/commands/checkout-release-branch.sh
 
       echo '--- :ruby: Set up Ruby Tools'

.buildkite/release-pipelines/publish-release.yml

@@ -8,7 +8,6 @@ steps:
       echo '--- :robot_face: Use bot for git operations'
       source use-bot-for-git
 
-      echo '--- :git: Checkout Release Branch'
       .buildkite/commands/checkout-release-branch.sh
 
       echo '--- :ruby: Setup Ruby Tools'