Retry release sync after concurrent branch updates

.github/scripts/test-merge-main-into-release.sh

@@ -7,6 +7,7 @@ unset GITHUB_STEP_SUMMARY
 
 SCRIPT_DIR="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd)"
 HELPER="$SCRIPT_DIR/merge-main-into-release.sh"
+WORKFLOW="$SCRIPT_DIR/../workflows/update-release-branch.yml"
 TEST_ROOT="$(mktemp -d "${TMPDIR:-/tmp}/release-sync-tests.XXXXXX")"
 trap 'cd "$SCRIPT_DIR"; rm -rf -- "$TEST_ROOT"' EXIT
 
@@ -15,6 +16,15 @@ fail() {
   exit 1
 }
 
+awk '
+  /^      - name: Merge main into release branch$/ { step = 1 }
+  step && /^        run: \|$/ { body = 1; next }
+  body && /^      - / { exit }
+  body { sub(/^          /, ""); print }
+' "$WORKFLOW" > "$TEST_ROOT/workflow.sh"
+[[ -s "$TEST_ROOT/workflow.sh" ]] || fail "Release sync workflow step was not found."
+bash -n "$TEST_ROOT/workflow.sh"
+
 write_file() {
   mkdir -p -- "$(dirname -- "$1")"
   printf '%s\n' "$2" > "$1"
@@ -208,3 +218,91 @@ run_helper
 assert_parents
 [[ "$(cat shared.txt)" == reconciled && -f next.txt ]] || fail "Next sync revisited repaired content."
 echo "PASS: real repair avoids repeating squash-era conflicts"
+
+new_workflow_repo() {
+  new_repo "$1"
+  git switch -q main
+  mkdir -p .github/scripts
+  cp "$HELPER" .github/scripts/merge-main-into-release.sh
+  commit_all helper
+  git switch -q release/test
+  write_file release.txt release
+  commit_all release
+  git switch -q main
+  write_file main.txt main
+  commit_all main
+  main_sha="$(git rev-parse HEAD)"
+  git clone -q --bare . "$TEST_ROOT/$1-remote.git"
+  git remote add origin "$TEST_ROOT/$1-remote.git"
+  git clone -q "$TEST_ROOT/$1-remote.git" "$TEST_ROOT/$1-concurrent"
+  concurrent_repo="$TEST_ROOT/$1-concurrent"
+  git -C "$concurrent_repo" config user.name "Release sync test"
+  git -C "$concurrent_repo" config user.email "release-sync@example.invalid"
+  git -C "$concurrent_repo" config commit.gpgsign false
+  mkdir "$TEST_ROOT/$1-runner"
+  runner_temp="$TEST_ROOT/$1-runner"
+}
+
+run_workflow() (
+  export PUSH_MODE="$1" CONCURRENT_REPO="$concurrent_repo"
+  export PUSH_COUNT="$runner_temp/push-count"
+  export RUNNER_TEMP="$runner_temp" BRANCH=release/test
+  printf '0\n' > "$PUSH_COUNT"
+
+  # Simulate another writer immediately before the workflow's real push.
+  git() {
+    if [[ "$1" == push && "$2" == origin ]]; then
+      local attempt
+      attempt="$(cat "$PUSH_COUNT")"
+      attempt=$((attempt + 1))
+      printf '%s\n' "$attempt" > "$PUSH_COUNT"
+      if [[ "$PUSH_MODE" == reject ]]; then
+        echo "Simulated push permission failure." >&2
+        return 1
+      fi
+      if [[ "$PUSH_MODE" == exhaust || "$attempt" -eq 1 ]]; then
+        command git -C "$CONCURRENT_REPO" checkout -q -B release/test origin/release/test
+        printf 'concurrent\n' > "$CONCURRENT_REPO/concurrent-$attempt.txt"
+        command git -C "$CONCURRENT_REPO" add -A
+        command git -C "$CONCURRENT_REPO" commit -qm concurrent-release
+        command git -C "$CONCURRENT_REPO" push -q origin release/test
+        if [[ "$attempt" -eq 1 ]]; then
+          command git -C "$CONCURRENT_REPO" checkout -q main
+          printf 'advanced\n' > "$CONCURRENT_REPO/advanced-main.txt"
+          command git -C "$CONCURRENT_REPO" add -A
+          command git -C "$CONCURRENT_REPO" commit -qm advanced-main
+          command git -C "$CONCURRENT_REPO" push -q origin main
+        fi
+      fi
+    fi
+    command git "$@"
+  }
+  export -f git
+  bash "$TEST_ROOT/workflow.sh" > "$TEST_ROOT/output" 2>&1
+)
+
+new_workflow_repo workflow-race
+run_workflow race || { cat "$TEST_ROOT/output"; fail "Workflow did not recover from a push race."; }
+[[ "$(cat "$runner_temp/push-count")" == 2 ]] || fail "Push race did not use exactly two attempts."
+[[ "$(git rev-parse origin/release/test)" == "$(git rev-parse HEAD)" ]] || fail "Retry merge was not pushed."
+[[ "$(git rev-parse HEAD^2)" == "$main_sha" ]] || fail "Retry changed the pinned main commit."
+[[ -f concurrent-1.txt && -f main.txt && -f release.txt ]] || fail "Retry lost concurrent or merged files."
+[[ ! -e advanced-main.txt ]] || fail "Retry merged an unpinned main commit."
+echo "PASS: workflow retries races, preserves concurrent work, and keeps main pinned"
+
+new_workflow_repo workflow-rejection
+if run_workflow reject; then
+  fail "Workflow ignored a non-race push failure."
+fi
+[[ "$(cat "$runner_temp/push-count")" == 1 ]] || fail "Workflow retried a non-race push failure."
+grep -q 'not retrying' "$TEST_ROOT/output" || fail "Missing non-race failure diagnostic."
+echo "PASS: workflow fails immediately when a rejected push leaves the release tip unchanged"
+
+new_workflow_repo workflow-exhaustion
+if run_workflow exhaust; then
+  fail "Workflow ignored exhausted push races."
+fi
+[[ "$(cat "$runner_temp/push-count")" == 3 ]] || fail "Workflow did not stop after three push attempts."
+grep -q 'Unable to push release sync after 3 attempts' "$TEST_ROOT/output" || fail "Missing retry exhaustion diagnostic."
+[[ "$(grep -c '::warning::.*retrying' "$TEST_ROOT/output")" == 2 ]] || fail "Workflow announced a retry after the last attempt."
+echo "PASS: workflow fails after three push races without announcing another retry"

.github/workflows/update-release-branch.yml

@@ -84,8 +84,30 @@ jobs:
           git fetch origin main
           MAIN_SHA="$(git rev-parse origin/main)"
 
-          # Checkout is on release; always run the helper from the pinned main
-          # commit, including when release has not received the helper yet.
+          # Always run the helper from the pinned main commit, including when
+          # release has not received the helper yet.
           git show "$MAIN_SHA:.github/scripts/merge-main-into-release.sh" > "$RUNNER_TEMP/merge-main-into-release.sh"
-          bash "$RUNNER_TEMP/merge-main-into-release.sh" "$MAIN_SHA" "$BRANCH"
-          git push origin "HEAD:refs/heads/$BRANCH"
+
+          for attempt in {1..3}; do
+            git fetch origin "$BRANCH"
+            RELEASE_SHA="$(git rev-parse "origin/$BRANCH")"
+            git checkout -B "$BRANCH" "origin/$BRANCH"
+
+            bash "$RUNNER_TEMP/merge-main-into-release.sh" "$MAIN_SHA" "$BRANCH"
+            if git push origin "HEAD:refs/heads/$BRANCH"; then
+              exit 0
+            fi
+
+            git fetch origin "$BRANCH"
+            if [[ "$(git rev-parse "origin/$BRANCH")" == "$RELEASE_SHA" ]]; then
+              echo "::error::Release sync push failed without a change to the remote release branch; not retrying."
+              exit 1
+            fi
+
+            if [[ "$attempt" -lt 3 ]]; then
+              echo "::warning::Release branch changed while syncing; retrying ($attempt/3)."
+            fi
+          done
+
+          echo "::error::Unable to push release sync after 3 attempts."
+          exit 1