Fix: Let main always fast-forward to the generated release

tools/release/sync-release.sh

@@ -78,14 +78,28 @@ fi
 old_local=""
 old_local="$(git rev-parse -q --verify "refs/heads/$TARGET^{commit}" 2>/dev/null)" || old_local=""
 
-# No-op only if the existing target has exactly this tree AND this parent.
+# main may hold commits develop lacks (earlier release merges, hotfixes). Then
+# release also takes main as a second parent: the tree stays the filtered source
+# tree, and main can always fast-forward to release -- the PR never conflicts.
+main_commit=""
+for ref in refs/remotes/origin/main refs/heads/main; do
+  main_commit="$(git rev-parse -q --verify "$ref^{commit}" 2>/dev/null)" && break
+  main_commit=""
+done
+parents="$src_commit"
+if [ -n "$main_commit" ] && ! git merge-base --is-ancestor "$main_commit" "$src_commit"; then
+  parents="$src_commit $main_commit"
+fi
+
+# No-op only if the existing target has exactly this tree AND these parents.
 if [ -n "$old_local" ] \
    && [ "$(git rev-parse "$old_local^{tree}")" = "$new_tree" ] \
-   && [ "$(git rev-parse -q --verify "$old_local^" 2>/dev/null || true)" = "$src_commit" ]; then
+   && [ "$(git rev-list --parents -n 1 "$old_local" | cut -d' ' -f2-)" = "$parents" ]; then
   echo "no changes: $TARGET already regenerated from $short"
   new_commit="$old_local"
 else
-  new_commit="$(git commit-tree "$new_tree" -p "$src_commit" -m "Release: sync from $SOURCE $short")"
+  pargs=""; for p in $parents; do pargs="$pargs -p $p"; done
+  new_commit="$(git commit-tree "$new_tree" $pargs -m "Release: sync from $SOURCE $short")"
   git update-ref "refs/heads/$TARGET" "$new_commit"
   echo "$TARGET -> $(git rev-parse --short "$new_commit") (parent $short)"
 fi

tools/release/test-sync-release.sh

@@ -130,6 +130,31 @@ git update-ref refs/remotes/origin/release "$stale"
 bash "$SYNC" --push >"$SCRATCH/push3.log" 2>&1
 check "push overwrites stale remote release" test "$(git --git-dir="$SCRATCH/remote.git" rev-parse refs/heads/release)" = "$(git rev-parse release)"
 
+# main has diverged from develop (a commit main has and develop lacks, with a
+# conflicting edit): release must still fast-forward main, keep develop's tree.
+git branch -D release -q 2>/dev/null
+base="$(git rev-parse HEAD~1)"
+git checkout -q -b main "$base"
+echo "main-only edit" > LICENSE; git add LICENSE; git commit -q -m "main-only"
+main_tip="$(git rev-parse HEAD)"
+git checkout -q develop
+dev_tip="$(git rev-parse HEAD)"
+bash "$SYNC" >"$SCRATCH/main1.log" 2>&1
+check "diverged main: first parent is source" test "$(git rev-parse release^1)" = "$dev_tip"
+check "diverged main: second parent is main" test "$(git rev-parse -q --verify release^2)" = "$main_tip"
+check "diverged main: main fast-forwards to release" git merge-base --is-ancestor "$main_tip" release
+check "diverged main: tree is develop's, not main's" test "$(git show release:LICENSE)" = "$(git show develop:LICENSE)"
+bash "$SYNC" >"$SCRATCH/main2.log" 2>&1
+h_main="$(git rev-parse release)"
+bash "$SYNC" >"$SCRATCH/main3.log" 2>&1
+check "diverged main: repeat run is a no-op" test "$(git rev-parse release)" = "$h_main"
+# once main is an ancestor of develop, no second parent is added
+git branch -f main "$base"
+git branch -D release -q
+bash "$SYNC" >"$SCRATCH/main4.log" 2>&1
+check "main behind develop: single parent" bash -c '! git rev-parse -q --verify release^2 >/dev/null'
+git branch -D main -q
+
 # Refuses when target is checked out
 git checkout -q release 2>/dev/null
 bash "$SYNC" >/dev/null 2>&1; rc=$?