add --chmod --dry-run should reject symlink index entries

[genedna]

Summary

libra add --chmod=+x --dry-run <symlink> exits 0 and silently treats the invalid chmod as a successful no-op. Git rejects applying an executable bit to a non-regular index entry, exits non-zero, and reports the affected path.

This is tracked by libra-testcases as TC-0008, based on upstream Git t/t3700-add.sh test 52 (git add --chmod --dry-run reports error for non regular files).

Versions

• Libra: 0.22.19
• Git control: 2.55.0.782.g1630431f32
• Platform: macOS/Darwin, case-insensitive APFS
• Upstream source: Git commit 1630431f326e15fcde608827b5ff38422528eb59, t/t3700-add.sh lines 508-513

Reproduction

set -eu

work=$(mktemp -d)
trap 'rm -rf "$work"' EXIT

"$LIBRA" init -q --initial-branch=main "$work/repo"
cd "$work/repo"

printf 'target\n' >foo
ln -s foo foo4
"$LIBRA" update-index --add foo4

"$LIBRA" ls-files --stage -- foo4
"$LIBRA" add --chmod=+x --dry-run foo4 >out 2>err
rc=$?
printf 'exit=%s\n' "$rc"
cat out
cat err
"$LIBRA" ls-files --stage -- foo4

The setup creates a symlink entry with index mode 120000.

Actual behavior

$ libra ls-files --stage -- foo4
120000 19102815663d23f8b75a47e7a01965dcdc96468c 0 foo4

$ libra add --chmod=+x --dry-run foo4
(dry run, no files were staged)
exit=0

stderr is empty. The index correctly remains unchanged at mode 120000, but the invalid operation is reported as successful.

Expected behavior

Git produces:

$ git add --chmod=+x --dry-run foo4
error: cannot chmod +x 'foo4'
exit=255

The index remains unchanged at mode 120000.

The exact numeric error code and prose do not need to be identical, but Libra should:

1. Exit with a regular non-zero status, not 0 or a timeout/signal status.
2. Identify foo4 and explain that +x/chmod cannot be applied to the non-regular entry.
3. Leave the index mode and object ID unchanged.
4. Continue to allow product-specific Error-Code, Hint, JSON, or other explanatory output.

Rationale

--dry-run suppresses index writes; it should not suppress validation. An index symlink uses mode 120000 and has no regular-file executable bit to change. Treating this as a successful no-op can make preflight checks accept an invalid operation.

Test tracking

• cases/tc0008-add-chmod-dry-run-nonregular/tc0008-add-chmod-dry-run-nonregular.sh
• libra-git-testcases-spec.md TC-0008
• results/libra-0.22.19/notes.md TC-0008
• libra-git-testcases-review.md TC-0008

[genedna]

Related issues

This issue is related to the following open issues (cross-linked):

• libra update-index: rejects the --index-info option/argument; rejects the --refresh option/argument; rejects the --skip-worktree option/argument #540: add --chmod symlink rejection relates to update-index --chmod

Linked to improve discoverability, triage, and tracking of shared root causes.