git branchless crashes on empty commits

[samueltardieu]

Description of the bug

git branchless refuses to rewrite an empty commit and crashes:

$ git init; git branchless init --main-branch main
$ echo foo > foo
$ git add foo
$ git commit -am "First commit"
$ git commit --allow-empty -m "Second commit with no changes"
$ git reword HEAD^ -f -m "Fix message of first commit"  
The application panicked (crashed).
Message:  assertion failed: !dest_oids.is_empty()
Location: git-branchless-lib/src/core/rewrite/plan.rs:1023

  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ SPANTRACE ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

   0: git_branchless_reword::reword with effects=<Output fancy=true> revsets=[Revset("HEAD^")] resolve_revset_options=ResolveRevsetOptions { show_hidden_commits: false } messages=Messages(["Fix message of first commit"]) git_run_info=<GitRunInfo path_to_git="git" working_directory="/tmp/z" env=not shown> force_rewrite_public_commits=true
      at git-branchless-reword/src/lib.rs:62

Backtrace omitted. Run with RUST_BACKTRACE=1 environment variable to display it.
Run with RUST_BACKTRACE=full to include source snippets.

Expected behavior

No error should happen. git branchless should acknowledge the existence of empty commits, or at least accept to rewrite empty commits as empty commits when restacking.

This is a behaviour I encountered when working with patch series created by b4: b4 creates an empty commit at the beginning of the patch series to track the cover letter and the changelog of the patch series.

While b4 + git-branchless could be the perfect combination for working with email-based git workflows, the systematic rejection of empty commits by git-branchless (even when an empty commit is being rewritten) makes it difficult to use.

Actual behavior

A crash happened

Version of rustc

No response

Automated bug report

Software version

git-branchless 0.8.0

Operating system

Linux 6.1.74

Command-line

/home/sam/.nix-profile/bin/git-branchless bug-report 

Environment variables

SHELL=/run/current-system/sw/bin/zsh
EDITOR=vim

Git version

> git version 
git version 2.43.0

Hooks

Show 7 hooks

Hook post-applypatch

#!/bin/sh
## START BRANCHLESS CONFIG

git branchless hook post-applypatch "$@"

## END BRANCHLESS CONFIG

Hook post-checkout

#!/bin/sh
## START BRANCHLESS CONFIG

git branchless hook post-checkout "$@"

## END BRANCHLESS CONFIG

Hook post-commit

#!/bin/sh
## START BRANCHLESS CONFIG

git branchless hook post-commit "$@"

## END BRANCHLESS CONFIG

Hook post-merge

#!/bin/sh
## START BRANCHLESS CONFIG

git branchless hook post-merge "$@"

## END BRANCHLESS CONFIG

Hook post-rewrite

#!/bin/sh
## START BRANCHLESS CONFIG

git branchless hook post-rewrite "$@"

## END BRANCHLESS CONFIG

Hook pre-auto-gc

#!/bin/sh
## START BRANCHLESS CONFIG

git branchless hook pre-auto-gc "$@"

## END BRANCHLESS CONFIG

Hook reference-transaction

#!/bin/sh
## START BRANCHLESS CONFIG

# Avoid canceling the reference transaction in the case that `branchless` fails
# for whatever reason.
git branchless hook reference-transaction "$@" || (
echo 'branchless: Failed to process reference transaction!'
echo 'branchless: Some events (e.g. branch updates) may have been lost.'
echo 'branchless: This is a bug. Please report it.'
)

## END BRANCHLESS CONFIG

Events

Show 5 events

Event ID: 7, transaction ID: 5 (message: reword)

1. RefUpdateEvent { timestamp: 1706552567.2591848, event_tx_id: EventTransactionId(5), ref_name: ReferenceName("refs/heads/main"), old_oid: 9202ffd46a48d38c42c8843eb6f22cb3dcc9f677, new_oid: ad2fd8941a2bb37f80c44b2ba7a25949a9489c7f, message: None }
2. RewriteEvent { timestamp: 1706552567.2686825, event_tx_id: EventTransactionId(5), old_commit_oid: 9202ffd46a48d38c42c8843eb6f22cb3dcc9f677, new_commit_oid: ad2fd8941a2bb37f80c44b2ba7a25949a9489c7f }
3. WorkingCopySnapshot { timestamp: 1706552567.2729092, event_tx_id: EventTransactionId(5), head_oid: 9202ffd46a48d38c42c8843eb6f22cb3dcc9f677, commit_oid: NonZeroOid(8dcb0ad411c163cc1eaeef0b84e5faf34e404812), ref_name: None }
4. RefUpdateEvent { timestamp: 1706552567.2799516, event_tx_id: EventTransactionId(5), ref_name: ReferenceName("HEAD"), old_oid: 9202ffd46a48d38c42c8843eb6f22cb3dcc9f677, new_oid: ad2fd8941a2bb37f80c44b2ba7a25949a9489c7f, message: None }

:
@ ad2fd89 4m (> main) xxxxxx xxxxxx xxxx xx xxxxxxx

Event ID: 6, transaction ID: 4 (message: post-commit)

1. CommitEvent { timestamp: 1706552548.0, event_tx_id: EventTransactionId(4), commit_oid: NonZeroOid(9202ffd46a48d38c42c8843eb6f22cb3dcc9f677) }

:
@ ad2fd89 4m (> main) xxxxxx xxxxxx xxxx xx xxxxxxx

Event ID: 4, transaction ID: 3 (message: reference-transaction)

1. RefUpdateEvent { timestamp: 1706552548.8607051, event_tx_id: EventTransactionId(3), ref_name: ReferenceName("HEAD"), old_oid: e12c3c1de973ed76b458946ac976eb0c28d30698, new_oid: 9202ffd46a48d38c42c8843eb6f22cb3dcc9f677, message: None }
2. RefUpdateEvent { timestamp: 1706552548.8607051, event_tx_id: EventTransactionId(3), ref_name: ReferenceName("refs/heads/main"), old_oid: e12c3c1de973ed76b458946ac976eb0c28d30698, new_oid: 9202ffd46a48d38c42c8843eb6f22cb3dcc9f677, message: None }

:
@ ad2fd89 4m (> main) xxxxxx xxxxxx xxxx xx xxxxxxx

Event ID: 3, transaction ID: 2 (message: post-commit)

1. CommitEvent { timestamp: 1706552541.0, event_tx_id: EventTransactionId(2), commit_oid: NonZeroOid(e12c3c1de973ed76b458946ac976eb0c28d30698) }

:
@ ad2fd89 4m (> main) xxxxxx xxxxxx xxxx xx xxxxxxx

Event ID: 1, transaction ID: 1 (message: reference-transaction)

1. RefUpdateEvent { timestamp: 1706552541.0822334, event_tx_id: EventTransactionId(1), ref_name: ReferenceName("HEAD"), old_oid: 0000000000000000000000000000000000000000, new_oid: e12c3c1de973ed76b458946ac976eb0c28d30698, message: None }
2. RefUpdateEvent { timestamp: 1706552541.0822334, event_tx_id: EventTransactionId(1), ref_name: ReferenceName("refs/heads/main"), old_oid: 0000000000000000000000000000000000000000, new_oid: e12c3c1de973ed76b458946ac976eb0c28d30698, message: None }

:
@ ad2fd89 4m (> main) xxxxxx xxxxxx xxxx xx xxxxxxx

Version of git-branchless

No response

Version of git

No response