[PATCH] sequencer: fix environment that 'exec' commands run under

From: Elijah Newren <newren@gmail.com>

Commands executed from `git rebase --exec` can give different behavior from within that environment than they would outside of it, due to the fact that sequencer.c exports both GIT_DIR and GIT_WORK_TREE. For example, if the relevant script calls something like

  git -C ../otherdir log --format=%H --no-walk

the user may be surprised to find that the command above does not show a commit hash from ../otherdir, because $GIT_DIR prevents automatic gitdir detection and makes the -C option useless.

This is a regression in behavior from the original legacy implemented-in-shell rebase. It is perhaps rare for it to cause problems in practice, especially since most small problems that were caused by this area of bugs has been fixed-up in the past in a way that masked the particular bug observed without fixing the real underlying problem.

How we arrived at the current situation is perhaps merited. The setting of GIT_DIR and GIT_WORK_TREE done by sequencer.c arose from a sequence of historical accidents:

* When rebase was implemented as a shell command, it would call
  git-sh-setup, which among other things would set GIT_DIR -- but not
  export it.  This meant that when rebase --exec commands were run via
      /bin/sh -c "$COMMAND"
  they would not inherit the GIT_DIR setting.  The fact that GIT_DIR
  was not set in the run $COMMAND is the behavior we'd like to restore.

* When the rebase--helper builtin was introduced to allow incrementally
  replacing shell with C code, we were in an implementation that was
  half shell, half C.  In particular, commit 18633e1a22 ("rebase -i: use
  the rebase--helper builtin", 2017-02-09) added calls to
      exec git rebase--helper ...
  which caused rebase--helper to inherit the GIT_DIR environment
  variable from the shell.  git's setup would change the environment
  variable from an absolute path to a relative one (".git"), but would
  leave it set.  This meant that when rebase --exec commands were run
  via
      run_command_v_opt(...)
  they would inherit the GIT_DIR setting.

* In commit 09d7b6c6fa ("sequencer: pass absolute GIT_DIR to exec
  commands", 2017-10-31), it was noted that the GIT_DIR caused problems
  with some commands; e.g.
      git rebase --exec 'cd subdir && git describe' ...
  would have GIT_DIR=.git which was invalid due to the change to the
  subdirectory.  Instead of questioning why GIT_DIR was set, that commit
  instead made sequencer change GIT_DIR to be an absolute path and
  explicitly export it via
      argv_array_pushf(&child_env, "GIT_DIR=%s", absolute_path(get_git_dir()));
      run_command_v_opt_cd_env(..., child_env.argv)

* In commit ab5e67d751 ("sequencer: pass absolute GIT_WORK_TREE to exec
  commands", 2018-07-14), it was noted that when GIT_DIR is set but
  GIT_WORK_TREE is not, that we do not discover GIT_WORK_TREE but just
  assume it is '.'.  That is incorrect if trying to run commands from a
  subdirectory.  However, rather than question why GIT_DIR was set, that
  commit instead also added GIT_WORK_TREE to the list of things to
  export.

Each of the above problems would have been fixed automatically when git-rebase become a full builtin, had it not been for the fact that sequencer.c started exporting GIT_DIR and GIT_WORK_TREE in the interim. Stop exporting them now.

Signed-off-by: Elijah Newren <newren@gmail.com>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
    sequencer: fix environment that 'exec' commands run under
    
    I'm not sure if the added regression tests make sense, or if t3409 is
    the best place to put them (taking over a recently removed t3409 that
    was used for the deprecated preserve merges option).

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1134%2Fnewren%2Ffix-rebase-exec-environ-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1134/newren/fix-rebase-exec-environ-v1
Pull-Request: https://github.com/git/git/pull/1134

 sequencer.c               |  9 +--------
 t/t3409-rebase-environ.sh | 23 +++++++++++++++++++++++
 2 files changed, 24 insertions(+), 8 deletions(-)
 create mode 100755 t/t3409-rebase-environ.sh