[PATCH v2] sequencer: do not export GIT_DIR and GIT_WORK_TREE for 'exec'

["Elijah Newren via GitGitGadget" <gitgitgadget@gmail.com>]

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.

An explanation of 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 had 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 became 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
    
    Changes since v1:
    
     * Fix wording in multiple locations pointed out by Johannes
       Altmanninger
    
    Note that Johaness Altmanninger also suggested some additional
    GIT_DIR/GIT_WORK_TREE and --git-dir/--work-tree testcases, but since
    those change the current working directory to the work tree so that
    GIT_WORK_TREE ends up being '.' in every case, I think it's straying a
    bit from the point of the test. If other feel strongly, I can add them
    in.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1134%2Fnewren%2Ffix-rebase-exec-environ-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1134/newren/fix-rebase-exec-environ-v2
Pull-Request: https://github.com/git/git/pull/1134

Range-diff vs v1:

 1:  12713c83705 ! 1:  c647c45375a sequencer: fix environment that 'exec' commands run under
     @@ Metadata
      Author: Elijah Newren <newren@gmail.com>
      
       ## Commit message ##
     -    sequencer: fix environment that 'exec' commands run under
     +    sequencer: do not export GIT_DIR and GIT_WORK_TREE for 'exec'
      
          Commands executed from `git rebase --exec` can give different behavior
          from within that environment than they would outside of it, due to the
     @@ Commit message
          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:
     +    An explanation of 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
     @@ Commit message
            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
     +      replacing shell with C code, we had 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
     @@ Commit message
            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
     +    git-rebase became 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.
      


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

diff --git a/sequencer.c b/sequencer.c
index ea96837cde3..9afdbe3e3d1 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -3495,17 +3495,12 @@ static int error_failed_squash(struct repository *r,
 
 static int do_exec(struct repository *r, const char *command_line)
 {
-	struct strvec child_env = STRVEC_INIT;
 	const char *child_argv[] = { NULL, NULL };
 	int dirty, status;
 
 	fprintf(stderr, _("Executing: %s\n"), command_line);
 	child_argv[0] = command_line;
-	strvec_pushf(&child_env, "GIT_DIR=%s", absolute_path(get_git_dir()));
-	strvec_pushf(&child_env, "GIT_WORK_TREE=%s",
-		     absolute_path(get_git_work_tree()));
-	status = run_command_v_opt_cd_env(child_argv, RUN_USING_SHELL, NULL,
-					  child_env.v);
+	status = run_command_v_opt(child_argv, RUN_USING_SHELL);
 
 	/* force re-reading of the cache */
 	if (discard_index(r->index) < 0 || repo_read_index(r) < 0)
@@ -3535,8 +3530,6 @@ static int do_exec(struct repository *r, const char *command_line)
 		status = 1;
 	}
 
-	strvec_clear(&child_env);
-
 	return status;
 }
 
diff --git a/t/t3409-rebase-environ.sh b/t/t3409-rebase-environ.sh
new file mode 100755
index 00000000000..83ffb39d9ff
--- /dev/null
+++ b/t/t3409-rebase-environ.sh
@@ -0,0 +1,23 @@
+#!/bin/sh
+
+test_description='git rebase interactive environment'
+
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	test_commit one &&
+	test_commit two &&
+	test_commit three
+'
+
+test_expect_success 'rebase --exec does not muck with GIT_DIR' '
+	git rebase --exec "printf %s \$GIT_DIR >environ" HEAD~1 &&
+	test_must_be_empty environ
+'
+
+test_expect_success 'rebase --exec does not muck with GIT_WORK_TREE' '
+	git rebase --exec "printf %s \$GIT_WORK_TREE >environ" HEAD~1 &&
+	test_must_be_empty environ
+'
+
+test_done

base-commit: 88d915a634b449147855041d44875322de2b286d
-- 
gitgitgadget