[PATCH 1/1] rebase: teach `--exec` about `GIT_REBASE_BRANCH`

[Kristoffer Haugsbakk <code@khaugsbakk.name>]

The command fed to `--exec` might need some contextual information from
the branch name. But there is no convenient access to the branch name
that we were on before starting the rebase; rebase operates in detached
HEAD mode so we cannot ask for it directly. This means that we need to
parse something like this from the first line of `git branch --list`:

    (no branch, rebasing <branch>)

This is a moderate amount of effort for something that git-rebase(1) can
store for us.

To that end, teach `--exec` about an env. variable which stores the
branch name for the rebase-in-progress, if applicable.

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
---
 Documentation/git-rebase.txt |  4 ++++
 builtin/rebase.c             | 15 ++++++++++++++-
 t/t3409-rebase-environ.sh    | 19 +++++++++++++++++++
 3 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/Documentation/git-rebase.txt b/Documentation/git-rebase.txt
index 06206521fc3..9b3d6ee8203 100644
--- a/Documentation/git-rebase.txt
+++ b/Documentation/git-rebase.txt
@@ -578,6 +578,10 @@ squash/fixup series.
 This uses the `--interactive` machinery internally, but it can be run
 without an explicit `--interactive`.
 +
+The command has access to the environment variable `GIT_REBASE_BRANCH`
+which stores the branch name that `HEAD` was pointing at when the rebase
+started, if applicable.
++
 See also INCOMPATIBLE OPTIONS below.
 
 --root::
diff --git a/builtin/rebase.c b/builtin/rebase.c
index 5b086f651a6..0202130c2d7 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -1044,6 +1044,17 @@ static int check_exec_cmd(const char *cmd)
 	return 0;
 }
 
+static void try_set_env_git_rebase_branch(void)
+{
+	const char *refname = resolve_ref_unsafe("HEAD", 0, NULL, NULL);
+	const char *shortname = NULL;
+
+	if (refname)
+		skip_prefix(refname, "refs/heads/", &shortname);
+	if (shortname)
+		xsetenv("GIT_REBASE_BRANCH", shortname, true);
+}
+
 int cmd_rebase(int argc, const char **argv, const char *prefix)
 {
 	struct rebase_options options = REBASE_OPTIONS_INIT;
@@ -1451,8 +1462,10 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 	if (gpg_sign)
 		options.gpg_sign_opt = xstrfmt("-S%s", gpg_sign);
 
-	if (options.exec.nr)
+	if (options.exec.nr) {
 		imply_merge(&options, "--exec");
+		try_set_env_git_rebase_branch();
+	}
 
 	if (options.type == REBASE_APPLY) {
 		if (ignore_whitespace)
diff --git a/t/t3409-rebase-environ.sh b/t/t3409-rebase-environ.sh
index acaf5558dbe..5b1d78a255a 100755
--- a/t/t3409-rebase-environ.sh
+++ b/t/t3409-rebase-environ.sh
@@ -21,4 +21,23 @@ test_expect_success 'rebase --exec does not muck with GIT_WORK_TREE' '
 	test_must_be_empty environ
 '
 
+test_expect_success 'rebase --exec cmd can access GIT_REBASE_BRANCH' '
+	write_script cmd <<-\EOF &&
+printf "%s\n" $GIT_REBASE_BRANCH >actual
+EOF
+	git branch --show-current >expect &&
+	git rebase --exec ./cmd HEAD~1 &&
+	test_cmp expect actual
+'
+
+test_expect_success 'rebase --exec cmd has no GIT_REBASE_BRANCH when on detached HEAD' '
+	test_when_finished git checkout - &&
+	git checkout --detach &&
+	write_script cmd <<-\EOF &&
+printf "%s" $GIT_REBASE_BRANCH >environ
+EOF
+	git rebase --exec ./cmd HEAD~1 &&
+	test_must_be_empty environ
+'
+
 test_done
-- 
2.44.0.64.g52b67adbeb2