[PATCH v2] var: add GIT_DEFAULT_BRANCH variable

["Thomas Weißschuh" <thomas@t-8ch.de>]

Introduce the builtin variable GIT_DEFAULT_BRANCH which represents the
the default branch name that will be used by git-init.

Currently this variable is equivalent to
    git config init.defaultbranch || 'master'

This however will break if at one point the default branch is changed as
indicated by `default_branch_name_advice` in `refs.c`.

By providing this command ahead of time users of git can make their
code forward-compatible.

Co-developed-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Thomas Weißschuh <thomas@t-8ch.de>
---

Changes from v1 ( https://lore.kernel.org/git/20211030140112.834650-1-thomas@t-8ch.de/ ):
* Replaced the custom subcommand with an internal variable
* Cleaned up the tests

@Johannes: I replaced BUG() with die() from your example because that seems to be
nicer for user facing messages.

 Documentation/git-var.txt |  3 +++
 builtin/var.c             | 13 +++++++++++++
 t/t0007-git-var.sh        | 19 +++++++++++++++++++
 3 files changed, 35 insertions(+)

diff --git a/Documentation/git-var.txt b/Documentation/git-var.txt
index 6072f936ab..387cc1b914 100644
--- a/Documentation/git-var.txt
+++ b/Documentation/git-var.txt
@@ -59,6 +59,9 @@ ifdef::git-default-pager[]
     The build you are using chose '{git-default-pager}' as the default.
 endif::git-default-pager[]
 
+GIT_DEFAULT_BRANCH::
+    The name of the first branch created in newly initialized repositories.
+
 SEE ALSO
 --------
 linkgit:git-commit-tree[1]
diff --git a/builtin/var.c b/builtin/var.c
index 6c6f46b4ae..d1d82b6c93 100644
--- a/builtin/var.c
+++ b/builtin/var.c
@@ -5,6 +5,7 @@
  */
 #include "builtin.h"
 #include "config.h"
+#include "refs.h"
 
 static const char var_usage[] = "git var (-l | <variable>)";
 
@@ -27,6 +28,17 @@ static const char *pager(int flag)
 	return pgm;
 }
 
+static const char *default_branch(int flag)
+{
+	const char *name = repo_default_branch_name(the_repository, 1);
+
+	if (!name)
+		die("could not determine the default branch name");
+
+	return name;
+}
+
+
 struct git_var {
 	const char *name;
 	const char *(*read)(int);
@@ -36,6 +48,7 @@ static struct git_var git_vars[] = {
 	{ "GIT_AUTHOR_IDENT",   git_author_info },
 	{ "GIT_EDITOR", editor },
 	{ "GIT_PAGER", pager },
+	{ "GIT_DEFAULT_BRANCH", default_branch },
 	{ "", NULL },
 };
 
diff --git a/t/t0007-git-var.sh b/t/t0007-git-var.sh
index 53af92d571..6b6852e35e 100755
--- a/t/t0007-git-var.sh
+++ b/t/t0007-git-var.sh
@@ -27,6 +27,25 @@ test_expect_success !FAIL_PREREQS,!AUTOIDENT 'requested identities are strict' '
 	)
 '
 
+test_expect_success 'get GIT_DEFAULT_BRANCH without configuration' '
+	(
+		sane_unset GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME &&
+		echo master >expect &&
+		git var GIT_DEFAULT_BRANCH >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'get GIT_DEFAULT_BRANCH with configuration' '
+	test_config init.defaultbranch foo &&
+	(
+		sane_unset GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME &&
+		echo foo >expect &&
+		git var GIT_DEFAULT_BRANCH >actual &&
+		test_cmp expect actual
+	)
+'
+
 # For git var -l, we check only a representative variable;
 # testing the whole output would make our test too brittle with
 # respect to unrelated changes in the test suite's environment.

base-commit: 0cddd84c9f3e9c3d793ec93034ef679335f35e49
-- 
2.33.1