[PATCH 1/9] init: allow overriding the default branch name for new repositories

["Don Goodman-Wilson via GitGitGadget" <gitgitgadget@gmail.com>]

From: Don Goodman-Wilson <don@goodman-wilson.com>

There is a growing number of projects trying to avoid the non-inclusive
name `master` in their repositories. For existing repositories, this
requires manual work. For new repositories, the only way to do that
automatically is by copying all of Git's template directory, then
hard-coding the desired default branch name into the `.git/HEAD` file,
and then configuring `init.templateDir` to point to those copied
template files.

To make this process much less cumbersome, let's introduce support for
`core.defaultBranchName`. That way, users won't need to keep their
copied template files up to date, and won't interfere with default hooks
installed by their administrators.

While at it, also let users set the default branch name via the
environment variable `GIT_TEST_DEFAULT_BRANCH_NAME`, in preparation for
adjusting Git's test suite to a more inclusive default branch name. As
is common in Git, the `GIT_TEST_*` variable takes precedence over the
config setting.

Note: we use the prefix `core.` instead of `init.` because we want to
adjust also `git clone`, `git fmt-merge-msg` and other commands over the
course of the next commits to respect this setting.

Helped-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Helped-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Don Goodman-Wilson <don@goodman-wilson.com>
---
 builtin/init-db.c |  8 +++++---
 refs.c            | 34 ++++++++++++++++++++++++++++++++++
 refs.h            |  6 ++++++
 t/t0001-init.sh   | 20 ++++++++++++++++++++
 4 files changed, 65 insertions(+), 3 deletions(-)

diff --git a/builtin/init-db.c b/builtin/init-db.c
index 0b7222e7188..99792adfd43 100644
--- a/builtin/init-db.c
+++ b/builtin/init-db.c
@@ -258,15 +258,17 @@ static int create_default_files(const char *template_path,
 		die("failed to set up refs db: %s", err.buf);
 
 	/*
-	 * Create the default symlink from ".git/HEAD" to the "master"
-	 * branch, if it does not exist yet.
+	 * Create the default symlink from ".git/HEAD" to the default
+	 * branch name, if it does not exist yet.
 	 */
 	path = git_path_buf(&buf, "HEAD");
 	reinit = (!access(path, R_OK)
 		  || readlink(path, junk, sizeof(junk)-1) != -1);
 	if (!reinit) {
-		if (create_symref("HEAD", "refs/heads/master", NULL) < 0)
+		char *default_ref = git_default_branch_name(0);
+		if (create_symref("HEAD", default_ref, NULL) < 0)
 			exit(1);
+		free(default_ref);
 	}
 
 	initialize_repository_version(fmt->hash_algo);
diff --git a/refs.c b/refs.c
index 224ff66c7bb..8499b3865cb 100644
--- a/refs.c
+++ b/refs.c
@@ -560,6 +560,40 @@ void expand_ref_prefix(struct argv_array *prefixes, const char *prefix)
 		argv_array_pushf(prefixes, *p, len, prefix);
 }
 
+char *git_default_branch_name(int short_name)
+{
+	const char *branch_name = getenv("GIT_TEST_DEFAULT_BRANCH_NAME");
+	char *from_config = NULL, *prefixed;
+
+	/*
+	 * If the default branch name was not specified via the environment
+	 * variable GIT_TEST_DEFAULT_BRANCH_NAME, retrieve it from the config
+	 * setting core.defaultBranchName. If neither are set, fall back to the
+	 * hard-coded default.
+	 */
+	if (!branch_name || !*branch_name) {
+		if (git_config_get_string("core.defaultbranchname",
+					  &from_config) < 0)
+			die(_("Could not retrieve `core.defaultBranchName`"));
+
+		if (from_config)
+			branch_name = from_config;
+		else
+			branch_name = "master";
+	}
+
+	if (short_name)
+		return from_config ? from_config : xstrdup(branch_name);
+
+	/* prepend "refs/heads/" to the branch name */
+	prefixed = xstrfmt("refs/heads/%s", branch_name);
+	if (check_refname_format(prefixed, 0))
+		die(_("invalid default branch name: '%s'"), branch_name);
+
+	free(from_config);
+	return prefixed;
+}
+
 /*
  * *string and *len will only be substituted, and *string returned (for
  * later free()ing) if the string passed in is a magic short-hand form
diff --git a/refs.h b/refs.h
index a92d2c74c83..e8d4f6e2f13 100644
--- a/refs.h
+++ b/refs.h
@@ -154,6 +154,12 @@ int repo_dwim_log(struct repository *r, const char *str, int len, struct object_
 int dwim_ref(const char *str, int len, struct object_id *oid, char **ref);
 int dwim_log(const char *str, int len, struct object_id *oid, char **ref);
 
+/*
+ * Retrieves the name of the default branch. If `short_name` is non-zero, the
+ * branch name will be prefixed with "refs/heads/".
+ */
+char *git_default_branch_name(int short_name);
+
 /*
  * A ref_transaction represents a collection of reference updates that
  * should succeed or fail together.
diff --git a/t/t0001-init.sh b/t/t0001-init.sh
index 1edd5aeb8f0..b144cd8f46b 100755
--- a/t/t0001-init.sh
+++ b/t/t0001-init.sh
@@ -464,4 +464,24 @@ test_expect_success MINGW 'redirect std handles' '
 	grep "Needed a single revision" output.txt
 '
 
+test_expect_success 'custom default branch name from config' '
+	git config --global core.defaultbranchname nmb &&
+	GIT_TEST_DEFAULT_BRANCH_NAME= git init custom-config &&
+	git config --global --unset core.defaultbranchname &&
+	git -C custom-config symbolic-ref HEAD >actual &&
+	grep nmb actual
+'
+
+test_expect_success 'custom default branch name from env' '
+	GIT_TEST_DEFAULT_BRANCH_NAME=nmb git init custom-env &&
+	git -C custom-env symbolic-ref HEAD >actual &&
+	grep nmb actual
+'
+
+test_expect_success 'invalid custom default branch name' '
+	test_must_fail env GIT_TEST_DEFAULT_BRANCH_NAME="with space" \
+		git init custom-invalid 2>err &&
+	test_i18ngrep "invalid default branch name" err
+'
+
 test_done
-- 
gitgitgadget