Re: [PATCH] builtin: add git-default-branch command

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

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Hi Thomas,

On Sat, 30 Oct 2021, Thomas Weißschuh wrote:

> Introduce command `default-branch` which allows to retrieve the branch
> that will be used by git-init.
>
> Currently this command 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`.

I am very sympathetic to the motivation for your patch, I have had to
resort to an ugly hack in Git for Windows' script that generates the
installer: the script creates a throw-away repository _just_ to determine
said branch name.

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

It is probably overkill to introduce a whole new command for just this
single purpose.

But we do have prior art in Git how to display similar information: `git
var -l` will list e.g. `GIT_PAGER`, even if it is not configured
explicitly.

Something like this should be a good start along those lines:

-- snip --
diff --git a/builtin/var.c b/builtin/var.c
index 6c6f46b4aea..937c63939d9 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 +28,16 @@ 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)
+		BUG("could not determine the default branch name");
+
+	return name;
+}
+
 struct git_var {
 	const char *name;
 	const char *(*read)(int);
@@ -36,6 +47,7 @@ static struct git_var git_vars[] = {
 	{ "GIT_AUTHOR_IDENT",   git_author_info },
 	{ "GIT_EDITOR", editor },
 	{ "GIT_PAGER", pager },
+	{ "GIT_DEFAULT_BRANCH", default_branch },
 	{ "", NULL },
 };

-- snap --

Thanks,
Johannes