[PATCH v4 0/6] Avoid problem where git_dir is set after alias expansion

When expanding an alias in a subdirectory, we setup the git_dir (gently), read the config, and then restore the "env" (e.g. the current working directory) so that the command specified by the alias can run correctly.

What we failed to reset was the git_dir, meaning that in the most common case, it was now pointing to a .git/ directory *in the subdirectory*.

This problem was identified in the GVFS fork, where a pre-command hook was introduced to allow pre-fetching missing blobs.

An early quick fix in the GVFS fork simply built on top of the save_env_before_alias() hack, introducing another hack that saves the git_dir and restores it after an alias is expanded:

	https://github.com/Microsoft/git/commit/2d859ba3b

That is very hacky, though, and it is much better (although much more involved, too) to fix this "properly", i.e. by replacing the ugly save/restore logic by simply using the early config code path.

However, aliases are strange beasts.

When an alias refers to a single Git command (originally the sole intention of aliases), the current working directory is restored to what it had been before expanding the alias.

But when an alias starts with an exclamation point, i.e. referring to a command-line to be interpreted by the shell, the current working directory is no longer in the subdirectory but instead in the worktree's top-level directory.

This is even true for worktrees added by `git worktree add`.

But when we are inside the .git/ directory, the current working directory is *restored* to the subdirectory inside the .git/ directory.

In short, the rules (what is the expected current working directory after expanding an alias and before actually running it) are a bit complicated.

This patch series does *not* address the problem identified by Brandon Williams where the early config machinery fails to look into the *commondir* in worktrees added by `git worktree add`. My hope is that Brandon is okay with applying this patch series before his config patch series.

Changes since v3:

- avoided the strbuf in alias_lookup() by using skip_prefix() and !strcmp()
  instead.

- fixed tyop ("read" instead of "reading", causing Junio to stumble) in the
  commit message of 6/6.

Johannes Schindelin (6):
  discover_git_directory(): avoid setting invalid git_dir
  config: report correct line number upon error
  help: use early config when autocorrecting aliases
  t1308: relax the test verifying that empty alias values are disallowed
  t7006: demonstrate a problem with aliases in subdirectories
  Use the early config machinery to expand aliases

 alias.c                | 29 +++++++++++++++++++-------
 config.c               |  3 ++-
 git.c                  | 55 ++++----------------------------------------------
 help.c                 |  2 +-
 setup.c                |  1 +
 t/t1300-repo-config.sh |  6 ++++++
 t/t1308-config-set.sh  |  4 +++-
 t/t7006-pager.sh       | 11 ++++++++++
 8 files changed, 50 insertions(+), 61 deletions(-)

base-commit: 02a2850ad58eff6de70eb2dc5f96345c463857ac
Published-As: https://github.com/dscho/git/releases/tag/alias-early-config-v4
Fetch-It-Via: git fetch https://github.com/dscho/git alias-early-config-v4

Interdiff vs v3:
 diff --git a/alias.c b/alias.c
 index 6bdc9363037..5df052ae4c4 100644
 --- a/alias.c
 +++ b/alias.c
 @@ -2,15 +2,16 @@
  
  struct config_alias_data
  {
 -	struct strbuf key;
 +	const char *alias;
  	char *v;
  };
  
  static int config_alias_cb(const char *key, const char *value, void *d)
  {
  	struct config_alias_data *data = d;
 +	const char *p;
  
 -	if (!strcmp(key, data->key.buf))
 +	if (skip_prefix(key, "alias.", &p) && !strcmp(p, data->alias))
  		return git_config_string((const char **)&data->v, key, value);
  
  	return 0;
 @@ -18,12 +19,9 @@ static int config_alias_cb(const char *key, const char *value, void *d)
  
  char *alias_lookup(const char *alias)
  {
 -	struct config_alias_data data = { STRBUF_INIT, NULL };
 +	struct config_alias_data data = { alias, NULL };
  
 -	strbuf_addf(&data.key, "alias.%s", alias);
 -	if (git_config_key_is_valid(data.key.buf))
 -		read_early_config(config_alias_cb, &data);
 -	strbuf_release(&data.key);
 +	read_early_config(config_alias_cb, &data);
  
  	return data.v;
  }