Re: [PATCH/RFC 3/4] config: read global scope via config_sequence

"Delilah Ashley Wu via GitGitGadget" [off-list ref]
writes:

From: Delilah Ashley Wu <redacted>

The output of `git config list --global` should include both the home
(`$HOME/.gitconfig`) and XDG (`$XDG_CONFIG_HOME/git/config`) configs,
but it only reads from the former.

", but" -> "to match the information given by the command without
--global, but".

This patch introduces a regression. If both global config files are
unreadable, then `git config list --global` should exit non-zero. This
is no longer the case, so mark the corresponding test as a "TODO known
breakage" and address the issue in the next patch, config: keep bailing
on unreadable global files.

That is rather unfortunate, as we do try hard to avoid deliberate
regressions in our history.  The reason why this step cannot be done
without first introducing a regression is...?

If the reason is "it would make a single patch too big", perhaps we
can do it in two steps, one preliminary "git_config_sequence() learns
an extra barf-if-no-input parameter that causes it to return error if
no files in the specified sequence exists" step, followed by this
change that starts using git_config_sequence() to handle "--global",
which uses that new flag to ensure that there won't be a regression?

 	if (opts->use_global_config) {
+		/*
+		 * Since global config is sourced from more than one location,
+		 * use `config.c#do_git_config_sequence()` with `opts->options`
+		 * to read it. However, writing global config should point to a
+		 * single destination, set in `opts->source.file`.
+		 */
+		opts->options.ignore_repo = 1;
+		opts->options.ignore_cmdline= 1;
+		opts->options.ignore_worktree = 1;
+		opts->options.ignore_system = 1;
+		opts->source.scope = CONFIG_SCOPE_GLOBAL;

Very nicely done.

Thanks.