config: keep bailing on unreadable global files

The behaviour for `git config list` is:
  A. Without `--global`, it should not bail on unreadable/non-existent
     global config files.

  B. With `--global`, it should bail when both `$HOME/.gitconfig` and
     `$XDG_CONFIG_HOME/git/config` are unreadable. It should not bail
     when one or more of them is readable.

The previous patch introduced a regression in scenario B: running
`git config list --global` would not fail when both global config files
are unreadable. For example,
`GIT_CONFIG_GLOBAL=does-not-exist git config list --global` would exit
with status code 0.

Assuming that `config_source->scope == CONFIG_SCOPE_GLOBAL` iff the
`--global` argument is specified, use this to determine whether to bail.
When reading only the global scope and both global config files are
unreadable, then adjust the return code to be non-zero.

Note: The logic to determine the exit code does not actually sum the
return codes of the underlying operations. Instead, it uses a single
decrement operation. If this is undesirable, we can change it to sum
the return codes of the underlying operations instead.

Signed-off-by: Delilah Ashley Wu <delilahwu@microsoft.com>

Author: delilahw

config.c

@@ -2014,11 +2014,13 @@ int git_config_system(void)
 }
 
 static int do_git_config_sequence(const struct config_options *opts,
-				  const struct repository *repo,
-				  config_fn_t fn, void *data)
+				  const struct repository *repo, config_fn_t fn,
+				  void *data, enum config_scope scope)
 {
 	int ret = 0;
 	char *system_config = git_system_config();
+	int global_config_success_count = 0;
+	int nonzero_ret_on_global_config_error = scope == CONFIG_SCOPE_GLOBAL;
 	char *xdg_config = NULL;
 	char *user_config = NULL;
 	char *repo_config;
@@ -2050,13 +2052,29 @@ static int do_git_config_sequence(const struct config_options *opts,
 	if (!opts->ignore_global) {
 		git_global_config_paths(&user_config, &xdg_config);
 
-		if (xdg_config && !access_or_die(xdg_config, R_OK, ACCESS_EACCES_OK))
-			ret += git_config_from_file_with_options(fn, xdg_config, data,
-						CONFIG_SCOPE_GLOBAL, NULL);
+		if (xdg_config &&
+		    !access_or_die(xdg_config, R_OK, ACCESS_EACCES_OK)) {
+			ret += git_config_from_file_with_options(fn, xdg_config,
+								 data,
+								 CONFIG_SCOPE_GLOBAL,
+								 NULL);
+			if (nonzero_ret_on_global_config_error && !ret)
+				++global_config_success_count;
+		}
+
+		if (user_config &&
+		    !access_or_die(user_config, R_OK, ACCESS_EACCES_OK)) {
+			ret += git_config_from_file_with_options(fn, user_config,
+								 data,
+								 CONFIG_SCOPE_GLOBAL,
+								 NULL);
+			if (nonzero_ret_on_global_config_error && !ret)
+				++global_config_success_count;
+		}
 
-		if (user_config && !access_or_die(user_config, R_OK, ACCESS_EACCES_OK))
-			ret += git_config_from_file_with_options(fn, user_config, data,
-						CONFIG_SCOPE_GLOBAL, NULL);
+		if (nonzero_ret_on_global_config_error &&
+		    !global_config_success_count)
+			--ret;
 
 		free(xdg_config);
 		free(user_config);
@@ -2117,7 +2135,10 @@ int config_with_options(config_fn_t fn, void *data,
 		ret = git_config_from_blob_ref(fn, repo, config_source->blob,
 					       data, config_source->scope);
 	} else {
-		ret = do_git_config_sequence(opts, repo, fn, data);
+		ret = do_git_config_sequence(opts, repo, fn, data,
+					     config_source ?
+						     config_source->scope :
+						     CONFIG_SCOPE_UNKNOWN);
 	}
 
 	if (inc.remote_urls) {