add: warn when asked to update SKIP_WORKTREE entries

`git add` already refrains from updating SKIP_WORKTREE entries, but it
silently exits with zero code when it is asked to do so. Instead, let's
warn the user and display a hint on how to update these entries.

Note that we only warn the user whey they give a pathspec item that
matches no eligible path for updating, but it does match one or more
SKIP_WORKTREE entries. A warning was chosen over erroring out right away
to reproduce the same behavior `add` already exhibits with ignored
files. This also allow users to continue their workflow without having
to invoke `add` again with only the eligible paths (as those will have
already been added).

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 20 insertions, 0 deletions

diff --git a/advice.c b/advice.c
index 1647423..0b9c89c 100644
--- a/advice.c
+++ b/advice.c
@@ -2,6 +2,7 @@
 #include "config.h"
 #include "color.h"
 #include "help.h"
+#include "string-list.h"
 
 int advice_fetch_show_forced_updates = 1;
 int advice_push_update_rejected = 1;
@@ -136,6 +137,7 @@ static struct {
 	[ADVICE_STATUS_HINTS]				= { "statusHints", 1 },
 	[ADVICE_STATUS_U_OPTION]			= { "statusUoption", 1 },
 	[ADVICE_SUBMODULE_ALTERNATE_ERROR_STRATEGY_DIE] = { "submoduleAlternateErrorStrategyDie", 1 },
+	[ADVICE_UPDATE_SPARSE_PATH]			= { "updateSparsePath", 1 },
 	[ADVICE_WAITING_FOR_EDITOR]			= { "waitingForEditor", 1 },
 };
 
@@ -284,6 +286,24 @@ void NORETURN die_conclude_merge(void)
 	die(_("Exiting because of unfinished merge."));
 }
 
+void advise_on_updating_sparse_paths(struct string_list *pathspec_list)
+{
+	struct string_list_item *item;
+
+	if (!pathspec_list->nr)
+		return;
+
+	fprintf(stderr, _("The following pathspecs didn't match any"
+			  " eligible path, but they do match index\n"
+			  "entries outside the current sparse checkout:\n"));
+	for_each_string_list_item(item, pathspec_list)
+		fprintf(stderr, "%s\n", item->string);
+
+	advise_if_enabled(ADVICE_UPDATE_SPARSE_PATH,
+			  _("Disable or modify the sparsity rules if you intend"
+			    " to update such entries."));
+}
+
 void detach_advice(const char *new_name)
 {
 	const char *fmt =