reset: warn when refresh_index() takes more than 2 seconds

refresh_index() is done after a reset command as an optimization.  Because
it can be an expensive call, warn the user if it takes more than 2 seconds
and tell them how to avoid it using the --quiet command line option or
reset.quiet config setting.

Signed-off-by: Ben Peart <benpeart@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -333,6 +333,10 @@ advice.*::
 	commitBeforeMerge::
 		Advice shown when linkgit:git-merge[1] refuses to
 		merge to avoid overwriting local changes.
+	resetQuiet::
+		Advice to consider using the `--quiet` option to linkgit:git-reset[1]
+		when the command takes more than 2 seconds to enumerate unstaged
+		changes after reset.
 	resolveConflict::
 		Advice shown by various commands when conflicts
 		prevent the operation from being performed.

advice.c

@@ -12,6 +12,7 @@ int advice_push_needs_force = 1;
 int advice_status_hints = 1;
 int advice_status_u_option = 1;
 int advice_commit_before_merge = 1;
+int advice_reset_quiet_warning = 1;
 int advice_resolve_conflict = 1;
 int advice_implicit_identity = 1;
 int advice_detached_head = 1;
@@ -65,6 +66,7 @@ static struct {
 	{ "statusHints", &advice_status_hints },
 	{ "statusUoption", &advice_status_u_option },
 	{ "commitBeforeMerge", &advice_commit_before_merge },
+	{ "resetQuiet", &advice_reset_quiet_warning },
 	{ "resolveConflict", &advice_resolve_conflict },
 	{ "implicitIdentity", &advice_implicit_identity },
 	{ "detachedHead", &advice_detached_head },

advice.h

@@ -12,6 +12,7 @@ extern int advice_push_needs_force;
 extern int advice_status_hints;
 extern int advice_status_u_option;
 extern int advice_commit_before_merge;
+extern int advice_reset_quiet_warning;
 extern int advice_resolve_conflict;
 extern int advice_implicit_identity;
 extern int advice_detached_head;

builtin/reset.c

@@ -25,6 +25,8 @@
 #include "submodule.h"
 #include "submodule-config.h"
 
+#define REFRESH_INDEX_DELAY_WARNING_IN_MS (2 * 1000)
+
 static const char * const git_reset_usage[] = {
 	N_("git reset [--mixed | --soft | --hard | --merge | --keep] [-q] [<commit>]"),
 	N_("git reset [-q] [<tree-ish>] [--] <paths>..."),
@@ -377,9 +379,19 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 			int flags = quiet ? REFRESH_QUIET : REFRESH_IN_PORCELAIN;
 			if (read_from_tree(&pathspec, &oid, intent_to_add))
 				return 1;
-			if (!quiet && get_git_work_tree())
+			if (!quiet && get_git_work_tree()) {
+				uint64_t t_begin, t_delta_in_ms;
+
+				t_begin = getnanotime();
 				refresh_index(&the_index, flags, NULL, NULL,
 					      _("Unstaged changes after reset:"));
+				t_delta_in_ms = (getnanotime() - t_begin) / 1000000;
+				if (advice_reset_quiet_warning && t_delta_in_ms > REFRESH_INDEX_DELAY_WARNING_IN_MS) {
+					printf(_("\nIt took %.2f seconds to enumerate unstaged changes after reset.  You can\n"
+						"use '--quiet' to avoid this.  Set the config setting reset.quiet to true\n"
+						"to make this the default.\n"), t_delta_in_ms / 1000.0);
+				}
+			}
 		} else {
 			int err = reset_index(&oid, reset_type, quiet);
 			if (reset_type == KEEP && !err)