status: modernize git-status "slow untracked files" advice

`git status` can be slow when there are a large number of
untracked files and directories since Git must search the entire
worktree to enumerate them.  When it is too slow, Git prints
advice with the elapsed search time and a suggestion to disable
the search using the `-uno` option.  This suggestion also carries
a warning that might scare off some users.

However, these days, `-uno` isn't the only option.  Git can reduce
the time taken to enumerate untracked files by caching results from
previous `git status` invocations, when the `core.untrackedCache`
and `core.fsmonitor` features are enabled.

Update the `git status` man page to explain these configuration
options, and update the advice to provide more detail about the
current configuration and to refer to the updated documentation.

Signed-off-by: Rudy Rigot <rudy.rigot@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

3 files changed, 153 insertions, 5 deletions

diff --git a/Documentation/git-status.txt b/Documentation/git-status.txt
index 5e438a7fdc..a051b1e8f3 100644
--- a/Documentation/git-status.txt
+++ b/Documentation/git-status.txt
@@ -457,6 +457,66 @@ during the write may conflict with other simultaneous processes, causing
 them to fail. Scripts running `status` in the background should consider
 using `git --no-optional-locks status` (see linkgit:git[1] for details).
 
+UNTRACKED FILES AND PERFORMANCE
+-------------------------------
+
+`git status` can be very slow in large worktrees if/when it
+needs to search for untracked files and directories. There are
+many configuration options available to speed this up by either
+avoiding the work or making use of cached results from previous
+Git commands. There is no single optimum set of settings right
+for everyone. We'll list a summary of the relevant options to help
+you, but before going into the list, you may want to run `git status`
+again, because your configuration may already be caching `git status`
+results, so it could be faster on subsequent runs.
+
+* The `--untracked-files=no` flag or the
+	`status.showUntrackedfiles=false` config (see above for both):
+	indicate that `git status` should not report untracked
+	files. This is the fastest option. `git status` will not list
+	the untracked files, so you need to be careful to remember if
+	you create any new files and manually `git add` them.
+
+* `advice.statusUoption=false` (see linkgit:git-config[1]):
+	setting this variable to `false` disables the warning message
+	given when enumerating untracked files takes more than 2
+	seconds.  In a large project, it may take longer and the user
+	may have already accepted the trade off (e.g. using "-uno" may
+	not be an acceptable option for the user), in which case, there
+	is no point issuing the warning message, and in such a case,
+	disabling the warning may be the best.
+
+* `core.untrackedCache=true` (see linkgit:git-update-index[1]):
+	enable the untracked cache feature and only search directories
+	that have been modified since the previous `git status` command.
+	Git remembers the set of untracked files within each directory
+	and assumes that if a directory has not been modified, then
+	the set of untracked files within has not changed.  This is much
+	faster than enumerating the contents of every directory, but still
+	not without cost, because Git still has to search for the set of
+	modified directories. The untracked cache is stored in the
+	`.git/index` file. The reduced cost of searching for untracked
+	files is offset slightly by the increased size of the index and
+	the cost of keeping it up-to-date. That reduced search time is
+	usually worth the additional size.
+
+* `core.untrackedCache=true` and `core.fsmonitor=true` or
+	`core.fsmonitor=<hook_command_pathname>` (see
+	linkgit:git-update-index[1]): enable both the untracked cache
+	and FSMonitor features and only search directories that have
+	been modified since the previous `git status` command.  This
+	is faster than using just the untracked cache alone because
+	Git can also avoid searching for modified directories.  Git
+	only has to enumerate the exact set of directories that have
+	changed recently. While the FSMonitor feature can be enabled
+	without the untracked cache, the benefits are greatly reduced
+	in that case.
+
+Note that after you turn on the untracked cache and/or FSMonitor
+features it may take a few `git status` commands for the various
+caches to warm up before you see improved command times.  This is
+normal.
+
 SEE ALSO
 --------
 linkgit:gitignore[5]
diff --git a/t/t7508-status.sh b/t/t7508-status.sh
index 2b7ef6c41a..aed07c5b62 100755
--- a/t/t7508-status.sh
+++ b/t/t7508-status.sh
@@ -1676,4 +1676,74 @@ test_expect_success 'racy timestamps will be fixed for dirty worktree' '
 	! test_is_magic_mtime .git/index
 '
 
+test_expect_success 'setup slow status advice' '
+	GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main git init slowstatus &&
+	(
+		cd slowstatus &&
+		cat >.gitignore <<-\EOF &&
+		/actual
+		/expected
+		/out
+		EOF
+		git add .gitignore &&
+		git commit -m "Add .gitignore" &&
+		git config advice.statusuoption true
+	)
+'
+
+test_expect_success 'slow status advice when core.untrackedCache and fsmonitor are unset' '
+	(
+		cd slowstatus &&
+		git config core.untrackedCache false &&
+		git config core.fsmonitor false &&
+		GIT_TEST_UF_DELAY_WARNING=1 git status >actual &&
+		cat >expected <<-\EOF &&
+		On branch main
+
+		It took 3.25 seconds to enumerate untracked files.
+		See '\''git help status'\'' for information on how to improve this.
+
+		nothing to commit, working tree clean
+		EOF
+		test_cmp expected actual
+	)
+'
+
+test_expect_success 'slow status advice when core.untrackedCache true, but not fsmonitor' '
+	(
+		cd slowstatus &&
+		git config core.untrackedCache true &&
+		git config core.fsmonitor false &&
+		GIT_TEST_UF_DELAY_WARNING=1 git status >actual &&
+		cat >expected <<-\EOF &&
+		On branch main
+
+		It took 3.25 seconds to enumerate untracked files.
+		See '\''git help status'\'' for information on how to improve this.
+
+		nothing to commit, working tree clean
+		EOF
+		test_cmp expected actual
+	)
+'
+
+test_expect_success 'slow status advice when core.untrackedCache true, and fsmonitor' '
+	(
+		cd slowstatus &&
+		git config core.untrackedCache true &&
+		git config core.fsmonitor true &&
+		GIT_TEST_UF_DELAY_WARNING=1 git status >actual &&
+		cat >expected <<-\EOF &&
+		On branch main
+
+		It took 3.25 seconds to enumerate untracked files,
+		but the results were cached, and subsequent runs may be faster.
+		See '\''git help status'\'' for information on how to improve this.
+
+		nothing to commit, working tree clean
+		EOF
+		test_cmp expected actual
+	)
+'
+
 test_done
diff --git a/wt-status.c b/wt-status.c
index 5813174896..b430d25da4 100644
--- a/wt-status.c
+++ b/wt-status.c
@@ -18,8 +18,10 @@
 #include "worktree.h"
 #include "lockfile.h"
 #include "sequencer.h"
+#include "fsmonitor-settings.h"
 
 #define AB_DELAY_WARNING_IN_MS (2 * 1000)
+#define UF_DELAY_WARNING_IN_MS (2 * 1000)
 
 static const char cut_line[] =
 "------------------------ >8 ------------------------\n";
@@ -1205,6 +1207,13 @@ static void wt_longstatus_print_tracking(struct wt_status *s)
 	strbuf_release(&sb);
 }
 
+static int uf_was_slow(struct wt_status *s)
+{
+	if (getenv("GIT_TEST_UF_DELAY_WARNING"))
+		s->untracked_in_ms = 3250;
+	return UF_DELAY_WARNING_IN_MS < s->untracked_in_ms;
+}
+
 static void show_merge_in_progress(struct wt_status *s,
 				   const char *color)
 {
@@ -1814,6 +1823,7 @@ static void wt_longstatus_print(struct wt_status *s)
 {
 	const char *branch_color = color(WT_STATUS_ONBRANCH, s);
 	const char *branch_status_color = color(WT_STATUS_HEADER, s);
+	enum fsmonitor_mode fsm_mode = fsm_settings__get_mode(s->repo);
 
 	if (s->branch) {
 		const char *on_what = _("On branch ");
@@ -1870,13 +1880,21 @@ static void wt_longstatus_print(struct wt_status *s)
 		wt_longstatus_print_other(s, &s->untracked, _("Untracked files"), "add");
 		if (s->show_ignored_mode)
 			wt_longstatus_print_other(s, &s->ignored, _("Ignored files"), "add -f");
-		if (advice_enabled(ADVICE_STATUS_U_OPTION) && 2000 < s->untracked_in_ms) {
+		if (advice_enabled(ADVICE_STATUS_U_OPTION) && uf_was_slow(s)) {
 			status_printf_ln(s, GIT_COLOR_NORMAL, "%s", "");
+			if (fsm_mode > FSMONITOR_MODE_DISABLED) {
+				status_printf_ln(s, GIT_COLOR_NORMAL,
+						_("It took %.2f seconds to enumerate untracked files,\n"
+						"but the results were cached, and subsequent runs may be faster."),
+						s->untracked_in_ms / 1000.0);
+			} else {
+				status_printf_ln(s, GIT_COLOR_NORMAL,
+						_("It took %.2f seconds to enumerate untracked files."),
+						s->untracked_in_ms / 1000.0);
+			}
 			status_printf_ln(s, GIT_COLOR_NORMAL,
-					 _("It took %.2f seconds to enumerate untracked files. 'status -uno'\n"
-					   "may speed it up, but you have to be careful not to forget to add\n"
-					   "new files yourself (see 'git help status')."),
-					 s->untracked_in_ms / 1000.0);
+					_("See 'git help status' for information on how to improve this."));
+			status_printf_ln(s, GIT_COLOR_NORMAL, "%s", "");
 		}
 	} else if (s->committable)
 		status_printf_ln(s, GIT_COLOR_NORMAL, _("Untracked files not listed%s"),