[PATCH 2/3] status: warn when a/b calculation takes too long

["Jeff Hostetler via GitGitGadget" <gitgitgadget@gmail.com>]

From: Jeff Hostetler <jeffhost@microsoft.com>

The ahead/behind calculation in 'git status' can be slow in some
cases. Users may not realize that there are ways to avoid this
computation, especially if they are not using the information.

Add a warning that appears if this calculation takes more than
two seconds. The warning can be disabled through the new config
setting advice.statusAheadBehind.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
 Documentation/config/advice.txt |  6 ++++++
 advice.c                        |  2 ++
 advice.h                        |  1 +
 wt-status.c                     | 17 +++++++++++++++++
 4 files changed, 26 insertions(+)

diff --git a/Documentation/config/advice.txt b/Documentation/config/advice.txt
index ec4f6ae658..4bc5b4c742 100644
--- a/Documentation/config/advice.txt
+++ b/Documentation/config/advice.txt
@@ -37,6 +37,12 @@ advice.*::
 		we can still suggest that the user push to either
 		refs/heads/* or refs/tags/* based on the type of the
 		source object.
+	statusAheadBehind::
+		Shown when linkgit:git-status[1] computes the ahead/behind
+		counts for a local ref compared to its remote tracking ref,
+		and that calculation takes longer than expected. Will not
+		appear if `status.aheadBehind` is false or the option
+		`--no-ahead-behind` is given.
 	statusHints::
 		Show directions on how to proceed from the current
 		state in the output of linkgit:git-status[1], in
diff --git a/advice.c b/advice.c
index ce5f374ecd..54f8dea30c 100644
--- a/advice.c
+++ b/advice.c
@@ -12,6 +12,7 @@ int advice_push_needs_force = 1;
 int advice_push_unqualified_ref_name = 1;
 int advice_status_hints = 1;
 int advice_status_u_option = 1;
+int advice_status_ahead_behind_warning = 1;
 int advice_commit_before_merge = 1;
 int advice_reset_quiet_warning = 1;
 int advice_resolve_conflict = 1;
@@ -68,6 +69,7 @@ static struct {
 	{ "pushUnqualifiedRefName", &advice_push_unqualified_ref_name },
 	{ "statusHints", &advice_status_hints },
 	{ "statusUoption", &advice_status_u_option },
+	{ "statusAheadBehindWarning", &advice_status_ahead_behind_warning },
 	{ "commitBeforeMerge", &advice_commit_before_merge },
 	{ "resetQuiet", &advice_reset_quiet_warning },
 	{ "resolveConflict", &advice_resolve_conflict },
diff --git a/advice.h b/advice.h
index e50f02cdfe..c86de9b9b8 100644
--- a/advice.h
+++ b/advice.h
@@ -12,6 +12,7 @@ extern int advice_push_needs_force;
 extern int advice_push_unqualified_ref_name;
 extern int advice_status_hints;
 extern int advice_status_u_option;
+extern int advice_status_ahead_behind_warning;
 extern int advice_commit_before_merge;
 extern int advice_reset_quiet_warning;
 extern int advice_resolve_conflict;
diff --git a/wt-status.c b/wt-status.c
index d2a1bec226..c94d43879a 100644
--- a/wt-status.c
+++ b/wt-status.c
@@ -19,6 +19,8 @@
 #include "lockfile.h"
 #include "sequencer.h"
 
+#define AB_DELAY_WARNING_IN_MS (2 * 1000)
+
 static const char cut_line[] =
 "------------------------ >8 ------------------------\n";
 
@@ -1085,14 +1087,29 @@ static void wt_longstatus_print_tracking(struct wt_status *s)
 	struct branch *branch;
 	char comment_line_string[3];
 	int i;
+	uint64_t t_begin = 0;
 
 	assert(s->branch && !s->is_initial);
 	if (!skip_prefix(s->branch, "refs/heads/", &branch_name))
 		return;
 	branch = branch_get(branch_name);
+
+	t_begin = getnanotime();
+
 	if (!format_tracking_info(branch, &sb, s->ahead_behind_flags))
 		return;
 
+	if (advice_status_ahead_behind_warning &&
+	    s->ahead_behind_flags == AHEAD_BEHIND_FULL) {
+		uint64_t t_delta_in_ms = (getnanotime() - t_begin) / 1000000;
+		if (t_delta_in_ms > AB_DELAY_WARNING_IN_MS) {
+			strbuf_addf(&sb, _("\n"
+					   "It took %.2f seconds to compute the branch ahead/behind values.\n"
+					   "You can use '--no-ahead-behind' to avoid this.\n"),
+				    t_delta_in_ms / 1000.0);
+		}
+	}
+
 	i = 0;
 	if (s->display_comment_prefix) {
 		comment_line_string[i++] = comment_line_char;
-- 
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