[PATCH v4 4/4] maintenance: use Windows scheduled tasks

["Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>]

From: Derrick Stolee <dstolee@microsoft.com>

Git's background maintenance uses cron by default, but this is not
available on Windows. Instead, integrate with Task Scheduler.

Tasks can be scheduled using the 'schtasks' command. There are several
command-line options that can allow for some advanced scheduling, but
unfortunately these seem to all require authenticating using a password.

Instead, use the "/xml" option to pass an XML file that contains the
configuration for the necessary schedule. These XML files are based on
some that I exported after constructing a schedule in the Task Scheduler
GUI. These options only run background maintenance when the user is
logged in, and more fields are populated with the current username and
SID at run-time by 'schtasks'.

Since the GIT_TEST_MAINT_SCHEDULER environment variable allows us to
specify 'schtasks' as the scheduler, we can test the Windows-specific
logic on a macOS platform. Thus, add a check that the XML file written
by Git is valid when xmllint exists on the system.

There is a deficiency in the current design. Windows has two kinds of
applications: GUI applications that start by "winmain()" and console
applications that start by "main()". Console applications are attached
to a new Console window if they are not already associated with a GUI
application. This means that every hour the scheudled task launches a
command window for the scheduled tasks. Not only is this visually
obtrusive, but it also takes focus from whatever else the user is
doing!

A simple fix would be to insert a GUI application that acts as a shim
between the scheduled task and Git. This is currently possible in Git
for Windows by setting the <Command> tag equal to

  C:\Program Files\Git\git-bash.exe

with options "--hide --no-needs-console --command=cmd\git.exe"
followed by the arguments currently used. Since git-bash.exe is not
included in Windows builds of core Git, I chose to leave out this
feature. My plan is to submit a small patch to Git for Windows that
converts the use of git.exe with this use of git-bash.exe in the
short term. In the long term, we can consider creating this GUI
shim application within core Git, perhaps in contrib/.

Co-authored-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
 Documentation/git-maintenance.txt |  22 ++++
 builtin/gc.c                      | 165 ++++++++++++++++++++++++++++++
 t/t7900-maintenance.sh            |  40 ++++++++
 3 files changed, 227 insertions(+)

diff --git a/Documentation/git-maintenance.txt b/Documentation/git-maintenance.txt
index f2d59f2bcc..e1adfff6db 100644
--- a/Documentation/git-maintenance.txt
+++ b/Documentation/git-maintenance.txt
@@ -313,6 +313,28 @@ To create more advanced customizations to your background tasks, see
 launchctl.plist(5) for more information.
 
 
+BACKGROUND MAINTENANCE ON WINDOWS SYSTEMS
+-----------------------------------------
+
+Windows does not support `cron` and instead has its own system for
+scheduling background tasks. The `git maintenance start` command uses
+the `schtasks` command to submit tasks to this system. You can inspect
+all background tasks using the Task Scheduler application. The tasks
+added by Git have names of the form `Git Maintenance (<frequency>)`.
+The Task Scheduler GUI has ways to inspect these tasks, but you can also
+export the tasks to XML files and view the details there.
+
+Note that since Git is a console application, these background tasks
+create a console window visible to the current user. This can be changed
+manually by selecting the "Run whether user is logged in or not" option
+in Task Scheduler. This change requires a password input, which is why
+`git maintenance start` does not select it by default.
+
+If you want to customize the background tasks, please rename the tasks
+so future calls to `git maintenance (start|stop)` do not overwrite your
+custom tasks.
+
+
 GIT
 ---
 Part of the linkgit:git[1] suite
diff --git a/builtin/gc.c b/builtin/gc.c
index 782769f243..f6c42f96c1 100644
--- a/builtin/gc.c
+++ b/builtin/gc.c
@@ -1671,6 +1671,167 @@ static int launchctl_update_schedule(int run_maintenance, int fd, const char *cm
 		return launchctl_remove_plists(cmd);
 }
 
+static char *schtasks_task_name(const char *frequency)
+{
+	struct strbuf label = STRBUF_INIT;
+	strbuf_addf(&label, "Git Maintenance (%s)", frequency);
+	return strbuf_detach(&label, NULL);
+}
+
+static int schtasks_remove_task(enum schedule_priority schedule, const char *cmd)
+{
+	int result;
+	struct strvec args = STRVEC_INIT;
+	const char *frequency = get_frequency(schedule);
+	char *name = schtasks_task_name(frequency);
+
+	strvec_split(&args, cmd);
+	strvec_pushl(&args, "/delete", "/tn", name, "/f", NULL);
+
+	result = run_command_v_opt(args.v, 0);
+
+	strvec_clear(&args);
+	free(name);
+	return result;
+}
+
+static int schtasks_remove_tasks(const char *cmd)
+{
+	return schtasks_remove_task(SCHEDULE_HOURLY, cmd) ||
+		schtasks_remove_task(SCHEDULE_DAILY, cmd) ||
+		schtasks_remove_task(SCHEDULE_WEEKLY, cmd);
+}
+
+static int schtasks_schedule_task(const char *exec_path, enum schedule_priority schedule, const char *cmd)
+{
+	int result;
+	struct child_process child = CHILD_PROCESS_INIT;
+	const char *xml;
+	char *xmlpath;
+	struct tempfile *tfile;
+	const char *frequency = get_frequency(schedule);
+	char *name = schtasks_task_name(frequency);
+
+	xmlpath =  xstrfmt("%s/schedule-%s.xml",
+			   the_repository->objects->odb->path,
+			   frequency);
+	tfile = create_tempfile(xmlpath);
+	if (!tfile || !fdopen_tempfile(tfile, "w"))
+		die(_("failed to create '%s'"), xmlpath);
+
+	xml = "<?xml version=\"1.0\" encoding=\"US-ASCII\"?>\n"
+	      "<Task version=\"1.4\" xmlns=\"http://schemas.microsoft.com/windows/2004/02/mit/task\">\n"
+	      "<Triggers>\n"
+	      "<CalendarTrigger>\n";
+	fputs(xml, tfile->fp);
+
+	switch (schedule) {
+	case SCHEDULE_HOURLY:
+		fprintf(tfile->fp,
+			"<StartBoundary>2020-01-01T01:00:00</StartBoundary>\n"
+			"<Enabled>true</Enabled>\n"
+			"<ScheduleByDay>\n"
+			"<DaysInterval>1</DaysInterval>\n"
+			"</ScheduleByDay>\n"
+			"<Repetition>\n"
+			"<Interval>PT1H</Interval>\n"
+			"<Duration>PT23H</Duration>\n"
+			"<StopAtDurationEnd>false</StopAtDurationEnd>\n"
+			"</Repetition>\n");
+		break;
+
+	case SCHEDULE_DAILY:
+		fprintf(tfile->fp,
+			"<StartBoundary>2020-01-01T00:00:00</StartBoundary>\n"
+			"<Enabled>true</Enabled>\n"
+			"<ScheduleByWeek>\n"
+			"<DaysOfWeek>\n"
+			"<Monday />\n"
+			"<Tuesday />\n"
+			"<Wednesday />\n"
+			"<Thursday />\n"
+			"<Friday />\n"
+			"<Saturday />\n"
+			"</DaysOfWeek>\n"
+			"<WeeksInterval>1</WeeksInterval>\n"
+			"</ScheduleByWeek>\n");
+		break;
+
+	case SCHEDULE_WEEKLY:
+		fprintf(tfile->fp,
+			"<StartBoundary>2020-01-01T00:00:00</StartBoundary>\n"
+			"<Enabled>true</Enabled>\n"
+			"<ScheduleByWeek>\n"
+			"<DaysOfWeek>\n"
+			"<Sunday />\n"
+			"</DaysOfWeek>\n"
+			"<WeeksInterval>1</WeeksInterval>\n"
+			"</ScheduleByWeek>\n");
+		break;
+
+	default:
+		break;
+	}
+
+	xml = "</CalendarTrigger>\n"
+	      "</Triggers>\n"
+	      "<Principals>\n"
+	      "<Principal id=\"Author\">\n"
+	      "<LogonType>InteractiveToken</LogonType>\n"
+	      "<RunLevel>LeastPrivilege</RunLevel>\n"
+	      "</Principal>\n"
+	      "</Principals>\n"
+	      "<Settings>\n"
+	      "<MultipleInstancesPolicy>IgnoreNew</MultipleInstancesPolicy>\n"
+	      "<Enabled>true</Enabled>\n"
+	      "<Hidden>true</Hidden>\n"
+	      "<UseUnifiedSchedulingEngine>true</UseUnifiedSchedulingEngine>\n"
+	      "<WakeToRun>false</WakeToRun>\n"
+	      "<ExecutionTimeLimit>PT72H</ExecutionTimeLimit>\n"
+	      "<Priority>7</Priority>\n"
+	      "</Settings>\n"
+	      "<Actions Context=\"Author\">\n"
+	      "<Exec>\n"
+	      "<Command>\"%s\\git.exe\"</Command>\n"
+	      "<Arguments>--exec-path=\"%s\" for-each-repo --config=maintenance.repo maintenance run --schedule=%s</Arguments>\n"
+	      "</Exec>\n"
+	      "</Actions>\n"
+	      "</Task>\n";
+	fprintf(tfile->fp, xml, exec_path, exec_path, frequency);
+	strvec_split(&child.args, cmd);
+	strvec_pushl(&child.args, "/create", "/tn", name, "/f", "/xml", xmlpath, NULL);
+	close_tempfile_gently(tfile);
+
+	child.no_stdout = 1;
+	child.no_stderr = 1;
+
+	if (start_command(&child))
+		die(_("failed to start schtasks"));
+	result = finish_command(&child);
+
+	delete_tempfile(&tfile);
+	free(xmlpath);
+	free(name);
+	return result;
+}
+
+static int schtasks_schedule_tasks(const char *cmd)
+{
+	const char *exec_path = git_exec_path();
+
+	return schtasks_schedule_task(exec_path, SCHEDULE_HOURLY, cmd) ||
+		schtasks_schedule_task(exec_path, SCHEDULE_DAILY, cmd) ||
+		schtasks_schedule_task(exec_path, SCHEDULE_WEEKLY, cmd);
+}
+
+static int schtasks_update_schedule(int run_maintenance, int fd, const char *cmd)
+{
+	if (run_maintenance)
+		return schtasks_schedule_tasks(cmd);
+	else
+		return schtasks_remove_tasks(cmd);
+}
+
 #define BEGIN_LINE "# BEGIN GIT MAINTENANCE SCHEDULE"
 #define END_LINE "# END GIT MAINTENANCE SCHEDULE"
 
@@ -1761,6 +1922,8 @@ static int crontab_update_schedule(int run_maintenance, int fd, const char *cmd)
 
 #if defined(__APPLE__)
 static const char platform_scheduler[] = "launchctl";
+#elif defined(GIT_WINDOWS_NATIVE)
+static const char platform_scheduler[] = "schtasks";
 #else
 static const char platform_scheduler[] = "crontab";
 #endif
@@ -1789,6 +1952,8 @@ static int update_background_schedule(int enable)
 
 	if (!strcmp(scheduler, "launchctl"))
 		result = launchctl_update_schedule(enable, lk.tempfile->fd, cmd);
+	else if (!strcmp(scheduler, "schtasks"))
+		result = schtasks_update_schedule(enable, lk.tempfile->fd, cmd);
 	else if (!strcmp(scheduler, "crontab"))
 		result = crontab_update_schedule(enable, lk.tempfile->fd, cmd);
 	else
diff --git a/t/t7900-maintenance.sh b/t/t7900-maintenance.sh
index 6d37312901..a26ff22541 100755
--- a/t/t7900-maintenance.sh
+++ b/t/t7900-maintenance.sh
@@ -453,6 +453,46 @@ test_expect_success !MINGW 'start and stop macOS maintenance' '
 	test_line_count = 0 actual
 '
 
+test_expect_success 'start and stop Windows maintenance' '
+	write_script print-args <<-\EOF &&
+	echo $* >>args
+	while test $# -gt 0
+	do
+		case "$1" in
+		/xml) shift; xmlfile=$1; break ;;
+		*) shift ;;
+		esac
+	done
+	test -z "$xmlfile" || cp "$xmlfile" .
+	EOF
+
+	rm -f args &&
+	GIT_TEST_MAINT_SCHEDULER="schtasks:./print-args" git maintenance start &&
+
+	# start registers the repo
+	git config --get --global maintenance.repo "$(pwd)" &&
+
+	printf "/create /tn Git Maintenance (%s) /f /xml .git/objects/schedule-%s.xml\n" \
+		hourly hourly daily daily weekly weekly >expect &&
+	test_cmp expect args &&
+
+	for frequency in hourly daily weekly
+	do
+		test_xmllint "schedule-$frequency.xml"
+	done &&
+
+	rm -f args &&
+	GIT_TEST_MAINT_SCHEDULER="schtasks:./print-args" git maintenance stop &&
+
+	# stop does not unregister the repo
+	git config --get --global maintenance.repo "$(pwd)" &&
+
+	rm expect &&
+	printf "/delete /tn Git Maintenance (%s) /f\n" \
+		hourly daily weekly >expect &&
+	test_cmp expect args
+'
+
 test_expect_success 'register preserves existing strategy' '
 	git config maintenance.strategy none &&
 	git maintenance register &&
-- 
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