gc: remove gc.pid file at end of execution

This file isn't really harmful, but isn't useful either, and can create
minor annoyance for the user:

* It's confusing, as the presence of a *.pid file often implies that a
  process is currently running. A user running "ls .git/" and finding
  this file may incorrectly guess that a "git gc" is currently running.

* Leaving this file means that a "git gc" in an already gc-ed repo is
  no-longer a no-op. A user running "git gc" in a set of repositories,
  and then synchronizing this set (e.g. rsync -av, unison, ...) will see
  all the gc.pid files as changed, which creates useless noise.

This patch unlinks the file after the garbage collection is done, so that
gc.pid is actually present only during execution.

Future versions of Git may want to use the information left in the gc.pid
file (e.g. for policies like "don't attempt to run a gc if one has
already been ran less than X hours ago"). If so, this patch can safely be
reverted. For now, let's not bother the users.
Explained-by: NMatthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Improved-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/gc.c

@@ -14,6 +14,7 @@
 #include "cache.h"
 #include "parse-options.h"
 #include "run-command.h"
+#include "sigchain.h"
 #include "argv-array.h"
 
 #define FAILED_RUN "failed to run %s"
@@ -35,6 +36,21 @@ static struct argv_array repack = ARGV_ARRAY_INIT;
 static struct argv_array prune = ARGV_ARRAY_INIT;
 static struct argv_array rerere = ARGV_ARRAY_INIT;
 
+static char *pidfile;
+
+static void remove_pidfile(void)
+{
+	if (pidfile)
+		unlink(pidfile);
+}
+
+static void remove_pidfile_on_signal(int signo)
+{
+	remove_pidfile();
+	sigchain_pop(signo);
+	raise(signo);
+}
+
 static int gc_config(const char *var, const char *value, void *cb)
 {
 	if (!strcmp(var, "gc.packrefs")) {
@@ -179,6 +195,10 @@ static const char *lock_repo_for_gc(int force, pid_t* ret_pid)
 	FILE *fp;
 	int fd, should_exit;
 
+	if (pidfile)
+		/* already locked */
+		return NULL;
+
 	if (gethostname(my_host, sizeof(my_host)))
 		strcpy(my_host, "unknown");
 
@@ -219,6 +239,10 @@ static const char *lock_repo_for_gc(int force, pid_t* ret_pid)
 	strbuf_release(&sb);
 	commit_lock_file(&lock);
 
+	pidfile = git_pathdup("gc.pid");
+	sigchain_push_common(remove_pidfile_on_signal);
+	atexit(remove_pidfile);
+
 	return NULL;
 }
 

t/t6500-gc.sh

@@ -9,6 +9,11 @@ test_expect_success 'gc empty repository' '
 	git gc
 '
 
+test_expect_success 'gc does not leave behind pid file' '
+	git gc &&
+	test_path_is_missing .git/gc.pid
+'
+
 test_expect_success 'gc --gobbledegook' '
 	test_expect_code 129 git gc --nonsense 2>err &&
 	test_i18ngrep "[Uu]sage: git gc" err