add option to update cache in a background process

the cli flag is undocumented for now, since it's experimental.

Closes: https://codeberg.org/nsxiv/nsxiv/issues/416
Closes: https://codeberg.org/nsxiv/nsxiv/pulls/425
Co-authored-by: explosion-mental <explosion0mental@gmail.com>

main.c

@@ -936,6 +936,23 @@ int main(int argc, char *argv[])
 	filecnt = fileidx;
 	fileidx = options->startnum < filecnt ? options->startnum : 0;
 
+	if (options->background_cache && !options->private_mode) {
+		pid_t ppid = getpid(); /* to check if parent is still alive or not */
+		switch (fork()) {
+		case 0:
+			tns_init(&tns, files, &filecnt, &fileidx, NULL);
+			while (filecnt > 0 && getppid() == ppid) {
+				tns_load(&tns, filecnt - 1, false, true);
+				remove_file(filecnt - 1, true);
+			}
+			exit(0);
+			break;
+		case -1:
+			error(0, errno, "fork failed");
+			break;
+		}
+	}
+
 	win_init(&win);
 	img_init(&img, &win);
 	arl_init(&arl);

nsxiv.h

@@ -274,6 +274,7 @@ struct opt {
 	bool thumb_mode;
 	bool clean_cache;
 	bool private_mode;
+	bool background_cache;
 };
 
 extern const opt_t *options;

options.c

@@ -81,7 +81,8 @@ void parse_options(int argc, char **argv)
 		 */
 		OPT_START = UCHAR_MAX,
 		OPT_AA,
-		OPT_AL
+		OPT_AL,
+		OPT_BG
 	};
 	static const struct optparse_long longopts[] = {
 		{ "framerate",      'A',     OPTPARSE_REQUIRED },
@@ -109,6 +110,8 @@ void parse_options(int argc, char **argv)
 		{ "null",           '0',     OPTPARSE_NONE },
 		{ "anti-alias",    OPT_AA,   OPTPARSE_OPTIONAL },
 		{ "alpha-layer",   OPT_AL,   OPTPARSE_OPTIONAL },
+		/* TODO: document this when it's stable */
+		{ "bg-cache",      OPT_BG,   OPTPARSE_OPTIONAL },
 		{ 0 }, /* end */
 	};
 
@@ -145,6 +148,7 @@ void parse_options(int argc, char **argv)
 	_options.thumb_mode = false;
 	_options.clean_cache = false;
 	_options.private_mode = false;
+	_options.background_cache = false;
 
 	if (argc > 0) {
 		s = strrchr(argv[0], '/');
@@ -264,6 +268,11 @@ void parse_options(int argc, char **argv)
 				error(EXIT_FAILURE, 0, "Invalid argument for option --alpha-layer: %s", op.optarg);
 			_options.alpha_layer = op.optarg == NULL;
 			break;
+		case OPT_BG:
+			if (op.optarg != NULL && !STREQ(op.optarg, "no"))
+				error(EXIT_FAILURE, 0, "Invalid argument for option --bg-cache: %s", op.optarg);
+			_options.background_cache = op.optarg == NULL;
+			break;
 		}
 	}