Add flag to enable easier cross-seeding

Author: rmsacks

init.c

@@ -274,6 +274,7 @@ static void print_help()
 	  "-v, --verbose                 : be verbose\n"
 	  "-w, --web-seed=<url>[,<url>]* : add web seed URLs\n"
 	  "                                additional -w adds more URLs\n"
+	  "-x, --cross-seed              : ensure info hash is unique for easier cross-seeding\n"
 #else
 	  "-a <url>[,<url>]* : specify the full announce URLs\n"
 	  "                    additional -a adds backup trackers\n"
@@ -296,6 +297,7 @@ static void print_help()
 	  "-v                : be verbose\n"
 	  "-w <url>[,<url>]* : add web seed URLs\n"
 	  "                    additional -w adds more URLs\n"
+	  "-x                : ensure info hash is unique for easier cross-seeding\n"
 #endif
 	  "\nPlease send bug reports, patches, feature requests, praise and\n"
 	  "general gossip about the program to: [email protected]\n");
@@ -427,6 +429,7 @@ EXPORT void init(struct metafile *m, int argc, char *argv[])
 #endif
 		{"verbose", 0, NULL, 'v'},
 		{"web-seed", 1, NULL, 'w'},
+		{"cross-seed", 0, NULL, 'x'},
 		{NULL, 0, NULL, 0}
 	};
 #endif
@@ -442,9 +445,9 @@ EXPORT void init(struct metafile *m, int argc, char *argv[])
 
 	/* now parse the command line options given */
 #ifdef USE_PTHREADS
-#define OPT_STRING "a:c:dfhl:n:o:ps:t:vw:"
+#define OPT_STRING "a:c:dfhl:n:o:ps:t:vw:x"
 #else
-#define OPT_STRING "a:c:dfhl:n:o:ps:vw:"
+#define OPT_STRING "a:c:dfhl:n:o:ps:vw:x"
 #endif
 #ifdef USE_LONG_OPTIONS
 	while ((c = getopt_long(argc, argv, OPT_STRING,
@@ -497,6 +500,9 @@ EXPORT void init(struct metafile *m, int argc, char *argv[])
 		case 'w':
 			ll_extend(m->web_seed_list, get_slist(optarg));
 			break;
+		case 'x':
+			m->cross_seed = 1;
+			break;
 		case '?':
 			fatal("use -h for help.\n");
 		}

main.c

@@ -18,12 +18,13 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
 */
 
 
-#include <stdlib.h>      /* exit() */
+#include <stdlib.h>      /* exit(), srandom() */
 #include <errno.h>       /* errno */
 #include <string.h>      /* strerror() */
 #include <stdio.h>       /* printf() etc. */
 #include <sys/stat.h>    /* S_IRUSR, S_IWUSR, S_IRGRP, S_IROTH */
 #include <fcntl.h>       /* open() */
+#include <time.h>        /* clock_gettime() */
 
 #include "export.h"
 #include "mktorrent.h"
@@ -121,6 +122,7 @@ int main(int argc, char *argv[])
 		0,    /* no_creation_date */
 		0,    /* private */
 		NULL, /* source string */
+		0,    /* cross_seed */
 		0,    /* verbose */
 		0,    /* force_overwrite */
 #ifdef USE_PTHREADS
@@ -136,6 +138,12 @@ int main(int argc, char *argv[])
 	/* print who we are */
 	printf("mktorrent " VERSION " (c) 2007, 2009 Emil Renner Berthing\n\n");
 
+	/* seed PRNG with current time */
+	struct timespec ts;
+	FATAL_IF(clock_gettime(CLOCK_REALTIME, &ts) == -1,
+		"failed to get time: %s\n", strerror(errno));
+	srandom(ts.tv_nsec ^ ts.tv_sec);
+
 	/* process options */
 	init(&m, argc, argv);
 

mktorrent.h

@@ -31,6 +31,7 @@ struct metafile {
 	int no_creation_date;      /* don't write the creation date */
 	int private;               /* set the private flag */
 	char *source;              /* set source for private trackers */
+	int cross_seed;            /* ensure info hash is unique for easier cross-seeding */
 	int verbose;               /* be verbose */
 	int force_overwrite;       /* overwrite existing output file */
 #ifdef USE_PTHREADS

output.c

@@ -23,6 +23,7 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
 #include <string.h>       /* strlen() etc. */
 #include <time.h>         /* time() */
 #include <inttypes.h>     /* PRIuMAX */
+#include <stdlib.h>       /* random() */
 
 #ifdef USE_OPENSSL
 #include <openssl/sha.h>  /* SHA_DIGEST_LENGTH */
@@ -179,6 +180,15 @@ EXPORT void write_metainfo(FILE *f, struct metafile *m, unsigned char *hash_stri
 	else
 		write_file_list(f, m->file_list);
 
+	if (m->cross_seed) {
+		fprintf(f, "12:x_cross_seed%u:mktorrent-", CROSS_SEED_RAND_LENGTH * 2 + 10);
+		for (int i = 0; i < CROSS_SEED_RAND_LENGTH; i++) {
+			unsigned char rand_byte = random();
+			fputc("0123456789ABCDEF"[rand_byte >> 4], f);
+			fputc("0123456789ABCDEF"[rand_byte & 0x0F], f);
+		}
+	}
+
 	/* the info section also contains the name of the torrent,
 	   the piece length and the hash string */
 	fprintf(f, "4:name%lu:%s12:piece lengthi%ue6:pieces%u:",

output.h

@@ -6,6 +6,8 @@
 #include "export.h"     /* EXPORT */
 #include "mktorrent.h"  /* struct metafile */
 
+#define CROSS_SEED_RAND_LENGTH 16
+
 EXPORT void write_metainfo(FILE *f, struct metafile *m,
 			unsigned char *hash_string);