Reduce chance of ogg serial number collision

When two ogg streams were made with two different calls to `flac`
within the same second, their serial numbers would be the same.
This caused problems with seeking in the test suite. While in
theory, libFLAC should be able to seek in a chained stream with
two equal serial numbers without fail, in practice seeking in
chained streams is hard enough. So, this commit makes sure the
chained test files have links with unequal serial numbers.

config.cmake.h.in

@@ -109,6 +109,9 @@
 /* Define to 1 if you have the <sys/stat.h> header file. */
 #cmakedefine HAVE_SYS_STAT_H
 
+/* Define to 1 if you have the <sys/time.h> header file. */
+#cmakedefine HAVE_SYS_TIME_H
+
 /* Define to 1 if you have the <sys/types.h> header file. */
 #cmakedefine HAVE_SYS_TYPES_H
 

configure.ac

@@ -53,7 +53,7 @@ AM_PROG_CC_C_O
 AC_C_INLINE
 AC_C_TYPEOF
 
-AC_CHECK_HEADERS([stdint.h stdbool.h inttypes.h byteswap.h sys/auxv.h sys/param.h sys/ioctl.h termios.h x86intrin.h cpuid.h arm_neon.h])
+AC_CHECK_HEADERS([stdint.h stdbool.h inttypes.h byteswap.h sys/auxv.h sys/param.h sys/ioctl.h sys/time.h termios.h x86intrin.h cpuid.h arm_neon.h])
 
 if test "x$ac_cv_header_stdint_h" != xyes -o "x$ac_cv_header_stdbool_h" != xyes; then
 AC_MSG_ERROR("Header stdint.h and/or stdbool.h not found")

src/flac/CMakeLists.txt

@@ -1,4 +1,5 @@
 check_include_file("sys/ioctl.h" HAVE_SYS_IOCTL_H)
+check_include_file("sys/time.h" HAVE_SYS_TIME_H)
 check_include_file("termios.h" HAVE_TERMIOS_H)
 
 add_executable(flacapp

src/flac/main.c

@@ -30,6 +30,10 @@
 #include <string.h>
 #include <time.h>
 
+#ifdef HAVE_SYS_TIME_H
+#include <sys/time.h>
+#endif
+
 #if !defined _MSC_VER && !defined __MINGW32__
 /* unlink is in stdio.h in VC++ */
 #include <unistd.h> /* for unlink() */
@@ -322,7 +326,30 @@ static int main_to_fuzz(int argc, char *argv[])
 	_setmode(fileno(stderr),_O_U8TEXT);
 #endif
 
-	srand((uint32_t)time(0));
+#ifdef HAVE_SYS_TIME_H
+	{
+		struct timeval tv;
+
+		if (gettimeofday(&tv, 0) < 0) {
+			srand(((uint32_t)time(0) << 8) + (uint32_t)clock());
+		}
+		else {
+			/* fall back when gettimeofday fails */
+			srand((uint32_t)(tv.tv_sec) * 1e6 + (uint32_t)tv.tv_usec);
+		}
+	}
+#else
+	/* time(0) does not have sufficient resolution when flac is invoked more than
+	 * once in quick succession (for example in the test suite). As far as I know,
+	 * clock() is the only sub-second portable alternative, but measures
+	 * execution time, which is often quite similar between runs. From limited
+	 * testing, it seems the value varies by about 100, so that would make
+	 * collision 100 times less likely than without. Therefore, use
+	 * both together to generate a random number seed.
+	 */
+	srand(((uint32_t)time(0) << 8) + (uint32_t)clock());
+#endif
+
 #ifdef _WIN32
 	{
 		const char *var;
@@ -1640,7 +1667,10 @@ int encode_file(const char *infilename, FLAC__bool is_first_file, FLAC__bool is_
 	encode_options.use_ogg = option_values.use_ogg;
 	/* set a random serial number if one has not yet been specified */
 	if(!option_values.has_serial_number) {
-		option_values.serial_number = rand();
+	        if (RAND_MAX < 0x7fffffff)
+			option_values.serial_number = (uint32_t)(rand() & 0x7fff) << 16 | (uint32_t)(rand());
+	        else
+			option_values.serial_number = rand();
 		option_values.has_serial_number = true;
 	}
 	encode_options.serial_number = option_values.serial_number++;