On SQLITE_BUSY, wait a random amount of time

If all contending processes wait a fixed amount of time (100 ms),
there is a good probability that they'll just collide again.

src/libmain/shared.cc

@@ -10,6 +10,7 @@
 #include <cctype>
 #include <exception>
 
+#include <sys/time.h>
 #include <sys/stat.h>
 #include <unistd.h>
 
@@ -135,6 +136,11 @@ static void initAndRun(int argc, char * * argv)
        everybody. */
     umask(0022);
 
+    /* Initialise the PRNG. */
+    struct timeval tv;
+    gettimeofday(&tv, 0);
+    srandom(tv.tv_usec);
+
     /* Process the NIX_LOG_TYPE environment variable. */
     string lt = getEnv("NIX_LOG_TYPE");
     if (lt != "") setLogType(lt);