CDRIVER-1639 seed the PRNG used to select servers

Author: ajdavis

NEWS

@@ -38,6 +38,7 @@ New features and bug fixes:
   * New CMake option ENABLE_TRACING allows debug output, which before had only
     been available with "configure --enable-tracing".
   * Bugfix: "PossiblePrimary"-type replicas could be selected for reads
+  * The random number generator used to select servers is now properly seeded.
 
 Removed configure flags:
   * --enable-experimental has been removed and all of its features

src/mongoc/mongoc-topology-description-private.h

@@ -48,6 +48,7 @@ struct _mongoc_topology_description_t
    char                              *compatibility_error;
    uint32_t                           max_server_id;
    bool                               stale;
+   unsigned int                       rand_seed;
 
    mongoc_apm_callbacks_t             apm_callbacks;
    void                              *apm_context;

src/mongoc/mongoc-topology-description.c

@@ -69,6 +69,7 @@ mongoc_topology_description_init (mongoc_topology_description_t      *descriptio
    description->compatible = true;
    description->compatibility_error = NULL;
    description->stale = true;
+   description->rand_seed = (unsigned int) bson_get_monotonic_time ();
 
    EXIT;
 }
@@ -82,6 +83,8 @@ mongoc_topology_description_init (mongoc_topology_description_t      *descriptio
  *       @dst must not already point to any allocated resources. Clean
  *       up with mongoc_topology_description_destroy.
  *
+ *       WARNING: @dst's rand_seed is not initialized.
+ *
  * Returns:
  *       None.
  *
@@ -678,6 +681,7 @@ mongoc_topology_description_select (mongoc_topology_description_t *topology,
 {
    mongoc_array_t suitable_servers;
    mongoc_server_description_t *sd = NULL;
+   int rand_n;
 
    ENTRY;
 
@@ -704,8 +708,9 @@ mongoc_topology_description_select (mongoc_topology_description_t *topology,
                                                  topology, read_pref,
                                                  local_threshold_ms);
    if (suitable_servers.len != 0) {
+      rand_n = MONGOC_RAND_R (&topology->rand_seed);
       sd = _mongoc_array_index(&suitable_servers, mongoc_server_description_t*,
-                               rand() % suitable_servers.len);
+                               rand_n % suitable_servers.len);
    }
 
    _mongoc_array_destroy (&suitable_servers);

src/mongoc/mongoc-util-private.h

@@ -38,6 +38,12 @@
 
 #define COALESCE(x, y) ((x == 0) ? (y) : (x))
 
+#ifdef _WIN32
+# define MONGOC_RAND_R rand_s
+#else
+# define MONGOC_RAND_R rand_r
+#endif
+
 BSON_BEGIN_DECLS