CDRIVER-699 introduce serverSelectionTryOnce option

Author: ajdavis

doc/mongoc_uri_t.page

@@ -69,6 +69,8 @@
     <table>
       <tr><td><p>heartbeatFrequencyMS</p></td><td><p>The interval between server monitoring checks. Defaults to 10 seconds in multi-threaded mode, 60 seconds in single threaded.</p></td></tr>
       <tr><td><p>serverSelectionTimeoutMS</p></td><td><p>A timeout in milliseconds to block for server selection before throwing an exception. The default is 30,000 (milliseconds).</p></td></tr>
+      <tr><td><p>serverSelectionTryOnce</p></td><td><p>If "true" (the default), the driver scans the topology exactly once after server selection fails, then either selects a server or raises an error. If it is false, then the driver repeatedly searches for an appropriate server for up to serverSelectionTimeoutMS milliseconds (pausing a half second between attempts).</p>
+      <p>Clients in a <code xref="mongoc_client_pool_t">mongoc_client_pool_t</code> share a topology scanner that runs on a background thread. Pooled clients ignore serverSelectionTryOnce; they signal the thread to rescan the topology every half-second until serverSelectionTimeoutMS expires.</p></td></tr>
       <tr><td><p>socketCheckIntervalMS</p></td><td><p>Only applicable to single threaded clients. If socket has not been used within this timeframe a isMaster call is attempted. Defaults to 5 seconds.</p></td></tr>
     </table>
     <note style="important">

src/mongoc/mongoc-topology-private.h

@@ -42,6 +42,7 @@ typedef struct _mongoc_topology_t
    mongoc_topology_description_t description;
    mongoc_uri_t                 *uri;
    mongoc_topology_scanner_t    *scanner;
+   bool                          server_selection_try_once;
 
    int64_t                       last_scan;
    int64_t                       timeout_msec;

src/mongoc/mongoc-topology.c

@@ -170,6 +170,23 @@ mongoc_topology_new (const mongoc_uri_t *uri,
                                                     _mongoc_topology_scanner_cb,
                                                     topology);
    topology->single_threaded = single_threaded;
+   if (single_threaded) {
+      /* Server Selection Spec:
+       *
+       *   "Single-threaded drivers MUST provide a "serverSelectionTryOnce"
+       *   mode, in which the driver scans the topology exactly once after
+       *   server selection fails, then either selects a server or raises an
+       *   error.
+       *
+       *   "The serverSelectionTryOnce option MUST be true by default."
+       */
+      topology->server_selection_try_once = mongoc_uri_get_option_as_bool (
+         uri,
+         "serverselectiontryonce",
+         true);
+   } else {
+      topology->server_selection_try_once = false;
+   }
 
    topology->timeout_msec = mongoc_uri_get_option_as_int32(
       topology->uri, "serverselectiontimeoutms",

src/mongoc/mongoc-uri.c

@@ -557,6 +557,7 @@ mongoc_uri_option_is_bool (const char *key)
    return !strcasecmp(key, "canonicalizeHostname") ||
               !strcasecmp(key, "journal") ||
               !strcasecmp(key, "safe") ||
+              !strcasecmp(key, "serverSelectionTryOnce") ||
               !strcasecmp(key, "slaveok") ||
               !strcasecmp(key, "ssl");
 }

tests/test-mongoc-topology.c

@@ -92,6 +92,44 @@ test_topology_client_pool_creation (void)
    mongoc_client_pool_destroy (pool);
 }
 
+static void
+test_server_selection_try_once_option (void)
+{
+   const char *uri_strings[3] = {
+      "mongodb://a",
+      "mongodb://a/?serverSelectionTryOnce=true",
+      "mongodb://a/?serverSelectionTryOnce=false" };
+
+   int i;
+   mongoc_client_t *client;
+   mongoc_uri_t *uri;
+   mongoc_client_pool_t *pool;
+
+   /* try_once is on by default for non-pooled, can be turned off */
+   client = mongoc_client_new (uri_strings[0]);
+   assert (client->topology->server_selection_try_once);
+   mongoc_client_destroy (client);
+
+   client = mongoc_client_new (uri_strings[1]);
+   assert (client->topology->server_selection_try_once);
+   mongoc_client_destroy (client);
+
+   client = mongoc_client_new (uri_strings[2]);
+   assert (! client->topology->server_selection_try_once);
+   mongoc_client_destroy (client);
+   
+   /* off for pooled clients, can't be enabled */
+   for (i = 0; i < sizeof (uri_strings) / sizeof (char *); i++) {
+      uri = mongoc_uri_new ("mongodb://a");
+      pool = mongoc_client_pool_new (uri);
+      client = mongoc_client_pool_pop (pool);
+      assert (!client->topology->server_selection_try_once);
+      mongoc_client_pool_push (pool, client);
+      mongoc_client_pool_destroy (pool);
+      mongoc_uri_destroy (uri);
+   }
+}
+
 static void
 test_topology_invalidate_server (void)
 {
@@ -402,6 +440,10 @@ test_topology_install (TestSuite *suite)
 {
    TestSuite_Add (suite, "/Topology/client_creation", test_topology_client_creation);
    TestSuite_Add (suite, "/Topology/client_pool_creation", test_topology_client_pool_creation);
+   TestSuite_Add (suite,
+                  "/Topology/server_selection_try_once_option",
+                  test_server_selection_try_once_option);
+
    TestSuite_Add (suite, "/Topology/invalidate_server", test_topology_invalidate_server);
    TestSuite_Add (suite, "/Topology/invalid_cluster_node", test_invalid_cluster_node);
    TestSuite_Add (suite, "/Topology/max_wire_version_race_condition", test_max_wire_version_race_condition);