CDRIVER-1685 add trace output for server selection

Author: ajdavis

src/mongoc/mongoc-read-prefs-private.h

@@ -46,6 +46,9 @@ typedef struct _mongoc_apply_read_prefs_result_t {
 
 #define READ_PREFS_RESULT_INIT { NULL, false, MONGOC_QUERY_NONE }
 
+const char *
+_mongoc_read_mode_as_str (mongoc_read_mode_t mode);
+
 void
 apply_read_preferences (const mongoc_read_prefs_t *read_prefs,
                         const mongoc_server_stream_t *server_stream,

src/mongoc/mongoc-read-prefs.c

@@ -164,8 +164,8 @@ mongoc_read_prefs_copy (const mongoc_read_prefs_t *read_prefs)
 }
 
 
-static const char *
-_get_read_mode_string (mongoc_read_mode_t mode)
+const char *
+_mongoc_read_mode_as_str (mongoc_read_mode_t mode)
 {
    switch (mode) {
    case MONGOC_READ_PRIMARY:
@@ -246,7 +246,7 @@ _apply_read_preferences_mongos (const mongoc_read_prefs_t *read_prefs,
 
       bson_append_document_begin (result->query_with_read_prefs,
                                   "$readPreference", 15, &child);
-      mode_str = _get_read_mode_string (mode);
+      mode_str = _mongoc_read_mode_as_str (mode);
       bson_append_utf8 (&child, "mode", 4, mode_str, -1);
       if (!bson_empty0 (tags)) {
          bson_append_array (&child, "tags", 4, tags);

src/mongoc/mongoc-server-description.c

@@ -695,6 +695,7 @@ mongoc_server_description_filter_stale (mongoc_server_description_t **sds,
             heartbeat_frequency_ms * 1000;
 
          if (staleness_usec > max_staleness_ms * 1000) {
+            TRACE ("Rejected stale RSSecondary [%s]", sds[i]->host.host_and_port);
             sds[i] = NULL;
          }
       }
@@ -721,6 +722,7 @@ mongoc_server_description_filter_stale (mongoc_server_description_t **sds,
                         heartbeat_frequency_ms;
 
          if (staleness_ms > max_staleness_ms) {
+            TRACE ("Rejected stale RSSecondary [%s]", sds[i]->host.host_and_port);
             sds[i] = NULL;
          }
       }
@@ -789,6 +791,10 @@ mongoc_server_description_filter_tags (mongoc_server_description_t **description
       if (found) {
          for (i = 0; i < description_len; i++) {
             if (! sd_matched[i]) {
+               TRACE ("Rejected [%s] [%s], doesn't match tags",
+                      mongoc_server_description_type (descriptions[i]),
+                      descriptions[i]->host.host_and_port);
+
                descriptions[i] = NULL;
             }
          }
@@ -800,6 +806,10 @@ mongoc_server_description_filter_tags (mongoc_server_description_t **description
    /* tried each */
    for (i = 0; i < description_len; i++) {
       if (! sd_matched[i]) {
+         TRACE ("Rejected [%s] [%s], reached end of tags array without match",
+                mongoc_server_description_type (descriptions[i]),
+                descriptions[i]->host.host_and_port);
+
          descriptions[i] = NULL;
       }
    }

src/mongoc/mongoc-topology-description.c

@@ -20,6 +20,7 @@
 #include "mongoc-topology-description-apm-private.h"
 #include "mongoc-trace-private.h"
 #include "mongoc-util-private.h"
+#include "mongoc-read-prefs-private.h"
 
 
 static void
@@ -365,7 +366,13 @@ _mongoc_replica_set_read_suitable_cb (void *item,
 
       /* add to our candidates */
       data->candidates[data->candidates_len++] = server;
+   } else {
+      TRACE ("Rejected [%s] [%s] for mode [%s]",
+             mongoc_server_description_type (server),
+             server->host.host_and_port,
+             _mongoc_read_mode_as_str (data->read_mode));
    }
+
    return true;
 }
 
@@ -529,6 +536,10 @@ mongoc_topology_description_suitable_servers (mongoc_array_t                *set
       server = (mongoc_server_description_t *)mongoc_set_get_item (topology->servers, 0);
       if (_mongoc_topology_description_server_is_candidate (server->type, read_mode, topology->type)) {
          _mongoc_array_append_val (set, server);
+      } else {
+         TRACE ("Rejected [%s] [%s] for read mode [%s] with topology type Single",
+                mongoc_server_description_type (server),
+                server->host.host_and_port, _mongoc_read_mode_as_str (read_mode));
       }
       goto DONE;
    }
@@ -574,6 +585,10 @@ mongoc_topology_description_suitable_servers (mongoc_array_t                *set
             for (i = 0; i < data.candidates_len; i++) {
                if (candidates[i] &&
                    candidates[i]->type != MONGOC_SERVER_RS_SECONDARY) {
+                  TRACE ("Rejected [%s] [%s] for mode [%s] with RS topology",
+                         mongoc_server_description_type (candidates[i]),
+                         candidates[i]->host.host_and_port,
+                         _mongoc_read_mode_as_str (read_mode));
                   candidates[i] = NULL;
                }
             }
@@ -670,7 +685,8 @@ mongoc_topology_description_select (mongoc_topology_description_t *topology,
 
    if (!topology->compatible) {
       /* TODO: check this in mongoc_topology_compatible (), CDRIVER-689 */
-      RETURN(NULL);
+      TRACE ("%s", "Incompatible topology");
+      RETURN (NULL);
    }
 
    if (topology->type == MONGOC_TOPOLOGY_SINGLE) {
@@ -679,7 +695,8 @@ mongoc_topology_description_select (mongoc_topology_description_t *topology,
       if (sd->has_is_master) {
          RETURN(sd);
       } else {
-         RETURN(NULL);
+         TRACE ("Topology type single, [%s] is down", sd->host.host_and_port);
+         RETURN (NULL);
       }
    }
 
@@ -695,7 +712,13 @@ mongoc_topology_description_select (mongoc_topology_description_t *topology,
 
    _mongoc_array_destroy (&suitable_servers);
 
-   RETURN(sd);
+   if (sd) {
+      TRACE ("Topology type [%s], selected [%s] [%s]",
+             mongoc_topology_description_type (topology),
+             mongoc_server_description_type (sd), sd->host.host_and_port);
+   }
+
+   RETURN (sd);
 }
 
 /*