Patch to fix mongo c driver to allow connection to secondary/slave mongodb instances. This patch will apply cleanly on revision 13808396bd1b128dfa03ea3ba4d3cdccb9d990c7 for https://github.com/mongodb/mongo-c-driver

patch-fix-slave-ok-flag.diff

diff --git a/src/gridfs.c b/src/gridfs.c
index f51b397..66f8d88 100644
--- a/src/gridfs.c
+++ b/src/gridfs.c
@@ -43,7 +43,7 @@ int gridfs_init( mongo *client, const char *dbname, const char *prefix,
 
     int options;
     bson b;
-    bson_bool_t success;
+    bson_bool_t success = 1;
 
     gfs->client = client;
 
@@ -76,7 +76,9 @@ int gridfs_init( mongo *client, const char *dbname, const char *prefix,
     bson_append_int( &b, "filename", 1 );
     bson_finish( &b );
     options = 0;
-    success = ( mongo_create_index( gfs->client, gfs->files_ns, &b, options, NULL ) == MONGO_OK );
+    // if not primary/master then ignore
+    if ( gfs->client->ismaster )
+        success = ( mongo_create_index( gfs->client, gfs->files_ns, &b, options, NULL ) == MONGO_OK );
     bson_destroy( &b );
     if ( !success ) {
         bson_free( ( char * )gfs->dbname );
@@ -91,7 +93,9 @@ int gridfs_init( mongo *client, const char *dbname, const char *prefix,
     bson_append_int( &b, "n", 1 );
     bson_finish( &b );
     options = MONGO_INDEX_UNIQUE;
-    success = ( mongo_create_index( gfs->client, gfs->chunks_ns, &b, options, NULL ) == MONGO_OK );
+    // if not primary/master then ignore
+    if ( gfs->client->ismaster )
+        success = ( mongo_create_index( gfs->client, gfs->chunks_ns, &b, options, NULL ) == MONGO_OK );
     bson_destroy( &b );
     if ( !success ) {
         bson_free( ( char * )gfs->dbname );
diff --git a/src/mongo.c b/src/mongo.c
index 021c46a..62f174a 100644
--- a/src/mongo.c
+++ b/src/mongo.c
@@ -139,6 +139,7 @@ static int mongo_check_is_master( mongo *conn ) {
     if ( mongo_simple_int_command( conn, "admin", "ismaster", 1, &out ) == MONGO_OK ) {
         if( bson_find( &it, &out, "ismaster" ) )
             ismaster = bson_iterator_bool( &it );
+            conn->ismaster = ismaster;
     } else {
         return MONGO_ERROR;
     }
@@ -164,7 +165,7 @@ void mongo_init( mongo *conn ) {
     conn->op_timeout_ms = 0;
 }
 
-int mongo_connect( mongo *conn , const char *host, int port ) {
+int mongo_connect( mongo *conn , const char *host, int port, int option ) {
     conn->primary = bson_malloc( sizeof( mongo_host_port ) );
     strncpy( conn->primary->host, host, strlen( host ) + 1 );
     conn->primary->port = port;
@@ -174,7 +175,7 @@ int mongo_connect( mongo *conn , const char *host, int port ) {
     if( mongo_socket_connect( conn, host, port ) != MONGO_OK )
         return MONGO_ERROR;
 
-    if( mongo_check_is_master( conn ) != MONGO_OK )
+    if( option == MONGO_PRIMARY_ONLY && mongo_check_is_master( conn ) != MONGO_OK )
         return MONGO_ERROR;
     else
         return MONGO_OK;
@@ -312,6 +313,7 @@ static int mongo_replset_check_host( mongo *conn ) {
     if ( mongo_simple_int_command( conn, "admin", "ismaster", 1, &out ) == MONGO_OK ) {
         if( bson_find( &it, &out, "ismaster" ) )
             ismaster = bson_iterator_bool( &it );
+            conn->ismaster = ismaster;
 
         if( bson_find( &it, &out, "setName" ) ) {
             set_name = bson_iterator_string( &it );
@@ -334,7 +336,7 @@ static int mongo_replset_check_host( mongo *conn ) {
     return MONGO_OK;
 }
 
-int mongo_replset_connect( mongo *conn ) {
+int mongo_replset_connect( mongo *conn, int option ) {
 
     int res = 0;
     mongo_host_port *node;
@@ -372,16 +374,20 @@ int mongo_replset_connect( mongo *conn ) {
             if( res == MONGO_OK ) {
                 if( mongo_replset_check_host( conn ) != MONGO_OK )
                     return MONGO_ERROR;
-
-                /* Primary found, so return. */
-                else if( conn->replset->primary_connected )
-                    return MONGO_OK;
-
-                /* No primary, so close the connection. */
                 else {
-                    mongo_close_socket( conn->sock );
-                    conn->sock = 0;
-                    conn->connected = 0;
+                    if ( option == MONGO_SECONDARY_OK )
+                        return MONGO_OK;
+
+                    /* Primary found, so return. */
+                    if( conn->replset->primary_connected )
+                        return MONGO_OK;
+
+                    /* No primary, so close the connection. */
+                    else {
+                        mongo_close_socket( conn->sock );
+                        conn->sock = 0;
+                        conn->connected = 0;
+                    }
                 }
             }
 
@@ -402,7 +408,7 @@ int mongo_set_op_timeout( mongo *conn, int millis ) {
     return MONGO_OK;
 }
 
-int mongo_reconnect( mongo *conn ) {
+int mongo_reconnect( mongo *conn, int option ) {
     int res;
     mongo_disconnect( conn );
 
@@ -410,7 +416,7 @@ int mongo_reconnect( mongo *conn ) {
         conn->replset->primary_connected = 0;
         mongo_replset_free_list( &conn->replset->hosts );
         conn->replset->hosts = NULL;
-        res = mongo_replset_connect( conn );
+        res = mongo_replset_connect( conn, option );
         return res;
     } else
         return mongo_socket_connect( conn, conn->primary->host, conn->primary->port );
@@ -731,6 +737,7 @@ mongo_cursor *mongo_find( mongo *conn, const char *ns, bson *query,
     mongo_cursor_set_fields( cursor, fields );
     mongo_cursor_set_limit( cursor, limit );
     mongo_cursor_set_skip( cursor, skip );
+    if ( !conn->ismaster ) options |= MONGO_SLAVE_OK;
     mongo_cursor_set_options( cursor, options );
 
     if( mongo_cursor_op_query( cursor ) == MONGO_OK )
@@ -1102,6 +1109,7 @@ bson_bool_t mongo_cmd_ismaster( mongo *conn, bson *realout ) {
         bson_iterator it;
         bson_find( &it, &out, "ismaster" );
         ismaster = bson_iterator_bool( &it );
+        conn->ismaster = ismaster;
     }
 
     if( realout )
diff --git a/src/mongo.h b/src/mongo.h
index 6f86174..2dbd361 100644
--- a/src/mongo.h
+++ b/src/mongo.h
@@ -80,6 +80,11 @@ enum mongo_cursor_opts {
     MONGO_PARTIAL = ( 1<<7 )          /**< Allow reads even if a shard is down. */
 };
 
+enum mongo_connect_opts {
+    MONGO_PRIMARY_ONLY,
+    MONGO_SECONDARY_OK
+};
+
 enum mongo_operations {
     MONGO_OP_MSG = 1000,
     MONGO_OP_UPDATE = 2001,
@@ -143,6 +148,8 @@ typedef struct mongo {
     char *errstr;              /**< String version of most recent driver error code. */
     int lasterrcode;           /**< getlasterror given by the server on calls. */
     char *lasterrstr;          /**< getlasterror string generated by server. */
+
+    int ismaster;
 } mongo;
 
 typedef struct {
@@ -184,7 +191,7 @@ void mongo_init( mongo *conn );
  * @return MONGO_OK or MONGO_ERROR on failure. On failure, a constant of type
  *   mongo_conn_return_t will be set on the conn->err field.
  */
-int mongo_connect( mongo *conn , const char *host, int port );
+int mongo_connect( mongo *conn , const char *host, int port, int option);
 
 /**
  * Set up this connection object for connecting to a replica set.
@@ -226,7 +233,7 @@ void mongo_parse_host( const char *host_string, mongo_host_port *host_port );
  * @return MONGO_OK or MONGO_ERROR on failure. On failure, a constant of type
  *   mongo_conn_return_t will be set on the conn->err field.
  */
-int mongo_replset_connect( mongo *conn );
+int mongo_replset_connect( mongo *conn, int option );
 
 /** Set a timeout for operations on this connection. This
  *  is a platform-specific feature, and only work on *nix
@@ -261,7 +268,7 @@ int mongo_check_connection( mongo *conn );
  * @return MONGO_OK or MONGO_ERROR and
  *   set the conn->err field.
  */
-int mongo_reconnect( mongo *conn );
+int mongo_reconnect( mongo *conn, int option );
 
 /**
  * Close the current connection to the server. After calling

• This will work well with nginx gridfs driver - https://gist.github.com/3263946

Disclaimer: This patch is definitely not perfect. Hardly done major testing but it works for my setup.