## Getting started building a mysqlnd plugin

It is important to remember that a `mysqlnd` plugin is itself a PHP extension.

The following code shows the basic structure of the MINIT function that will be used in the typical `mysqlnd` plugin:

    /* my_php_mysqlnd_plugin.c */

     static PHP_MINIT_FUNCTION(mysqlnd_plugin) {
      /* globals, ini entries, resources, classes */

      /* register mysqlnd plugin */
      mysqlnd_plugin_id = mysqlnd_plugin_register();

      conn_m = mysqlnd_get_conn_methods();
      memcpy(org_conn_m, conn_m,
        sizeof(struct st_mysqlnd_conn_methods));

      conn_m->query = MYSQLND_METHOD(mysqlnd_plugin_conn, query);
      conn_m->connect = MYSQLND_METHOD(mysqlnd_plugin_conn, connect);
    }

    /* my_mysqlnd_plugin.c */

     enum_func_status MYSQLND_METHOD(mysqlnd_plugin_conn, query)(/* ... */) {
      /* ... */
    }
    enum_func_status MYSQLND_METHOD(mysqlnd_plugin_conn, connect)(/* ... */) {
      /* ... */
    }

Task analysis: from C to userspace

     class proxy extends mysqlnd_plugin_connection {
      public function connect($host, ...) { .. }
    }
    mysqlnd_plugin_set_conn_proxy(new proxy());

Process:

1. PHP: user registers plugin callback

2. PHP: user calls any PHP MySQL API to connect to MySQL

3. C: ext/*mysql* calls mysqlnd method

4. C: mysqlnd ends up in ext/mysqlnd_plugin

5. C: ext/mysqlnd_plugin

   1. Calls userspace callback

   2. Or original `mysqlnd` method, if userspace callback not set

You need to carry out the following:

1. Write a class "mysqlnd_plugin_connection" in C

2. Accept and register proxy object through "mysqlnd_plugin_set_conn_proxy()"

3. Call userspace proxy methods from C (optimization - zend_interfaces.h)

Userspace object methods can either be called using `call_user_function()` or you can operate at a level closer to the Zend Engine and use `zend_call_method()`.

Optimization: calling methods from C using zend_call_method

The following code snippet shows the prototype for the `zend_call_method` function, taken from zend_interfaces.h.

     ZEND_API zval* zend_call_method(
      zval **object_pp, zend_class_entry *obj_ce,
      zend_function **fn_proxy, char *function_name,
      int function_name_len, zval **retval_ptr_ptr,
      int param_count, zval* arg1, zval* arg2 TSRMLS_DC
    );

Zend API supports only two arguments. You may need more, for example:

     enum_func_status (*func_mysqlnd_conn__connect)(
      MYSQLND *conn, const char *host,
      const char * user, const char * passwd,
      unsigned int passwd_len, const char * db,
      unsigned int db_len, unsigned int port,
      const char * socket, unsigned int mysql_flags TSRMLS_DC
    );

To get around this problem you will need to make a copy of `zend_call_method()` and add a facility for additional parameters. You can do this by creating a set of `MY_ZEND_CALL_METHOD_WRAPPER` macros.

Calling PHP userspace

This code snippet shows the optimized method for calling a userspace function from C:

    /* my_mysqlnd_plugin.c */

    MYSQLND_METHOD(my_conn_class,connect)(
      MYSQLND *conn, const char *host /* ... */ TSRMLS_DC) {
      enum_func_status ret = FAIL;
      zval * global_user_conn_proxy = fetch_userspace_proxy();
      if (global_user_conn_proxy) {
        /* call userspace proxy */
        ret = MY_ZEND_CALL_METHOD_WRAPPER(global_user_conn_proxy, host, /*...*/);
      } else {
        /* or original mysqlnd method = do nothing, be transparent */
        ret = org_methods.connect(conn, host, user, passwd,
              passwd_len, db, db_len, port,
              socket, mysql_flags TSRMLS_CC);
      }
      return ret;
    }

Calling userspace: simple arguments

    /* my_mysqlnd_plugin.c */

     MYSQLND_METHOD(my_conn_class,connect)(
      /* ... */, const char *host, /* ...*/) {
      /* ... */
      if (global_user_conn_proxy) {
        /* ... */
        zval* zv_host;
        MAKE_STD_ZVAL(zv_host);
        ZVAL_STRING(zv_host, host, 1);
        MY_ZEND_CALL_METHOD_WRAPPER(global_user_conn_proxy, zv_retval, zv_host /*, ...*/);
        zval_ptr_dtor(&zv_host);
        /* ... */
      }
      /* ... */
    }

Calling userspace: structs as arguments

    /* my_mysqlnd_plugin.c */

    MYSQLND_METHOD(my_conn_class, connect)(
      MYSQLND *conn, /* ...*/) {
      /* ... */
      if (global_user_conn_proxy) {
        /* ... */
        zval* zv_conn;
        ZEND_REGISTER_RESOURCE(zv_conn, (void *)conn, le_mysqlnd_plugin_conn);
        MY_ZEND_CALL_METHOD_WRAPPER(global_user_conn_proxy, zv_retval, zv_conn, zv_host /*, ...*/);
        zval_ptr_dtor(&zv_conn);
        /* ... */
      }
      /* ... */
    }

The first argument of many `mysqlnd` methods is a C "object". For example, the first argument of the connect() method is a pointer to `MYSQLND`. The struct MYSQLND represents a `mysqlnd` connection object.

The `mysqlnd` connection object pointer can be compared to a standard I/O file handle. Like a standard I/O file handle a `mysqlnd` connection object shall be linked to the userspace using the PHP resource variable type.

From C to userspace and back

     class proxy extends mysqlnd_plugin_connection {
      public function connect($conn, $host, ...) {
        /* "pre" hook */
        printf("Connecting to host = '%s'\n", $host);
        debug_print_backtrace();
        return parent::connect($conn);
      }

      public function query($conn, $query) {
        /* "post" hook */
        $ret = parent::query($conn, $query);
        printf("Query = '%s'\n", $query);
        return $ret;
      }
    }
    mysqlnd_plugin_set_conn_proxy(new proxy());

PHP users must be able to call the parent implementation of an overwritten method.

As a result of subclassing it is possible to refine only selected methods and you can choose to have "pre" or "post" hooks.

Buildin class: mysqlnd_plugin_connection::connect()

    /*  my_mysqlnd_plugin_classes.c */

     PHP_METHOD("mysqlnd_plugin_connection", connect) {
      /* ... simplified! ... */
      zval* mysqlnd_rsrc;
      MYSQLND* conn;
      char* host; int host_len;
      if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "rs",
        &mysqlnd_rsrc, &host, &host_len) == FAILURE) {
        RETURN_NULL();
      }
      ZEND_FETCH_RESOURCE(conn, MYSQLND* conn, &mysqlnd_rsrc, -1,
        "Mysqlnd Connection", le_mysqlnd_plugin_conn);
      if (PASS == org_methods.connect(conn, host, /* simplified! */ TSRMLS_CC))
        RETVAL_TRUE;
      else
        RETVAL_FALSE;
    }