Saving new node with a specific nid?

I think I've been able to get this to work. Basically you need to override/edit the node_save() function and the function it calls called drupal_write_record().

node_save() - edited and replaced with node_insert_with_defined_nid(&$node)
drupal_write_record() - edited and replaced with drupal_write_record_via_insert_with_defined_id($table, &$object, $update = array())

Importantly - before creating the new record double check that a node with that nid doesn't already exist.

Call the function with:

node_insert_with_defined_nid($node);

Here are the modified functions:

/**
 * Implements saving a node with a defined nid value.
 *
 * This function is basically an override of the standard node_save()
 * function.
 */
function node_insert_with_defined_nid(&$node) {

  // Here we will remove the nid from the object to trick the rest of
  // the function into thinking it is dealing with a normal insert.
  // We will then re-introduce the nid when needed.
  $defined_nid = $node->nid;
  unset($node->nid);


  // Let modules modify the node before it is saved to the database.
  node_invoke_nodeapi($node, 'presave');
  global $user;

  // Insert a new node.
  $node->is_new = empty($node->nid);

  if ($node->is_new || !empty($node->revision)) {
    // When inserting a node, $node->log must be set because
    // {node_revisions}.log does not (and cannot) have a default
    // value.  If the user does not have permission to create
    // revisions, however, the form will not contain an element for
    // log so $node->log will be unset at this point.
    if (!isset($node->log)) {
      $node->log = '';
    }
  }

  // For the same reasons, make sure we have $node->teaser and
  // $node->body set.
  if (!isset($node->teaser)) {
    $node->teaser = '';
  }
  if (!isset($node->body)) {
    $node->body = '';
  }

  $time = time();
  if (empty($node->created)) {
    $node->created = $time;
  }
  // The changed timestamp is always updated for bookkeeping purposes
  // (revisions, searching, ...)
  $node->changed = $time;

  $node->timestamp = $time;
  $node->format = isset($node->format) ? $node->format : FILTER_FORMAT_DEFAULT;

  // Generate the node table query and the node_revisions table query.
  if ($node->is_new) {
    _node_save_revision($node, $user->uid);

    // Call our modified function after putting the nid back into the
    // object.
    $node->nid = $defined_nid;
    drupal_write_record_via_insert_with_defined_id('node', $node);

    db_query('UPDATE {node_revisions} SET nid = %d WHERE vid = %d', $node->nid, $node->vid);
    $op = 'insert';
  }

  // Call the node specific callback (if any).
  node_invoke($node, $op);
  node_invoke_nodeapi($node, $op);

  // Update the node access table for this node.
  node_access_acquire_grants($node);

  // Clear the page and block caches.
  cache_clear_all();
}

/**
 * Implements inserting a record with a defined id value.
 *
 * Overrides the standard drupal_write_record() function.
 */
function drupal_write_record_via_insert_with_defined_id($table, &$object, $update = array()) {
  // Standardize $update to an array.
  if (is_string($update)) {
    $update = array($update);
  }

  $schema = drupal_get_schema($table);
  if (empty($schema)) {
    return FALSE;
  }

  // Convert to an object if needed.
  if (is_array($object)) {
    $object = (object) $object;
    $array = TRUE;
  }
  else {
    $array = FALSE;
  }

  $fields = $defs = $values = $serials = $placeholders = array();

  // Go through our schema, build SQL, and when inserting, fill in defaults for
  // fields that are not set.
  foreach ($schema['fields'] as $field => $info) {

    // For inserts, populate defaults from Schema if not already provided.
    if (!isset($object->$field) && !count($update) && isset($info['default'])) {
      $object->$field = $info['default'];
    }

    // Build arrays for the fields, placeholders, and values in our query.
    if (isset($object->$field)) {
      $fields[] = $field;
      $placeholders[] = db_type_placeholder($info['type']);

      if (empty($info['serialize'])) {
        $values[] = $object->$field;
      }
      else {
        $values[] = serialize($object->$field);
      }
    }
  }

  // Build the SQL.
  $query = '';
  if (!count($update)) {
    $query = "INSERT INTO {" . $table . "} (" . implode(', ', $fields) . ') VALUES (' . implode(', ', $placeholders) . ')';
    $return = SAVED_NEW;
  }

  // Execute the SQL.
  if (!db_query($query, $values)) {
    $return = FALSE;
  }

  // If we began with an array, convert back so we don't surprise the caller.
  if ($array) {
    $object = (array) $object;
  }

  return $return;
}

You're right!

Was using a rather quick hack to run the php code, which was running the code twice.
The error was getting logged on the 2nd run.

The code works fine.
Also used some of the code from the D7 docs for setting default etc.

Thanks a lot!

Here's the whole code:

$node = new stdClass();

$node->type = "page";

// Sets some defaults. Invokes hook_prepare() and hook_node_prepare().
node_object_prepare($node); 

// Or e.g. 'en' if locale is enabled.
$node->language = LANGUAGE_NONE; 
$node->uid = 234; 

// Status is 1 or 0; published or not.
$node->status = 1; 

// Promote is 1 or 0; promoted to front page or not.
$node->promote = 0; 

// Comment is 0, 1, 2; 0 = disabled, 1 = read only, or 2 = read/write.
$node->comment = 0; 

$node->nid =123;
$node->title = 'title 123';
$node->is_new = true;

$node = node_submit($node); // Prepare node for saving
node_save($node);