Give advice on the "can't connect to local MySQL server through socket" error

Seems like based on your experience that the line should just be removed entirely. Since it's turning into a no-op in your case.

Yes, the patch is the same as removing the modified line. I'm not sure why the original code switches from 'mysqli' to just 'mysql', or what the affect for others would be if this modification was removed. I'm guessing that it's not a good idea.

At present the code does not use PDO (#571352: false positives verifying database credentials in validation of DRUSH_BOOTSTRAP_DRUPAL_DATABASE) and looks like:

      // Verify connection settings.
      switch ($type) {
        case 'mysql':
        case 'mysqli':
          $connection = @mysql_connect($creds['host'], $creds['user'], $creds['pass']);
          if (!$connection || !mysql_select_db($creds['name'])) {
            drush_log(mysql_error(), 'bootstrap');
            return FALSE;
          }
          break;
        case 'pgsql':

I was lazy to add explicit mysqli support. Test with HEAD version. If it doesn't work I'll move to implement the mysqli case.

mysqli support was already implemented: http://drupal.org/node/571352#comment-3856212

This bug is back. For me drush worked well with 4.x and not with 5.x it's broken. I'm in a jailed linux shell and I do not have permission to access mysqld.sock. This worked all flawless in past and the database is configured and running :-). The update module is also enabled and works in the website.

:/web$ drush up --debug
Bootstrap to phase 0. [0.01 sec, 3.69 MB]                            [bootstrap]
Drush bootstrap phase : _drush_bootstrap_drush() [0.01 sec, 3.91 MB] [bootstrap]
Cache HIT cid: 5.4-commandfiles-0-d617a3ae39a107c58e49bc9e629d9ca6 [0.02 sec, 3.93 MB]                                     [debug]
Bootstrap to phase 0. [0.06 sec, 8.74 MB]                                                                              [bootstrap]
Bootstrap to phase 6. [0.07 sec, 8.74 MB]                                                                              [bootstrap]
Drush bootstrap phase : _drush_bootstrap_drupal_root() [0.07 sec, 8.75 MB]                                             [bootstrap]
Initialized Drupal 7.14 root directory at /web [0.09 sec, 11.9 MB]                                                        [notice]
Drush bootstrap phase : _drush_bootstrap_drupal_site() [0.09 sec, 11.91 MB]                                            [bootstrap]
Initialized Drupal site default at sites/default [0.09 sec, 11.91 MB]                                                     [notice]
Cache HIT cid: 5.4-commandfiles-2-c32acd775c18a90c5b41d0fc9accf5ab [0.1 sec, 11.92 MB]                                     [debug]
Drush bootstrap phase : _drush_bootstrap_drupal_configuration() [0.1 sec, 11.92 MB]                                    [bootstrap]
SQLSTATE[HY000] [2002] Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (111) [0.1 sec,[bootstrap]
11.95 MB]
Bootstrap to phase 5. [0.1 sec, 11.94 MB]                                                                              [bootstrap]
Command pm-update needs a higher bootstrap level to run - you will need invoke drush from a more functional Drupal     [error]
environment to run this command. [0.1 sec, 11.94 MB]
Command pm-update needs the following modules installed/enabled to run: update. [0.1 sec, 11.94 MB]                    [error]
The drush command 'up' could not be executed. [0.1 sec, 11.94 MB]                                                      [error]
Drush was not able to start (bootstrap) the Drupal database.                                                           [error]
Hint: This error often occurs when Drush is trying to bootstrap a site that has not been installed or does not have a
configured database.

Drush was attempting to connect to :
  Drupal version    : 7.14
  Site URI          : http://default
  Database driver   : mysql
  Database hostname : localhost
  Database username : foo
  Database name     : foo
  Default theme     : garland
  Administration theme: garland
  PHP configuration : /etc/php5/cli/php.ini
  Drush version     : 5.4
  Drush configuration:
  Drupal root       : /web
  Site path         : sites/default
  Modules path      : sites/all/modules
  Themes path       : sites/all/themes
  File directory path: sites/default/files
  %paths            : Array

You can select another site with a working database setup by specifying the URI to use with the --uri parameter on the
command line or $options['uri'] in your drushrc.php file.
 [0.11 sec, 11.95 MB]

I also believe that %paths : Array is a bug, too.

Mysql should not use the sock way by default as this may not allowed in jailed environments. -h must work always with the configured site settings or the site would otherwise not work.

A patch that fixed the problem would be very useful.

It seems your jailed shell has no access to the mysql socket file. I don't see the relation with mysqldump and -h cli option since the problem happens when drush tries to connect the database, with php. Also it already passes the hostname to the connection function. See drush_valid_db_credentials()

"localhost" means socket connection. To use tcp/ip hostname must be 127.0.0.1(*). So I don't see how it was working for you with drush 4.x and not with 5.x. Anyway the difference between 4.x and 5.x in this respect is the addition of support for unix_socket added in #437642: support for different MYSQL socket. Is this related to your setup?

(*) reference issue #1428638: Forcing 127.0.0.1 instead of localhost for drush

The reason that it no longer works is my hoster has limited the permissions to socks file. In past I had permission and now he suggested to use the IP address with -h 127.0.0.1. I was not aware that localhost is a special case for mysql dump.

Like the original reporter, who has 100% the same situation (localhost) including the same error message, I'm verbose now, we should configure Drupal to use "127.0.0.1" for the database connection and NOT "localhost. I'm fine with this, but it's something that should be documented!

The better would be if Drush adds the special case documented in #1428638: Forcing 127.0.0.1 instead of localhost for drush.

I agree we should add some notice in the error output near this:

Hint: This error often occurs when Drush is trying to bootstrap a site that has not been installed or does not have a configured database.

perhaps show the message conditionally based on the error number and/or hostname = localhost.

Added a message with reference to #1428638: Forcing 127.0.0.1 instead of localhost for drush

http://drupalcode.org/project/drush.git/commit/432713f