Bug #90420 Contribution by Facebook: Query attributes

Description:
Background innformation provided by Facebook:

Abstract:

This adds a way to tag COM_QUERY requests.
Currently, queries sent over the network are encoded as follows:
      total packet length
      COM_QUERY
      query string

The length of the query string is implicit from and calculated from the packet length. This change adds a new command called COM_QUERY_ATTRS with the following structure
       total packet length
       COM_QUERY_ATTRS
       total attributes length
       attributes string
       query string

Note that query string length is still implicit. In order to avoid conflicting RPC values with Oracle, COM_QUERY_ATTRS was put at the end of the valid RPC range. Its value is set at 255.

For upstream purposes, COM_QUERY_ATTRS and COM_QUERY can probably be merged, using capability flags to help with the format change.

Repo: https://github.com/mysql/mysql-server
Patch on top of 8.0.4: https://github.com/mysql/mysql-server/commit/577fbcf856cd693434d554545d499559bcdd3695
Facebook commits: https://github.com/facebook/mysql-5.6/commit/a619992

How to repeat:
See description

Suggested fix:
See contribution code attached

Query attributes 
(*) This code is contributed under the Facebook agreement

Contribution: fb_patch_43.txt (text/plain), 20.84 KiB.

Query attributes for 8.0.11 
(*) This code is contributed under the Facebook agreement

Contribution: fb_patch_83.txt (text/plain), 22.16 KiB.

Implemented as WL#12542 in 8.0.23

Posted by developer:
 
Fixed in 8.0.23.

For some applications, it may be useful to define metadata on a
per-query basis. Examples include the URL of the page that produced a
query, or extra processing information to be passed with a query for
use by a plugin such as an audit plugin or query rewrite plugin.
MySQL now supports this capability without the use of workarounds
such as specially formatted comments included in query strings:

* On the client side, the mysql_bind_param() C API function enables
  defining query attributes. These attributes apply to the next SQL
  statement sent to the server for execution. Additionally, the mysql and
  mysqltest clients have a query_attributes command that enables defining
  query attributes.

* On the server side, a component service provides access to query
  attributes. A component named query_attributes uses this service to
  implement a mysql_query_attribute_string() user-defined function (UDF)
  that enables obtaining attribute values within SQL statements. The
  query_attributes component is optional but must be installed for the UDF
  to be available.

Thanks to Facebook for suggesting the idea (and for contributing
code, although it was not used).