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[MDEV-34009] Introduce server-initiated instant failover mechanism #3231

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[MDEV-34009] Introduce server-initiated instant failover mechanism #3231

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Commits on Apr 29, 2024

  1. [MDEV-34009] Basic framework for instant failover mechanism 

    This adds the system variables `INSTANT_FAILOVER_TARGET` and
`INSTANT_FAILOVER_MODE` (`OFF`/`ON`/`ALL`), and the error code
`ER_INSTANT_FAILOVER`.

When `INSTANT_FAILOVER_MODE=ON`, the server will immediately respond
to newly-connected clients with an error packet containing the error code
4196 (`ER_INSTANT_FAILOVER`) and a specially-formatted message:

    |Arbitary human-readable message|value of INSTANT_FAILOVER_TARGET

For example:

    |Server is directing clients to the alternative server 'other-mariadb-server.company.com:3307'.|other-mariadb-server.company.com:3307

Updated and compatible clients can parse this message and redirect
appropriately, or display the human-readable message if they do not wish to
follow this redirection.  Older clients will display the message in its
entirety, and end users should at least have an idea of what's going on.

In my earliest implementation, the sending of the `ER_INSTANT_FAILOVER`
error packet by the MariaDB server depended on the exploitation of the
client vulnerability https://jira.mariadb.org/browse/CONC-648 (“Client
improperly accepts error packets prior to TLS handshake”), which I
discovered during its implementation.

The server should obviously be able to redirect clients which don't suffer
from this severe vulnerability.

In order to do this, we need to move the redirection handling into
`native_password_authentication()`.  This awkward arrangement is
necessitated by the total entanglement of the APPLICATION-layer
authentication code (e.g. username+password for "native" authentication)
with the TRANSPORT-layer security mechanism (TLS literally stands for
Transport Layer Security).

An unfortunate consequence of this is that the redirection mechanism will
not work unless the client is using the "native" authentication plugin, or
until other authentication plugins are similarly updated similarly.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer Amazon Web
Services, Inc.
    @dlenski
    dlenski committed Apr 29, 2024
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  2. Exclude connections to the EXTRA_PORT from INSTANT_FAILOVER_MODE=ON 

    MariaDB servers can optionally listen for client connections on an
additional TCP port (configured with the system variable `EXTRA_PORT`) in
addition to the "normal" TCP port (default 3306).  See
https://mariadb.com/kb/en/thread-pool-in-mariadb/#configuring-the-extra-port
for more information.

This `EXTRA_PORT` is intended for emergency and/or administrative use, and
we should therefore include it from redirection in
`INSTANT_FAILOVER_MODE=ON`, just as we do for non-network based connections
(Unix sockets and named pipes).

The code required to check for connections to the EXTRA_PORT is greatly
complicated by the fact that the `thd->net->vio->mysql_socket` and
`thd->net->vio->local` data structures are not reliably or fully populated
at the point where they are passed into `parse_client_handshake_packet.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer Amazon Web
Services, Inc.
    @dlenski
    dlenski committed Apr 29, 2024
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  3. Use libmariadb as updated for instant failover 

    The updated libmariadb parses the `ER_INSTANT_FAILOVER` error packets (with
the message component formatted as
`|Human-readable message|value of INSTANT_FAILOVER_TARGET system variable`)
and reconnects accordingly.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer Amazon Web
Services, Inc.
    @dlenski
    dlenski committed Apr 29, 2024
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  4. Add --follow-instant-failovers option to MariaDB CLI 

    This option is ENABLED by default.

The `main.mysqld--help` MTR test is updated as well, to reflect the addition
of this option to the `mysqld --help` output.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer Amazon Web
Services, Inc.
    @dlenski
    dlenski committed Apr 29, 2024
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  5. Add main.instant_failover MTR 

    This MTR test verifies the basic functionality of the
INSTANT_FAILOVER_{MODE,TARGET} system variables, including:

1. `INSTANT_FAILOVER_MODE=ON` as well as `ALL`
2. The handling of the extra port and local-socket connections (redirected
   in mode `ALL`, but not in mode `ON`)
3. The behavior of the client both with `--follow-instant-failovers` (the
   default) and with `--disable-follow-instant-failovers`
4. The client's handling of redirect loops (>=8 redirections causes
   `ER_INSTANT_FAILOVER`)
5. The ability to turn `INSTANT_FAILOVER_MODE` back to `OFF`, and to
   resume connections without redirection.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer Amazon Web
Services, Inc.
    @dlenski
    dlenski committed Apr 29, 2024
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