[RFC] V2.1.1: support HAProxy PROXY protocol V1 on MySQL frontends

[noahwilliamsson]

This adds basic support for the HAProxy PROXY protocol V1 on MySQL frontend
client connections. This allows the true client IP to be seen in the output
of SHOW FULL PROCESSLIST in ProxySQL's admin frontend when the ProxySQL
servers sit behind a load balancer such as HAProxy or AWS classic ELBs with
the PROXY protocol feature enabled.

I believe this should resolve #2497 (Support for proxy_protocol for proxysql behind aws load balancer).

The patch adds a Proxy_Protocol class which handles:

• parsing of PROXY protocol headers (only the V1 header supported for now)
• matching client IPs against a list of configured network CIDRs

A new option, proxy_protocol_frontend_nets (list of network CIDRs), is
introduced to allow the PROXY protocol feature to be selectively turned on for
specific subnets where e.g. load balancers are running. This is similar to how
the PROXY protocol is implemented in MariaDB.

This variable can be set either in the config file or via the ProxySQL Admin
interface. Configuration file example:

mysql_variables=
{
    ...
    proxy_protocol_frontend_nets="10.42.0.0/28 127.10.0.0/16"
    ...
}

ProxySQL Admin interface example:

UPDATE global_variables
SET variable_value="192.168.42.0/24"
WHERE variable_name="mysql-proxy_protocol_frontend_nets";
LOAD MYSQL VARIABLES TO RUNTIME;

The network CIDRs are stored as a thread-local variable in the MySQL_Thread
class. It's only updated by MySQL_Thread::refresh_variables().

A new MySQL Data Stream state (DSS), STATE_PROXY_PROTOCOL, is introduced to
handle the initial parsing of the PROXY protocol header. Once this has
completed, the state is reset back to STATE_SERVER_HANDSHAKE.

The patch hooks into the MySQL_Thread's main loop where accept() is called
to handle incoming connections. Next, the actual parsing of the PROXY protocol
header is invoked from the MySQL_Data_Stream::read_pkts() method, which is
also called from MySQL_Thread's main loop (where incoming data is handled).

I've only tested this with a HAProxy configuration block like:

listen proxysql-with-proxy
    bind *:6030
    option tcp-check
    mode tcp
    timeout server 6h
    timeout client 6h
    balance roundrobin
    server proxysql-backend-1 127.0.0.1:6033 check send-proxy
listen proxysql-without-proxy
    bind *:6031
    option tcp-check
    mode tcp
    timeout server 6h
    timeout client 6h
    balance roundrobin
    server proxysql-backend-2 192.168.1.123:6033 check

NOTE: I took the liberty to reindent a block of code in MySQL_Thread.cpp
so please review with whitespace changes hidden.

[pondix]

Automated message: PR pending admin approval for build testing

[noahwilliamsson]

I assume v2.1 is where new development happens so I rebased the changes on top of v2.1.1.

[mirostauder]

Can one of the admins verify this patch?

[noahwilliamsson]

Closing since this appears to have been implemented in #4600 and released with ProxySQL v2.7.0: https://github.com/sysown/proxysql/releases/tag/v2.7.0.