pgloader fails parsing standard Ubuntu /etc/mysql/my.cnf containing !includedir

Recent pgloader v3 builds fail on Ubuntu when /etc/mysql/my.cnf contains the standard MySQL !includedir directives.

The error is:

KABOOM!
MISSING-SECTION-HEADER-ERROR: Missing section header; found ! instead. at line 32

An unhandled error condition has been signalled:
   Missing section header; found ! instead. at line 32

This happens before the actual migration starts.

Environment

• Ubuntu 24.04
• MySQL 8.4
• PostgreSQL 16
• pgloader built from source:

pgloader version "3.6.ea152ef"
compiled with SBCL 2.2.9.debian

/etc/mysql/my.cnf

The standard Ubuntu MySQL configuration contains:

# * IMPORTANT: Additional settings that can override those from this file!
#   The files must end with '.cnf', otherwise they'll be ignored.

!includedir /etc/mysql/conf.d/
!includedir /etc/mysql/mysql.conf.d/

In my case:

32  #
33  !includedir /etc/mysql/conf.d/
34  !includedir /etc/mysql/mysql.conf.d/

The reported line number corresponds to this area of /etc/mysql/my.cnf, not to the pgloader command file.

Reproduction

With a normal pgloader command file:

/usr/src/pgloader/build/bin/pgloader /root/migration.load

pgloader immediately exits with:

MISSING-SECTION-HEADER-ERROR: Missing section header; found ! instead. at line 32

There is no ! character anywhere in migration.load:

grep -n '!' /root/migration.load

returns no results.

Changing HOME does not help:

HOME=/root/pgloader-home \
  /usr/src/pgloader/build/bin/pgloader /root/migration.load

produces the same error.

Expected behavior

pgloader should either:

1. support MySQL's !include / !includedir syntax when parsing /etc/mysql/my.cnf, or
2. ignore those directives, or
3. avoid parsing system-wide MySQL configuration files when all connection parameters are already supplied explicitly in the pgloader connection URI.

Why this is important

!includedir is part of the standard /etc/mysql/my.cnf shipped on Ubuntu/Debian installations, so automatically parsing this file with a generic INI parser makes current pgloader builds fail on a very common MySQL setup.

Older pgloader builds / the Mattermost pgloader Docker image do not exhibit this particular parsing error.

This appears to be related to the recently added automatic .my.cnf / /etc/mysql/my.cnf parsing in the v3 code path.

A simple fix may be to skip lines starting with !, similarly to how MySQL-specific include directives are handled elsewhere, or to implement support for !include and !includedir.

Reproduce the failure with /usr/src/pgloader/build/bin/pgloader /root/migration.load and the Ubuntu /etc/mysql/my.cnf containing !includedir. Trace the automatic .my.cnf and /etc/mysql/my.cnf parsing that runs before migration, then verify that these directives are handled or skipped and that an explicitly configured migration starts successfully.