Add option to skip archive_command validation during standby clone

[acastro2]

Description:

Problem

repmgr standby clone performs a hard validation check that fails when archive_mode = on but archive_command = '' (empty string). This creates a conflict with backup solutions that require archive_command to be empty so they can manage WAL archiving themselves.

Error:

ERROR: parameter "archive_command" must be set to a valid command

Relevant code (from repmgr-action-standby.c):

https://github.com/EnterpriseDB/repmgr/blame/e696e28f438fcc73beaaec9a17238286f9a77115/repmgr-action-standby.c#L6443

if (guc_set(conn, "archive_mode", "!=", "off"))
{
    i = guc_set(conn, "archive_command", "!=", "");
    if (i == 0 || i == -1)
    {
        log_error(_("parameter \"archive_command\" must be set to a valid command"));
        exit(ERR_BAD_CONFIG);
    }
}

Real-World Use Case

Veeam Backup & Replication explicitly requires PostgreSQL instances to have:

archive_mode = on
archive_command = ''  # MUST be empty

[Official Veeam documentation](https://helpcenter.veeam.com/docs/backup/vsphere/postgresql_backup.html) states: "The archive_command parameter must not contain any values in PostgreSQL instances."

Veeam manages WAL archiving through its own mechanisms and sets archive_command dynamically during backup operations. Having a pre-configured command interferes with this process.

Current Workarounds (All Problematic)

1. Temporarily set /bin/true then change back after clone

   • Requires manual intervention or complex automation
   • Creates a window where the configuration is wrong for Veeam
   • Not suitable for automated deployments

2. Manual clone with pg_basebackup then register

   • Bypasses repmgr's useful pre-flight checks
   • More error-prone
   • Loses the benefits of repmgr standby clone

Proposed Solution

Add a flag to skip archive_command validation:

repmgr standby clone --skip-archive-validation
# or
repmgr standby clone --no-check-archive-command

This would allow users who understand the implications (and are using external backup solutions) to proceed with empty archive_command.

Alternative Solutions

1. Make the validation a warning instead of a hard error
2. Add a configuration option in repmgr.conf: validate_archive_command = false
3. Only validate if NOT using streaming replication (where archive_command is less critical)

Environment

• PostgreSQL: 15
• repmgr: 5.x
• Backup solution: Veeam Backup & Replication
• OS: Ubuntu 24

Additional Context

According to [PostgreSQL documentation](https://www.postgresql.org/docs/current/runtime-config-wal.html#GUC-ARCHIVE-COMMAND), setting archive_command = '' is a valid configuration for temporarily disabling archiving while keeping archive_mode = on. Many enterprise backup solutions (Veeam, Commvault, etc.) rely on this pattern.

The hard validation in repmgr makes it incompatible with these enterprise backup solutions without workarounds, which is a significant limitation for production deployments.