mariadb-backup compatible wrappers for BACKUP SERVER

The server-side BACKUP SERVER statement (MDEV-14992) is a fundamentally different way of performing backups, compared to the current mariadb-backup tool. Likewise, streaming backup (MDEV-38362) will necessarily be quite different from what we have now.

While many concepts are different, it should be possible to implement a partial compatibility layer for the mariadb-backup and mbstream tools, so that existing scripts around backup can continue to work with minimal changes.

The mariadb-backup compatible tool would invoke either BACKUP SERVER in a client connection for backup operation, or mariadbd itself for prepare operation. Some operations, such as copy_back or move_back, could be implemented by invoking mysys functions. The tool would also have to examine the metadata in the specified directory.

This will have to be designed in detail once MDEV-14992 has been completed.

Designing and implementing support for streaming operations is obviously blocked by MDEV-38362. The mbstream compatible tool would be a wrapper for tar x, assuming that we adopt the POSIX tar format.

Currently, mbstream supported options are

-x   or --extract

-c  or  --create

-C  or --directory

-v  or --verbose

-p  or --parallel

-?  or --help

All options except --parallel or -p are available in the GNU tar tool.

A minimal mbstream work-a-like would be a shell script like this:

#!/bin/sh

# Note: We do not try to reject options that are valid in tar and invalid in mbstream

# Note: --parallel is not valid in GNU tar

# Note: -p (--preserve-permissions) in GNU tar does not take an argument like it does in mbstream

exec tar "$@"

An alternative for interactive use would be to add the following to $HOME/.bashrc or the start-up of a compatible shell:

alias mbstream=tar

To have a true drop-in replacement, a better wrapper script that properly filters out p and -parallel can be built around the getopt(1) utility. We will need a test suite that demonstrates that all parameters are parsed in a compatible way. Special attention needs to be paid on $IFS (for example, if some arguments contain spaces) and on variants of passing arguments: mbstream -xp4 and mbstream -p4 x are valid, mbstream -p4x or mbstream -px 4 are not.

For mariadb-backup we would create a nontrivial Bourne shell script that would invoke tools like mariadbd, mariadb, cp, mv as needed.

These wrapper scripts would be called mariadb-backup and mbstream just like their legacy counterparts, but unlike them, installed in a directory that is not in the $PATH. To put them into use, one would execute a command like the following:

export PATH=/path/to/the/wrappers:"$PATH"

or

PATH=/path/to/the/wrappers:"$PATH" my-legacy-backup-script

When a tweaked $PATH is in effect, the command names mariadb-backup and mbstream would invoke the wrapper scripts, not the legacy utilities.