FR - allow fetching username and password

[mikegriffin]

It might be useful in some environments, where credentials are not stored locally, to allow arbitrary commands to fetch them, eg:

[mysql:client]
credential-method=fetch
user-fetch-cmd=curl foo1 | base64 --decode
passwd-fetch-cmd=curl foo2 | base64 --decode

These results should probably be stored in a memory, ie not in an environment variable or file

[mikegriffin]

If this is to be implemented, it could probably be similar to *-backup-command hooks in commands/backup.py:

def call_hooks(event, entry):
    """
    Rerun pre or post hooks
    """
    hook = event + "-command"

    if entry.config["holland:backup"][hook] is not None:
        cmd = entry.config["holland:backup"][hook]
        try:
            cmd = Template(cmd).safe_substitute(
                hook=hook, backupset=entry.backupset, backupdir=entry.path
            )
            LOG.info(" [%s]> %s", hook, cmd)
            process = Popen(
                cmd,
                shell=True,
                stdin=open("/dev/null", "r"),
                stdout=PIPE,
                stderr=PIPE,
                close_fds=True,
            )
            output, errors = process.communicate()
        except OSError as exc:
            raise BackupError("%s" % exc)

        for line in errors.splitlines():
            LOG.error(" ! %s", line)
        for line in output.splitlines():
            LOG.info(" + %s", line)
        if process.returncode != 0:
            raise BackupError("%s command failed" % hook)
    return 0