op-mode: T6471: add optimized get_config_dict

[jestabro]

Change Summary

Add a faster get_config_dict for use by op-mode commands. This function avoids the overhead of reading the full config-dict in Config (implicitly used by ConfigTreeQuery) --- the overhead is necessary for config-mode, and is moderated by vyos-configd and caching; it is unnecessary and slow in op-mode, which has no daemon (yet).

Note that a low-level Popen is used instead of a vyos.utils.process function, as it was noticed that as well incurred a heavy performance hit: this will be investigated in a separate task.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):
  Performance improvement available for op-mode scripts.

Related Task(s)

Related PR(s)

Component(s) name

Proposed changes

How to test

Loading a non-trivial config (using vyos-1x/smoketest/configs/bgp-big-as-cloud), test with a Python script, as one would call in a op-mode show command, with interpreter startup and import overhead intact:

#!/usr/bin/env python3

import argparse

from vyos.configquery import ConfigTreeQuery
from vyos.configquery import op_mode_config_dict

parser = argparse.ArgumentParser()
parser.add_argument('--path', nargs='*', help='path to subtree')
parser.add_argument('--op-mode', action='store_true', help='use op-mode version')
parser.add_argument('--no-print', action='store_true', help='no output, for timing test')
args = parser.parse_args()

path = args.path

if args.op_mode:
    out = op_mode_config_dict(path)
else:
    out = ConfigTreeQuery().get_config_dict(path)
        
if not args.no_print:
    print(out)

Results (representative example over repeated runs):

vyos@vyos:~$ time ./config_dict.py --path interfaces ethernet --no-print

real    0m0.373s
user    0m0.156s
sys     0m0.212s
vyos@vyos:~$ time ./config_dict.py --path interfaces ethernet --no-print --op-mode

real    0m0.133s
user    0m0.083s
sys     0m0.050s

Smoketest result

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

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