A general summary of the issue and proposed solution to avoid for future migrations has been added to subtask T5996, however even when implemented for 1.4 it will control, but not avoid, the problem when migrating from 1.3 --- short story: this is an incompatibility between the legacy and modern parser, and a need to refine the approach towards compatibility.

@L0crian there are no changes that would allow this in 1.4-rc1 and not 1.4-rc3 but, perhaps, for a matter of timing in bringing the interface up (see below), but if you confirm that is the case and can provide the container portion of your config, I can attempt a reproducer.

The examples in T5939 reveal a general issue that, although uncommon, needs to be rationalized. The modern parser requires single backslashes to be escaped, if in quoted strings, however (1) the legacy parser (unlike the modern parser) is inconsistent in quoting values (2) the existing solution escapes all single backslashes not a part of control sequences and makes no attempt to restore single backslashes when possible --- this is safe, but leads to redundant escaping of backslashes as revealed in the examples in T5939.

This turns out to be non-trivial, due to the interaction between legacy and modern behavior; nonetheless, a simple preliminary fix is in testing, and the task will be updated with the details when the PR is complete.

This is resolved in T5923; any other related concerns, reopen or open a new task.

PR:
https://github.com/vyos/vyos-1x/pull/2941

The working implementation is linked below; performance is not yet comparable to the legacy loadFile, though there are obvious points of optimization to be made. The general framework will be submitted for PR to support other development, with load_config as an illustrative example, but not yet a replacement for loadFile.

PR:
https://github.com/vyos/vyos-1x/pull/2902

Firstly, note that this is a failure in boot configuration, and is not related to migration: the log output of vyos-router is misleading, reporting the steps following initialization. On success:
Starting VyOS router: migrate configure
on failure:
Starting VyOS router: migrate configure failed !

Sub-task merged and backported; I will close this as 'Resolved N/A', as described above.

Sub-task T5996.

PR:
https://github.com/vyos/vyos-1x/pull/2899

PR:
https://github.com/vyos/vyos-1x/pull/2898

Okay, I can reproduce the error with an errant copy-paste; not sure how that happened in the reported lines, but if I paste the url as the image name, the resulting error matches. I will leave open for the submitter's response/comments, but otherwise close as 'invalid'.

That is an odd one: I am not able to reproduce with those 1.5-rolling versions, but will raise priority for investigation.

PR
https://github.com/vyos/vyos-1x/pull/2893

An initial implementation of this was put on hold, as there were several missing pieces, namely reload http-api withing an existing configession (T2612) and soft-rollback/soft-commit-confirm (T5249/T5976). With those in place, this can proceed.

@adestis
https://vyos.dev/T5976

Fix linked task.

Closed; work continues in T5528.