Rename reindex and friend's `size` parameter to `max_docs`

[nik9000]

Reindex's size parameter is very easy to confuse with batch size. We should rename it. Standard deprecate in some 5.x release and rename in 6.0 logic should apply here, I think.

[nik9000]

I've marked this adoptme because I'm not sure when I'll get to it. I'll keep it on my list but if someone else wants to start playing with reindex I'd be happy to provide moral support and reviews.

[glefloch]

Hi @nik9000, can I start working on it?

[nik9000]

@glefloch, sure! This is probably a two PR thing - one to add the new name and deprecate the old name that'll be applied to master and 5.x and a followup to remove the old name that is only applied to master.

Adding the new name is a thing you'd do in RestReindexAction and AbstractBulkByQueryRestHandler. At least, I'd start with those classes and dig from there.

[glefloch]

@nik9000, by deprecating the old name, you mean handling both settings and maybe add some logs if the old setting is still in use?

[nik9000]

@nik9000, by deprecating the old name, you mean handling both settings and maybe add some logs if the old setting is still in use?

Right. We have lots of examples of code that does this. The change to RestReindexAction might be as simple as adding the new name to the ParseField definition and testing it. The change to AbstractBulkByQueryRestHandler is probably a bit more complex.

[glefloch]

@nik9000, After looking at the code, the first first modification is really easy. For AbstractBulkByQueryRestHandler class, the size is set from search request is that normal?
Is there any modification to do in this class?

[nik9000]

The second modification is indeed a lot trickier because of reindex's reuse of the search APIs for parsing. I'd like to figure out a nicer way to reuse that one day, but it is low on my list. I'm not totally sure the right way to handle it. It might require that cleanup I've been putting off.