On Mon, 2021-04-26 at 11:46 +0200, Quentin Schulz wrote:

Hi all,

I submitted a presentation about OVERRIDES, _append, +=, =. and others
for YP Summit 2021 in a month. While sharing the description with some
people in the Yocto community, I've been made aware that I'm missing
some (history) bits about OVERRIDES.

I've been told that it was added as a temporary measure/hack

Not sure who told you that but OVERRIDES has been around since bitbake
(then oemake) was split out from openembedded which is probably around
2004.

and that some had tried to get it removed/reconsidered back in 2015
(been given this link:
https://www.openembedded.org/wiki/OEDEM_2015#Agenda) but it was

already largely (ab)used?

Reading that agenda item, I suspect I was the one who added and discussed
it and it was less about removing OVERRIDES and more about considering
whether there was some better operator/format to clearly differentiate
between a variable name and an override. It was a way to see if anyone had
ideas, no great replacement was identified (but was worth asking IMO).

So now, my questions:
  - why was OVERRIDES implemented in the first place? What was it
trying to resolve?

They're one of the ways OE tried to handle conditional configuration changes
which are needed on a machine or policy (distro) basis and as a way of
allowing customisation. I'd say on balance they've been highly successful at it.

  - why was it considered for removal/reimplementation back in 2015
(or even earlier?)?

Not removal, considered for reimplementation. Is there a better syntax
which could be used? It is an open question, we've not identified one.

  - what made you decide to not go for it? Already too widely used?

We'd need to demonstrate that there was an issue being solved, that there
was enough of a benefit for people to take any pain of migration. The fact
they are widely used is a big factor.

  - is there something you'd have done differently?

I suspect lots of things. Specifically with overrides, I've personally wondered
about whether a specific operator syntax would make sense.

  - what are you thoughts on OVERRIDES today?
  - is there a plan to code a new implementation of OVERRIDES or a
similar mechanism? If so, what are the ideas you have currently on how
to do it? (hence the clickbait title :) )

I think you might find there was actually a newly coded backend
implementation of overrides around the 2015 timeframe as I quite radically
changed the way the datastore worked behind the scenes. There were some
user visible changes but they were minor and mostly corrections making the
behaviour consistent.

What changed behind the scenes was the datastore became dynamic.
Previously you updated OVERRIDES and then called dd.data.update_data().
This was removed in:

http://git.yoctoproject.org/cgit.cgi/poky/commit/bitbake/lib/bb/data_smart
.py?id=e7ccd9071233d66afb0bc72774b0032fb8229fe4

We also changed the default on getVar to expand by default in 2016.

Reading the history of:

http://git.yoctoproject.org/cgit.cgi/poky/log/bitbake/lib/bb/data_smart.py

may give some insight.

Another big change was dependency tracking of variables allowing hash
calculation for signatures. We also mandated overrides by lower case, which
helps parsing as it becomes easier for the code to identify or ignore possible
overrides.

Basically, we needed those fixes for various reasons such as sstate hashes, or
to fix parsing performance issues but we were able to make the existing APIs
mostly work so we didn't change the user visible side. That removed much of
the pressure to change the user visible implementation.

Just FWIW, much of our parsing speed pain now is in the tons of anonymous
python people keep adding, thinking little of the overhead about trying to
parse it all for dependencies and run it all...

Whether my presentation is selected does not matter, still curious
nonetheless :)

Cheers,

Richard

Hi,

I took the occasion to check what is the mechanism of overriding.

Below what myself found in Bitbake User Manual, ch. 3.3. Conditional Syntax (Overrides) "You can use OVERRIDES to ... conditionally append or prepend the value of a variable."
This is the point where I wonder why overriding is discussed in context of append and prepend.
Which element of appending and prepending operations has the nature of overriding other things?

BR
k.d.