Gentoo Archives: gentoo-dev

From: Brian Harring <ferringb@×××××.com>
To: Zac Medico <zmedico@g.o>
Cc: gentoo-dev@l.g.o
Subject: Re: [gentoo-dev] RFD: EAPI specification in ebuilds
Date: Mon, 12 Mar 2012 08:37:14
Message-Id: 20120312083612.GF7579@localhost
In Reply to: Re: [gentoo-dev] RFD: EAPI specification in ebuilds by Zac Medico
On Sun, Mar 11, 2012 at 09:08:24PM -0700, Zac Medico wrote:
> On 03/11/2012 06:55 PM, Brian Harring wrote: > > On Sat, Mar 10, 2012 at 08:06:50AM -0800, Zac Medico wrote: > >> Yeah. Another way of putting it is that the requirement to spawn a bash > >> process and source the ebuild adds a ridiculous amount of unnecessary > >> complexity, in violation of the KISS principle [1]. > > > > This statement is incorrect. > > > > Even if EAPI could be parsed via some non sourcing approach, we > > *still* have to source the ebuild to get the metadata for when the > > EAPI is supported (the vast majority of usage). That complexity is > > there one way or another- we wouldn't be trying to extract the EAPI > > from the ebuild unless the cache was invalid/missing. > > There are a couple of other cases worth considering: > > 1) User downloads an overlay that doesn't provide cache. We want the > package manager to give a pretty "EAPI unsupported" message, rather than > spit out some bash syntax errors.
This criticsm pretty much applies *strictly to the existing implementation*. It's disenguous busting it in this fashion. EAPI as a function explicitly gives it an out before hitting any of that, eliminating your entire critique. Same goes for parsing it out of the ebuild, or renaming the extension.
> 2) You're assuming that a package manager can validate cache for an EAPI > that it doesn't necessarily support.
Actually, I'm not.
> That's a somewhat fragile > assumption, given the complexities of cache validation, which involve > verification all files that affect metadata and those files may vary > depending on the EAPI.
This is a fair bit of handwavey bullshit. The same "complexities of cache validation" we have to by default deal with for valid/supported EAPIs; the case for unsupported EAPIs is actually simpler than for supported EAPis, and up until 7ddb7d30, was likely working fine in portage. Tweaking portage to restore that support is pretty easy. Also note that with the sole exception of g55, every implementation has to deal with this specific issue, even g55 isn't fully exempt from it since in cleansing a cache, there is the open question of pruning cache entries for ebuilds known, but not understood by the local PM. Either way, the algo is as follows: PM pulls EAPI from the ebuild (regardless of method); PM recognizes that it can't handle it- thus it stores EAPI:-$EAPI, and the normal cache ebuild checksumming; whether it be mtime, md5 for md5-dict, or whatever form new caches choose to use. For attempts to load, the cache subsystem checks that validation before trusting the cache entry- this is in place and has been for a very, very long time. This isn't anything new. So... the entry point is the ebuild; we have checksums for it, and a prefixed versions of it's EAPI stored. The sole deal breaker here is if we were to decide to allow eclasses to set EAPI (rather than the current "you're not supposed to, although we don't explicitly block it"- which could be tightened to be disallowed at the implementation level for EAPI>4). The following scenarios exist: 1) PM still doesn't support that EAPI, looks at the cache/ebuild: checksums are the same, thus the stored EAPI is trustable, leading to the PM knowing it still can't process that ebuild and masking it appropriately. 2) PM still doesn't support that EAPI, looks, sees that the checksums no longer match the ebuild. Re-pulls metadata/EAPI; if the ebuild has been changed to a supported EAPI, continues on its way. If not, stores the checksum/negated EAPI, and masks appropriately. 3) PM now supports that EAPI, looks, sees the negated eapi and recognizes it as one it knowns, and forces regeneration. All of those scenarios are there, and easy enough to handle- this applies for the existing EAPI implementation additionally. Now, this isn't to say that someone can't go and try to structure new features explicitly to defeat the existing validations. They could. The question here is whether or not new features would actually *break* that w/out intentionally going out of their way to do so (aka, skip the strawman arguments, actual examples required).
> > Phrasing it more bluntly: you can only avoid the sourcing step if you > > can isolate that the EAPI is unsupported (which is extremely rare in > > terms of actual user experience). For the rest of the time (well past > > the 99% mark) sourcing is the immediate step following. > > For the sake of being robust in all possible cases, it's just a lot > simpler if we can obtain the EAPI simply and reliably, without spawning > bash to source the ebuild.
This still is arguable; any performant PM is going to have to do the cache dance I mentioned above, or maintain an in memory list of ebuilds it knows it can't handle (which if you want to talk robust, fails miserably if the API isn't designed for sync invalidation of that global list). Complexity, oh my. That's the kicker from where I'm sitting; you're so focused on avoiding bash that you ignore the complexities surrounding it that are unavoidable regardless. The EAPI function route resolve this issue w/out penalizing the normal (aka supported) pathways, and does so at the same robustness as everything sans g55 (which aims to allow us to use things other than bash such as the mocked xml ebuild format). Frankly, the function route gives us a bit more robustness- rather than allowing the scenario of a PM that doesn't parse seeing a version that requires it, via the function route doing `eapi 5 || die` has no such failure mode.
> > Also, stop referencing wikipedia. People know what "trivial > > objection" and "KISS" is. > > You can't assume that. On this list we've got potential to have readers > and responders with lots of different backgrounds.
You might want to include links to Linux, gentoo, and bash while you're at it. Ya know, just in case.
> Your insistence on > using bash to obtain the EAPI would make me wonder if you understood the > KISS principle, if I didn't know you better.
Zing. What I'd like to see, is accuracy in this discussion. Skip the handwavey "complexity! complexity! complexity!" crap, same for selective robustness definitions. Past attempts at this discussion mostly failed due to people pulling crap like this and frankly it just pisses people off. Like it or not, each solution has technical pros/cons, more importantly, it has an impact on developers meaning what they *like* carries weight (rather than what you want them to do). Shebang/parsing EAPI w/out bash, EAPI as a function, all are reliant on the same cache mechanism. That's fact. G55 is the sole exception. The question of what mechanism to use is dependant on what people actually want, and how far we're willing to stretch the ebuild format (versus defining a new one rather than new versions of it). For the lifespan of gentoo thus far, our format has been bash; trying to shift away from bash is fine, but as long as it *is* bash we should be designing w/ that flow, including what is least likely to have a developer screw up. Pragmatic reality, especially since we're still stuck w/ compatibility issues getting to the new mechanism. Frankly I'm starting to think I should just write up a table of pros/cons, extending what Ulm has started; in particular, including accurate statement of the transition costs of each (something people seem to be ignoring- compatibility mechanisms are *not* cheap to get in place). ~harring


Subject Author
Re: [gentoo-dev] RFD: EAPI specification in ebuilds Zac Medico <zmedico@g.o>
Re: [gentoo-dev] RFD: EAPI specification in ebuilds Ciaran McCreesh <ciaran.mccreesh@××××××××××.com>