Re: RFC: Modularity Simplified

Hey Igor, On Tue, 26 Nov 2019 at 17:00, Igor Gnatenko <ignatenkobrain(a)fedoraproject.org&gt; wrote: > > Hello fellows, > > After last publication on LWN about Fedora Modularity mess, I think it > is time to describe the idea I was proposing internally with few other > folks (Adam Samalik, Brian Exelbierd) back in the RH times. > > Before I actually go deep, I'll try to answer main questions to myself > (so that you can understand why I am proposing this particular thing). > > 1. Do we want to package multiple streams only for "leaf" software or > any kind of it? > I believe that we need both, and we do support both. However, it might > not look as nice as it could: > * Need to create multiple repos for different "streams" > * Need to maintain epel7/epel8/f30/f31/master branches > * Package names have to be "mangled" (e.g. mozjs38, mozjs60) and it is > manual work > * However, those are supposed to be (according to the guidelines) > parallel-installable (and not be conflicting in any way) > Doing git merge / git cherry-pick and maintaining many git remotes > requires some advanced git knowledge and a time. And we can't actually > have multiple versions of a package with same name (without "mangled" > names) in a repo due to the way how our buildsystem works (and not > only buildsystem, with some caveats). > We do have some kind of a solution for multiple releases building from > one branch (package.cfg), however this work has been never finished, > thus there are many problems with this approach. > > 2. Do we want to support buildtime-only packages? > I would rather generalize this category as "less-supported packages". > I maintain 800+ Rust packages and very often I need to update them to > an incompatible version. In Rawhide I just do it, update all dependent > packages to use new version, and if I can't do that for some reason, > create "compat" package. Obviously, all patches are sent to the > upstream. > Upstreams are removing features, I need to deal with Obsoletes but I > simply can't continuously add new Obsoletes into the > fedora-obsolete-packages. And what for if they are used only during > build of other, more important, packages? Why do I have to spend time > with upgradepaths? I definitely want some mechanism which will tell to > user that "THIS PACKAGE IS NOT FULLY SUPPORTED." > Obviously, for packages which are used in runtime need a proper > support as we do today for all packages to share work (that's the > place where I agree with Kevin Kofler.) > > 3. Can we have different lifecycles for the software in Fedora? > Right now we have to keep all versions of software which was there at > GA point. And from the updates repo. We never remove packages > entirely. That said, if package was there at GA time, it will have to > be "supported" until the end of that Fedora release. > I don't think we can (should?) do much in this regard. If we make > packages to build from "stream" branch, we can put an information to > that branch that this package should be built for all distributions > (Fedora, EPEL) until this date. Or even store this info somewhere else > like PDC (yes, I know we want to kill it). fedpkg build will take this > into account and submit proper builds. And we can design some API in > infra which would tell until when some particular stream of a package > is supported. Much like it is done with Fedora releases & GNOME > Software. > > 4. Do we want to have some kind of "stream expansion" where software > builds against all Pythons, Perls and whatnot? > I think this should be conscious choice of a distribution, and in > specific cases, maintainer. Using some examples from previous threads, > why does bugzilla have to be built against 2 different versions of > perl and users could choose? I think maintainer should choose one > version of perl and let bugzilla use it. Being able to build > combinations of software is definitely nice, but I don't think this > should be standard practice. openSUSE does that with ruby and python > (they build all modules automatically for all versions) and I like > this. But having all packages built against all is just combinatoric > explosion. Given how many updates have feedback in Bodhi, I'm pretty > sure 99% of combination won't be tested (or even installed?) ever. > > 5. Are we still trying to be a Linux distribution or we are just > letting people to do whatever they want in our infrastructure? > This question bothers me from time to time and I don't have answer. > For example, Modularity is very flexible and I very often find people > saying that you can expand this and that, but in Fedora we limit its > usefulness. Do we actually need to develop something like this > (knowing in advance that probably nobody outside of Fedora/RHEL will > be using this software / technology)? Are we trying to create > technologies which would be very extensible and used in other > distributions instead of solving some specific problems? If former, > why don't we talk to others about things in advance and not getting > other people to work on these cool things? > > === > > How I would imagine having multiple streams: > * rpms/nodejs has multiple branches - 10, 11, 12 > * in all of them, nodejs.spec exists as-is without "mangled" name > (just with different versions and such) > * each has package.cfg or some alternative which specifies what is the > EOL and/or for which releases to build > * when builds are submitted, Koji automatically adds suffix to a main > package and subpackages containing branch name > * during the rpmbuild, extra provides are added (for the unversioned > names and indication of a stream) and conflicts (possibly depending on > some macro which user can disable) automatically > > Notes: > * If software needs to specify that it wants 9 ≤ nodejs ≤ 11 it can > describe this in standard way (Requires: (nodejs >= 9 with nodejs <= > 11)) and one will be picked up automatically > * If user wants to switch from 9 to 10, he can run `dnf swap nodejs9 nodejs10` > * If that requires some conflicting dependency to be switched, it will > be switched automatically > * Packages produced from nodejs.src have Provides: stream(nodejs) > stream(nodejs:9) and Conflicts: stream(nodejs) so that it is > explicitly not possible to install 9 and 10 at the same time > * If desired, packages can depend on a specific stream via Requires: > stream(nodejs:10) without actually depending on nodejs (this requires > some small piece of code in libsolv) > * In the very similar way, user can lock themselves to a specific > stream by writing some conf file which DNF would read (if point above > is implemented, about just tens of lines of the code in libdnf, or > even libsolv can be teached about that as well) > * DNF can be teached about these special things so that you can > automatically swap conflicting dependencies, but lock yourself to some > streams of a package > * All standard Conflicts/Requires/Obsoletes/… will simply work > * With side tags on demand it is now much easier > * Mass rebuild scripts will be teached about stream branches and will > build packages properly > * After branching, there will be a script which untags all packages > which should not be supported in the new release > * fedpkg will be teached about stream branches and will show which > branches build where, so that maintainers can easily maintain them > * Packaging guidelines should be adopted to accept conflicting > packages and tooling should be improved to show the conflicts and how > to resolve them > * fedora-release will have Suggests: stream(nodejs:10) so that when > user types dnf install nodejs, for example nodejs10 would be picked up > (instead of nodejs12) > * Some macros can be added to generate virtual package with "default > stream", so that there will be actual nodejs package depending on > nodejs12 if that would be default in given Fedora release > * Going further, I would extend Koji with fedora plugin which would > deal with the build submission instead of having this logic on clients > so that all people get same result > > === > > Now let me quote Matthew Miller with the list of requirements and > answer how we achieve them. > > 1. Users should have alternate streams of software available. > I think this is achieved with what I have said above. We will ship > nodejs9, nodejs10 at the same time. > > 2. Those alternate streams should be able to have different lifecycles. > nodejs 9 and 10 can automatically build for all distros until EOL > (which would be stored somewhere). EOL-date aware tooling (fedpkg) > would not build for excluded versions of distros and build for distros > where EOL of distro is further than EOL of component. > > 3. Packaging an individual stream for multiple outputs should be > easier than before. > As I wrote above, we will have one nodejs rpm repo with multiple > branches (aka streams) and no name mangling needed. fedpkg build will > spawn builds for all releases matching criteria (EOL date, excluded > distributions). > > === > > This is not the exact approach we have tried back in the days, but it > is very similar to that. Unfortunately I did not save any documents > from those times but Adam or Brian probably still can find them (since > they are RH internal). I agree that the approach is you described is good. From you have written, it seems to me that Stream could be an explicit rpm property. Then the names wouldn't be mangled just extended: ENSVRA. Anyway, i would just go with your approach for start. That way, one can avoid bothering rpm team unless necessary. This is really the only sensible implementation of modularity (i.e. providing multiple version of software) and the reason is that it provides the functionality required while leveraging already existing binary format to provide the new content: RPM, which makes matters on user-side much easier. It would be good to make this change as it will send Fedora (and all the other distros that use modularity) in the right direction again. Persistently going the wrong way and pretending it is the right one is not smart.