On Sat, May 23, 2015 at 02:46:23PM +0200, Johannes Schauer wrote: […] > does exactly what I expect would happen by default: It takes only the > packages from experimental where it needs to and gets the rest of the > packages from the repository with higher pin priority (unstable). > > Should it not be possible to do this with apt alone?
tl;dr: Yes it should, but it isn't currently – or not completely and it is kinda unlikely to change soon. The interface this has/will have is pkg/experimental as "pkg" and "pkg -t experimental" have a defined behaviour already which would suprise others if we change here. Long answer: apt-get install pkg/experimental manages to get very simple situations done like if pkg depends on foo>=experimental it will pick the experimental version for foo and continue this for foos versioned dependency on bar. As said, simple situations. It e.g. doesn't work if bar conflicts with foo at the moment. Has problems with provides. The error message if it can't figure something out is borderline useless, … All in all its implemented ATM as a hack. I added it around the time we got ButAutomaticUpgrades and for the same user-group. Also, while reading the title I was actually expecting a step further: Having three repositories pin 900, 800 and 700 doesn't give you a "pick the highest, but make it work in any case". That totally outside anything you can express with apt at all currently… The reason is partly in the design of APT itself: At startup it will figure out the pin value for each version and based on the rules will figure out which version is the installation candidate. This decision is never questioned again. Either all candidates play nicely together or they don't (In other words -t experimental, /experimental or =1.2.3.4 do not say: Install this package from experimental, but create a temporary infinite positive pin for this package version which makes this version the installation candidate). Then, first pass solving is installing all requested packages and acting according to the dependencies these have aka installing the candidates of these packages, too. The last step is the actual resolving step, which we call "problem resolver": It looks if any dependency is unsatisfied after the first pass. That happens usually then two packages conflict with each other for example. This resolver does 1vs1 comparisons (I nickname these cagefights) and does either hold back or remove. Especially note that it can't decide to install anything (else) / not in other versions for fear of computational explosions. The idea behind this solver is as old as apt, so I would actually be surprised if 17 years of research wouldn't have achieved anything.¹ Some screws are changed over the years of course, but nothing entirely ground shacking. This has some benefits actually as many people (and tools) are used to apt functioning a certain way, but the disadvantages are obvious. The reason I worked on EDSP was in fact to be able to overcome this problem in the long run by making it possible to change the resolver used by libapt without throwing away the entire ecosystem as this tends to be a very painful thing just to change the resolver (see yum vs. dnf). ¹ And frankly, also the requirements changed. I think 17 years ago any kind of dependency solving was magic. It still very much is, but we are so much used to it now, that we expect more and more. Unfortunately without purring more resources into it. Instead we declare package management a solved problem… Best regards David Kalnischkies
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