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Reimplement strict pinning by rejecting the non-candidates when
translating the problem from the depcache to the solver. This is
substantially better than restricting the list of alternatives for
an or group to only include allowed ones for debugging purposes,
albeit a bit slower.
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This is the first part of changing from Install() to Enqueue() for
installs, affecting only the versions. For packages, we still have
to resolve the group changing: When propagating cleanly, we don't
have the information as to which group the package was part of,
hence we are no longer able to queue the version selection of
upgrades before obsolete packages, for example, which needs
solving.
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This captures the meaning better
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Long term we should have a propagate queue, this is the minimal change
to keep the behavior identical, a first step on the road.
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These are taken roughly from the MiniSAT paper. We still have a bit
to go in actually encoding all clauses so the reasons are still
variables, and Assume() isn't fully working yet.
Adjust the existing Install()/Reject() code to use these functions,
we already see additional lines in the log that we failed to log
before, and this ensures more consistency.
This is sort of still the wrong direction: Install()/Reject() do the
propagation too; but that is tbd.
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Adopt MiniSAT's strategy for dealing with assignments and choices,
having a single step undoOne() function to undo one and record them
all on the queue; this should likely speed up backtracking since we
no longer need to rescan everything.
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Basically this boils down to checking that the priority of the
source candidate candidate is higher or equal than the priority
of the package's candidate that is being under consideration.
It stands to reason if we maybe we should actually calculate a
source candidate version; this will look similar but may perhaps
perform slightly different.
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More code but nicer to read
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This makes more sense, as all package versions are obsolete that
are not the candidate, usually.
Pay special attention to no-strict-pinning: If we don't have
strict pinning, a package without a candidate version may still
be non-obsolete if the latest version is not obsolete.
Likewise, in no-strict-pinning any later source version that exists
will cause the package to be considered obsolete, rather than just
candidates.
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This has two aspects:
1. For a dependency A | B | C we order the obsolete packages last,
that is, if A is obsolete, this gets reordered to B | C | A,
such that we try to pick non-obsolete packages first to ease
upgrade calculation.
2. When comparing two dependencies, we order dependencies into three
groups: First we satisfy dependencies mentioning only non-installed
(NEW) packages, then we satisfy "normal" dependencies, and finally
we satisfy any dependencies mentioning obsolete packages.
This means for example if you have obsolete libfoo1 and a new
libfoo1t64, that we will see Depends: libfoo1t64 before any
Depends: libfoo1 (which may expand to libfoo1 | libfoo1t64),
so we effectively will have selected "replacement" packages
this way already before getting to older packages where we
would have to choose between the obsolete package and its
replacement.
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Our backtracking is chronological, so we will first
try alternative choices or skip optional items in
later groups.
So installing manually installed packages before automatically
installed ones allows manually installed packages to remove
automatically installed packages easily. If we did automatic
packages first, we'd keep back upgrades for manual packages
or change choices for their dependencies, or would have to
backtrack harder to get back to the right decision level.
That's silly.
Ordering automatically installed packages last also allows
us to calculate autoremovable packages. Since we will have
installed all dependencies from manually installed packages
by the point where we get to automatically installed packages,
everything that will be installed in those Auto groups is
inherently garbage.
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This is a simple backtracking brute-force solver with heurisitcs,
this initial version has the following known gaps:
- Errors are not kept from branches, the error reporting after
backtracking isn't particularly useful.
- We cannot show automatically removed packages
- We cannot replace packages with others
- We do not have conflict-driven clause learning yet
Untested:
- Multi-arch
This solver is fundamentally different in key aspects:
- It solves smaller dependency groups before larger ones, leading
us to avoid installing A in A|B if B is installed more often and
more consistently.
- It only keeps the automatic packages reachable via the strongest
path. Currently it only implements autoremoval, but not display
of autoremoval as we simply enqueue all automatically installed
packages at the end when not doing automatic removal.
This will need some translation where we Solve() first, and then
Solve() again with the automatically installed packages added such
that we can mark them as Garbage for display purposes.
- It does not remove manually installed packages.
Hook the solver in via the EDSP framework, this allows us to achieve
easy initial integration without lots of issues.
A lot of this work was planned and executed in my free time and then
some leaked into work time I suppose.
Implementation notes:
- Restore the full backlog of items
The annoying thing is that we record only when an item was enqueued
and not the level at which it was installed, so when going back a
decision level we might have to reinstall packages that were queued
at an earlier decision level because they were only installed at a
later decision level.
- When picking one version, reject the others
- Propagate conflicts up to reverse dependencies
This will recursively mark every reverse dependency that can
no longer be satisfied as MUSTNOT.
Also make sure to recursively call Reject(Ver) from Reject(Pkg)
to make sure we trigger the Rejections there.
This means we now end up having Recursion in the algorithm. An
alternative approach would be to push *reject* items to the heap
and then do them, but this is not entirely straight forward and
it may simply not be necessary.
- Sort upgrades before other optional installs containing subsets
If I want to upgrade a package A, I schedule A3|A2|A1; if another
thing depends specifically on A1; we'd not be installed. Hence we
need to sort upgrades first.
This only is needed for optional packages; manual packages will
figure this out naturally.
- Rescoring is lazily implemented. Instead of calling make_heap()
after rescoring items, we just mark the items as dirty and reinsert
them. We also only rescore from the main solve loop, Reject() marks
the heap as needing a rescore due to a Conflict (as some versions will
no longer be installable), and RescoreWorkIfNeeded() then will do the
rescoring.
- Recursive unit propagation: Install() and Reject() recursively call
each other to promote decisions across single-version dependencies
(or across not-anymore satisfiable reverse-depends).
- Make Reason constructors explicit, this enhances readability
This makes calls like the one in here be
Reject(object, Reason(otherObject))
Ensuring that it's clear that the 2nd argument is a reason at the
caller side.
- Split Decision into Decision and Hint vs. first draft
When branching/deciding, we do not want to override SHOULD and MAY.
We do not actually use them yet, and we do actually clean them when
backtracking, but let's at least keep the data structure correct.
Convert the enum to a 16-bit integer so we can still fit in the same
space as before.
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