zeroinstall/injector/solver.py

   1 """
   2 Chooses a set of components to make a running program.
   3 """
   4
   5 # Copyright (C) 2009, Thomas Leonard
   6 # See the README file for details, or visit http://0install.net.
   7
   8 from zeroinstall import _
   9 import locale
  10 from logging import debug, warn, info
  11
  12 from zeroinstall.injector.reader import MissingLocalFeed
  13 from zeroinstall.injector.arch import machine_groups
  14 from zeroinstall.injector import model, sat, selections
  15
  16 class CommandInfo:
  17         def __init__(self, name, command, impl, arch):
  18                 self.name = name
  19                 self.command = command
  20                 self.impl = impl
  21                 self.arch = arch
  22
  23         def __repr__(self):
  24                 name = "%s_%s_%s_%s" % (self.impl.feed.get_name(), self.impl.get_version(), self.impl.arch, self.name)
  25                 return name.replace('-', '_').replace('.', '_')
  26
  27 class ImplInfo:
  28         is_dummy = False
  29
  30         def __init__(self, iface, impl, arch, dummy = False):
  31                 self.iface = iface
  32                 self.impl = impl
  33                 self.arch = arch
  34                 if dummy:
  35                         self.is_dummy = True
  36
  37         def __repr__(self):
  38                 name = "%s_%s_%s" % (self.impl.feed.get_name(), self.impl.get_version(), self.impl.arch)
  39                 return name.replace('-', '_').replace('.', '_')
  40
  41 def _get_command_name(runner):
  42         """Returns the 'command' attribute of a <runner>, or 'run' if there isn't one."""
  43         return runner.qdom.attrs.get('command', 'run')
  44
  45 class _DummyImpl(object):
  46         requires = []
  47         version = None
  48         arch = None
  49         commands = {}
  50
  51         def __repr__(self):
  52                 return "dummy"
  53
  54         feed = property(lambda self: self)
  55
  56         def get_version(self):
  57                 return "dummy"
  58
  59         def get_name(self):
  60                 return "dummy"
  61
  62 class Solver(object):
  63         """Chooses a set of implementations to satisfy the requirements of a program and its user.
  64         Typical use:
  65          1. Create a Solver object and configure it
  66          2. Call L{solve}.
  67          3. If any of the returned feeds_used are stale or missing, you may like to start downloading them
  68          4. If it is 'ready' then you can download and run the chosen versions.
  69         @ivar selections: the chosen implementation of each interface
  70         @type selections: L{selections.Selections}
  71         @ivar requires: the selected dependencies for each chosen version
  72         @type requires: {L{model.Interface}: [L{model.Dependency}]}
  73         @ivar feeds_used: the feeds which contributed to the choice in L{selections}
  74         @type feeds_used: set(str)
  75         @ivar record_details: whether to record information about unselected implementations
  76         @type record_details: {L{Interface}: [(L{Implementation}, str)]}
  77         @ivar details: extra information, if record_details mode was used
  78         @type details: {str: [(Implementation, comment)]}
  79         """
  80         __slots__ = ['selections', 'requires', 'feeds_used', 'details', 'record_details', 'ready']
  81
  82         def __init__(self):
  83                 self.selections = self.requires = self.feeds_used = self.details = None
  84                 self.record_details = False
  85                 self.ready = False
  86
  87         def solve(self, root_interface, root_arch, command_name = 'run'):
  88                 """Get the best implementation of root_interface and all of its dependencies.
  89                 @param root_interface: the URI of the program to be solved
  90                 @type root_interface: str
  91                 @param root_arch: the desired target architecture
  92                 @type root_arch: L{arch.Architecture}
  93                 @param command_name: which <command> element to select
  94                 @type command_name: str | None
  95                 @postcondition: self.ready, self.selections and self.feeds_used are updated"""
  96                 raise NotImplementedError("Abstract")
  97
  98 class SATSolver(Solver):
  99         """Converts the problem to a set of pseudo-boolean constraints and uses a PB solver to solve them.
 100         @ivar langs: the preferred languages (e.g. ["es_ES", "en"]). Initialised to the current locale.
 101         @type langs: str"""
 102
 103         __slots__ = ['_failure_reason', 'config', 'extra_restrictions', '_lang_ranks', '_langs']
 104
 105         @property
 106         def iface_cache(self):
 107                 return self.config.iface_cache  # (deprecated; used by 0compile)
 108
 109         def __init__(self, config, extra_restrictions = None):
 110                 """
 111                 @param config: policy preferences (e.g. stability), the iface_cache and the stores to use
 112                 @type config: L{policy.Config}
 113                 @param extra_restrictions: extra restrictions on the chosen implementations
 114                 @type extra_restrictions: {L{model.Interface}: [L{model.Restriction}]}
 115                 """
 116                 Solver.__init__(self)
 117                 assert not isinstance(config, str), "API change!"
 118                 self.config = config
 119                 self.extra_restrictions = extra_restrictions or {}
 120
 121                 # By default, prefer the current locale's language first and English second
 122                 self.langs = [locale.getlocale()[0] or 'en', 'en']
 123
 124         def set_langs(self, langs):
 125                 """Set the preferred languages.
 126                 @param lang: languages (and regions), first choice first
 127                 @type lang: [str]
 128                 """
 129                 # _lang_ranks is a map from locale string to score (higher is better)
 130                 _lang_ranks = {}
 131                 score = 0
 132                 i = len(langs)
 133                 # (is there are duplicates, the first occurance takes precedence)
 134                 while i > 0:
 135                         i -= 1
 136                         lang = langs[i].replace('_', '-')
 137                         _lang_ranks[lang.split('-')[0]] = score
 138                         _lang_ranks[lang] = score + 1
 139                         score += 2
 140                 self._langs = langs
 141                 self._lang_ranks = _lang_ranks
 142
 143         langs = property(lambda self: self._langs, set_langs)
 144
 145         def compare(self, interface, b, a, arch):
 146                 """Compare a and b to see which would be chosen first.
 147                 Does not consider whether the implementations are usable (check for that yourself first).
 148                 @param interface: The interface we are trying to resolve, which may
 149                 not be the interface of a or b if they are from feeds.
 150                 @rtype: int"""
 151
 152                 # Languages we understand come first
 153                 a_langs = (a.langs or 'en').split()
 154                 b_langs = (b.langs or 'en').split()
 155                 my_langs = self._lang_ranks
 156
 157                 r = cmp(max(my_langs.get(l.split('-')[0], -1) for l in a_langs),
 158                         max(my_langs.get(l.split('-')[0], -1) for l in b_langs))
 159                 if r: return r
 160
 161                 a_stab = a.get_stability()
 162                 b_stab = b.get_stability()
 163
 164                 # Preferred versions come first
 165                 r = cmp(a_stab == model.preferred, b_stab == model.preferred)
 166                 if r: return r
 167
 168                 stores = self.config.stores
 169                 if self.config.network_use != model.network_full:
 170                         r = cmp(a.is_available(stores), b.is_available(stores))
 171                         if r: return r
 172
 173                 # Packages that require admin access to install come last
 174                 r = cmp(b.requires_root_install, a.requires_root_install)
 175                 if r: return r
 176
 177                 # Stability
 178                 stab_policy = interface.stability_policy
 179                 if not stab_policy:
 180                         if self.config.help_with_testing: stab_policy = model.testing
 181                         else: stab_policy = model.stable
 182
 183                 if a_stab >= stab_policy: a_stab = model.preferred
 184                 if b_stab >= stab_policy: b_stab = model.preferred
 185
 186                 r = cmp(a_stab, b_stab)
 187                 if r: return r
 188
 189                 # Newer versions come before older ones
 190                 if a.id.startswith('package:') != b.id.startswith('package:'):
 191                         # If one of packages is native, do not compare full versions since
 192                         # it is useless to compare native and 0install version revisions
 193                         r = cmp(a.version[0], b.version[0])
 194                         if r: return r
 195                         # Othewise, prefer native package
 196                         return cmp(a.id.startswith('package:'), b.id.startswith('package:'))
 197                 else:
 198                         r = cmp(a.version, b.version)
 199                         if r: return r
 200
 201                 # Get best OS
 202                 r = cmp(arch.os_ranks.get(b.os, None),
 203                         arch.os_ranks.get(a.os, None))
 204                 if r: return r
 205
 206                 # Get best machine
 207                 r = cmp(arch.machine_ranks.get(b.machine, None),
 208                         arch.machine_ranks.get(a.machine, None))
 209                 if r: return r
 210
 211                 # Slightly prefer languages specialised to our country
 212                 # (we know a and b have the same base language at this point)
 213                 r = cmp(max(my_langs.get(l, -1) for l in a_langs),
 214                         max(my_langs.get(l, -1) for l in b_langs))
 215                 if r: return r
 216
 217                 # Slightly prefer cached versions
 218                 if self.config.network_use == model.network_full:
 219                         r = cmp(a.is_available(stores), b.is_available(stores))
 220                         if r: return r
 221
 222                 return cmp(a.id, b.id)
 223
 224         def solve(self, root_interface, root_arch, command_name = 'run', closest_match = False):
 225                 # closest_match is used internally. It adds a lowest-ranked
 226                 # by valid implementation to every interface, so we can always
 227                 # select something. Useful for diagnostics.
 228
 229                 # TODO: We need some way to figure out which feeds to include.
 230                 # Currently, we include any feed referenced from anywhere but
 231                 # this is probably too much. We could insert a dummy optimial
 232                 # implementation in stale/uncached feeds and see whether it
 233                 # selects that.
 234                 iface_cache = self.config.iface_cache
 235
 236                 problem = sat.SATProblem()
 237
 238                 impl_to_var = {}        # Impl -> sat var
 239                 self.feeds_used = set()
 240                 self.requires = {}
 241                 self.ready = False
 242                 self.details = self.record_details and {}
 243
 244                 self.selections = None
 245                 self._failure_reason = None
 246
 247                 ifaces_processed = set()
 248
 249                 impls_for_machine_group = {0 : []}              # Machine group (e.g. "64") to [impl] in that group
 250                 for machine_group in machine_groups.values():
 251                         impls_for_machine_group[machine_group] = []
 252
 253                 impls_for_iface = {}    # Iface -> [impl]
 254
 255                 group_clause_for = {}   # Iface URI -> AtMostOneClause | bool
 256                 group_clause_for_command = {}   # (Iface URI, command name) -> AtMostOneClause | bool
 257
 258                 # Return the dependencies of impl that we should consider.
 259                 # Skips dependencies if the use flag isn't what we need.
 260                 # (note: impl may also be a model.Command)
 261                 def deps_in_use(impl, arch):
 262                         for dep in impl.requires:
 263                                 use = dep.metadata.get("use", None)
 264                                 if use not in arch.use:
 265                                         continue
 266                                 yield dep
 267
 268                 # Add a clause so that if requiring_impl_var is True then an implementation
 269                 # matching 'dependency' must also be selected.
 270                 # Must have already done add_iface on dependency.interface.
 271                 def find_dependency_candidates(requiring_impl_var, dependency):
 272                         dep_iface = iface_cache.get_interface(dependency.interface)
 273                         dep_union = [sat.neg(requiring_impl_var)]       # Either requiring_impl_var is False, or ...
 274                         for candidate in impls_for_iface[dep_iface]:
 275                                 for r in dependency.restrictions:
 276                                         if candidate.__class__ is not _DummyImpl and not r.meets_restriction(candidate):
 277                                                 #warn("%s rejected due to %s", candidate.get_version(), r)
 278                                                 if candidate.version is not None:
 279                                                         break
 280                                 else:
 281                                         c_var = impl_to_var.get(candidate, None)
 282                                         if c_var is not None:
 283                                                 dep_union.append(c_var)
 284                                         # else we filtered that version out, so ignore it
 285
 286                         assert dep_union
 287                         problem.add_clause(dep_union)
 288
 289                 def is_unusable(impl, restrictions, arch):
 290                         """@return: whether this implementation is unusable.
 291                         @rtype: bool"""
 292                         return get_unusable_reason(impl, restrictions, arch) != None
 293
 294                 def get_unusable_reason(impl, restrictions, arch):
 295                         """
 296                         @param impl: Implementation to test.
 297                         @type restrictions: [L{model.Restriction}]
 298                         @return: The reason why this impl is unusable, or None if it's OK.
 299                         @rtype: str
 300                         @note: The restrictions are for the interface being requested, not the feed
 301                         of the implementation; they may be different when feeds are being used."""
 302                         for r in restrictions:
 303                                 if not r.meets_restriction(impl):
 304                                         return _("Incompatible with another selected implementation")
 305                         stability = impl.get_stability()
 306                         if stability <= model.buggy:
 307                                 return stability.name
 308                         if (self.config.network_use == model.network_offline or not impl.download_sources) and not impl.is_available(self.config.stores):
 309                                 if not impl.download_sources:
 310                                         return _("No retrieval methods")
 311                                 return _("Not cached and we are off-line")
 312                         if impl.os not in arch.os_ranks:
 313                                 return _("Unsupported OS")
 314                         if impl.machine not in arch.machine_ranks:
 315                                 if impl.machine == 'src':
 316                                         return _("Source code")
 317                                 return _("Unsupported machine type")
 318                         return None
 319
 320                 def usable_feeds(iface, arch):
 321                         """Return all feeds for iface that support arch.
 322                         @rtype: generator(ZeroInstallFeed)"""
 323                         yield iface.uri
 324
 325                         for f in iface_cache.get_feed_imports(iface):
 326                                 # Note: when searching for src, None is not in machine_ranks
 327                                 if f.os in arch.os_ranks and \
 328                                    (f.machine is None or f.machine in arch.machine_ranks):
 329                                         yield f.uri
 330                                 else:
 331                                         debug(_("Skipping '%(feed)s'; unsupported architecture %(os)s-%(machine)s"),
 332                                                 {'feed': f, 'os': f.os, 'machine': f.machine})
 333
 334                 def add_iface(uri, arch):
 335                         """Name implementations from feed and assert that only one can be selected."""
 336                         if uri in ifaces_processed: return
 337                         ifaces_processed.add(uri)
 338
 339                         iface = iface_cache.get_interface(uri)
 340
 341                         impls = []
 342                         for f in usable_feeds(iface, arch):
 343                                 self.feeds_used.add(f)
 344                                 debug(_("Processing feed %s"), f)
 345
 346                                 try:
 347                                         feed = iface_cache.get_feed(f)
 348                                         if feed is None: continue
 349                                         #if feed.name and iface.uri != feed.url and iface.uri not in feed.feed_for:
 350                                         #       info(_("Missing <feed-for> for '%(uri)s' in '%(feed)s'"), {'uri': iface.uri, 'feed': f})
 351
 352                                         if feed.implementations:
 353                                                 impls.extend(feed.implementations.values())
 354
 355                                         distro_feed_url = feed.get_distro_feed()
 356                                         if distro_feed_url:
 357                                                 self.feeds_used.add(distro_feed_url)
 358                                                 distro_feed = iface_cache.get_feed(distro_feed_url)
 359                                                 if distro_feed.implementations:
 360                                                         impls.extend(distro_feed.implementations.values())
 361                                 except MissingLocalFeed, ex:
 362                                         warn(_("Missing local feed; if it's no longer required, remove it with:") +
 363                                                         '\n0install remove-feed ' + iface.uri + ' ' + f,
 364                                                 {'feed': f, 'interface': iface, 'exception': ex})
 365                                 except Exception, ex:
 366                                         warn(_("Failed to load feed %(feed)s for %(interface)s: %(exception)s"), {'feed': f, 'interface': iface, 'exception': ex})
 367                                         #raise
 368
 369                         impls.sort(lambda a, b: self.compare(iface, a, b, arch))
 370
 371                         impls_for_iface[iface] = filtered_impls = []
 372
 373                         my_extra_restrictions = self.extra_restrictions.get(iface, [])
 374
 375                         if self.record_details:
 376                                 self.details[iface] = [(impl, get_unusable_reason(impl, my_extra_restrictions, arch)) for impl in impls]
 377
 378                         rank = 1
 379                         var_names = []
 380                         for impl in impls:
 381                                 if is_unusable(impl, my_extra_restrictions, arch):
 382                                         continue
 383
 384                                 filtered_impls.append(impl)
 385
 386                                 assert impl not in impl_to_var
 387                                 v = problem.add_variable(ImplInfo(iface, impl, arch))
 388                                 impl_to_var[impl] = v
 389                                 rank += 1
 390                                 var_names.append(v)
 391
 392                                 if impl.machine and impl.machine != 'src':
 393                                         impls_for_machine_group[machine_groups.get(impl.machine, 0)].append(v)
 394
 395                                 for d in deps_in_use(impl, arch):
 396                                         debug(_("Considering dependency %s"), d)
 397
 398                                         add_iface(d.interface, arch.child_arch)
 399
 400                                         # Must choose one version of d if impl is selected
 401                                         find_dependency_candidates(v, d)
 402
 403                         if closest_match:
 404                                 dummy_impl = _DummyImpl()
 405                                 dummy_var = problem.add_variable(ImplInfo(iface, dummy_impl, arch, dummy = True))
 406                                 var_names.append(dummy_var)
 407                                 impl_to_var[dummy_impl] = dummy_var
 408                                 filtered_impls.append(dummy_impl)
 409
 410                         # Only one implementation of this interface can be selected
 411                         if uri == root_interface:
 412                                 if var_names:
 413                                         clause = problem.at_most_one(var_names)
 414                                         problem.add_clause(var_names)   # at least one
 415                                 else:
 416                                         problem.impossible()
 417                                         clause = False
 418                         elif var_names:
 419                                 clause = problem.at_most_one(var_names)
 420                         else:
 421                                 # Don't need to add to group_clause_for because we should
 422                                 # never get a possible selection involving this.
 423                                 return
 424
 425                         assert clause is not True
 426                         assert clause is not None
 427                         if clause is not False:
 428                                 group_clause_for[uri] = clause
 429
 430                 def add_command_iface(uri, arch, command_name):
 431                         """Add every <command> in interface 'uri' with this name.
 432                         Each one depends on the corresponding implementation and only
 433                         one can be selected. If closest_match is on, include a dummy
 434                         command that can always be selected."""
 435
 436                         # Check whether we've already processed this (interface,command) pair
 437                         existing = group_clause_for_command.get((uri, command_name), None)
 438                         if existing is not None:
 439                                 return existing.lits
 440
 441                         # First ensure that the interface itself has been processed
 442                         # We'll reuse the ordering of the implementations to order
 443                         # the commands too.
 444                         add_iface(uri, arch)
 445
 446                         iface = iface_cache.get_interface(uri)
 447                         filtered_impls = impls_for_iface[iface]
 448
 449                         var_names = []
 450                         for impl in filtered_impls:
 451                                 command = impl.commands.get(command_name, None)
 452                                 if not command:
 453                                         if not isinstance(impl, _DummyImpl):
 454                                                 # Mark implementation as unselectable
 455                                                 problem.add_clause([sat.neg(impl_to_var[impl])])
 456                                         continue
 457
 458                                 # We have a candidate <command>. Require that if it's selected
 459                                 # then we select the corresponding <implementation> too.
 460                                 command_var = problem.add_variable(CommandInfo(command_name, command, impl, arch))
 461                                 problem.add_clause([impl_to_var[impl], sat.neg(command_var)])
 462
 463                                 var_names.append(command_var)
 464
 465                                 runner = command.get_runner()
 466                                 for d in deps_in_use(command, arch):
 467                                         if d is runner:
 468                                                 # With a <runner>, we depend on a command rather than on an
 469                                                 # implementation. This allows us to support recursive <runner>s, etc.
 470                                                 debug(_("Considering command runner %s"), d)
 471                                                 runner_command_name = _get_command_name(d)
 472                                                 runner_vars = add_command_iface(d.interface, arch.child_arch, runner_command_name)
 473
 474                                                 # If the parent command is chosen, one of the candidate runner commands
 475                                                 # must be too. If there aren't any, then this command is unselectable.
 476                                                 problem.add_clause([sat.neg(command_var)] + runner_vars)
 477                                         else:
 478                                                 debug(_("Considering command dependency %s"), d)
 479                                                 add_iface(d.interface, arch.child_arch)
 480
 481                                         # Must choose one version of d if impl is selected
 482                                         find_dependency_candidates(command_var, d)
 483
 484                         # Tell the user why we couldn't use this version
 485                         if self.record_details:
 486                                 def new_reason(impl, old_reason):
 487                                         if command_name in impl.commands:
 488                                                 return old_reason
 489                                         return old_reason or (_('No %s command') % command_name)
 490                                 self.details[iface] = [(impl, new_reason(impl, reason)) for (impl, reason) in self.details[iface]]
 491
 492                         if closest_match:
 493                                 dummy_command = problem.add_variable(None)
 494                                 var_names.append(dummy_command)
 495
 496                         if var_names:
 497                                 # Can't select more than one of them.
 498                                 assert (uri, command_name) not in group_clause_for_command
 499                                 group_clause_for_command[(uri, command_name)] = problem.at_most_one(var_names)
 500
 501                         return var_names
 502
 503                 if command_name is None:
 504                         add_iface(root_interface, root_arch)
 505                 else:
 506                         commands = add_command_iface(root_interface, root_arch, command_name)
 507                         if len(commands) > int(closest_match):
 508                                 # (we have at least one non-dummy command)
 509                                 problem.add_clause(commands)            # At least one
 510                         else:
 511                                 # (note: might be because we haven't cached it yet)
 512                                 info("No %s <command> in %s", command_name, root_interface)
 513
 514                                 impls = impls_for_iface[iface_cache.get_interface(root_interface)]
 515                                 if impls == [] or (len(impls) == 1 and isinstance(impls[0], _DummyImpl)):
 516                                         # There were no candidates at all.
 517                                         self._failure_reason = _("Interface '%s' has no usable implementations") % root_interface
 518                                 else:
 519                                         # We had some candidates implementations, but none for the command we need
 520                                         self._failure_reason = _("Interface '%s' cannot be executed directly; it is just a library "
 521                                                     "to be used by other programs (or missing '%s' command)") % (root_interface, command_name)
 522
 523                                 problem.impossible()
 524
 525                 # Require m<group> to be true if we select an implementation in that group
 526                 m_groups = []
 527                 for machine_group, impls in impls_for_machine_group.iteritems():
 528                         m_group = 'm%d' % machine_group
 529                         group_var = problem.add_variable(m_group)
 530                         if impls:
 531                                 for impl in impls:
 532                                         problem.add_clause([group_var, sat.neg(impl)])
 533                         m_groups.append(group_var)
 534                 if m_groups:
 535                         m_groups_clause = problem.at_most_one(m_groups)
 536                 else:
 537                         m_groups_clause = None
 538
 539                 def decide():
 540                         """Recurse through the current selections until we get to an interface with
 541                         no chosen version, then tell the solver to try the best version from that."""
 542
 543                         def find_undecided_dep(impl_or_command, arch):
 544                                 # Check for undecided dependencies of impl_or_command
 545                                 for dep in deps_in_use(impl_or_command, arch):
 546                                         if dep.qdom.name == 'runner':
 547                                                 dep_lit = find_undecided_command(dep.interface, _get_command_name(dep))
 548                                         else:
 549                                                 dep_lit = find_undecided(dep.interface)
 550                                         if dep_lit is not None:
 551                                                 return dep_lit
 552                                 return None
 553
 554                         seen = set()
 555                         def find_undecided(uri):
 556                                 if uri in seen:
 557                                         return  # Break cycles
 558                                 seen.add(uri)
 559
 560                                 group = group_clause_for[uri]
 561                                 #print "Group for %s = %s" % (uri, group)
 562                                 lit = group.current
 563                                 if lit is None:
 564                                         return group.best_undecided()
 565                                 # else there was only one choice anyway
 566
 567                                 # Check for undecided dependencies
 568                                 lit_info = problem.get_varinfo_for_lit(lit).obj
 569                                 return find_undecided_dep(lit_info.impl, lit_info.arch)
 570
 571                         def find_undecided_command(uri, name):
 572                                 if name is None: return find_undecided(uri)
 573
 574                                 group = group_clause_for_command[(uri, name)]
 575                                 lit = group.current
 576                                 if lit is None:
 577                                         return group.best_undecided()
 578                                 # else we've already chosen which <command> to use
 579
 580                                 # Check for undecided command-specific dependencies, and then for
 581                                 # implementation dependencies.
 582                                 lit_info = problem.get_varinfo_for_lit(lit).obj
 583                                 if lit_info is None:
 584                                         assert closest_match
 585                                         return None     # (a dummy command added for better diagnostics; has no dependencies)
 586                                 return find_undecided_dep(lit_info.command, lit_info.arch) or \
 587                                        find_undecided_dep(lit_info.impl, lit_info.arch)
 588
 589                         best = find_undecided_command(root_interface, command_name)
 590                         if best is not None:
 591                                 return best
 592
 593                         # If we're chosen everything we need, we can probably
 594                         # set everything else to False.
 595                         for group in group_clause_for.values() + group_clause_for_command.values() + [m_groups_clause]:
 596                                 if group.current is None:
 597                                         best = group.best_undecided()
 598                                         if best is not None:
 599                                                 return sat.neg(best)
 600
 601                         return None                     # Failed to find any valid combination
 602
 603                 ready = problem.run_solver(decide) is True
 604
 605                 if not ready and not closest_match:
 606                         # We failed while trying to do a real solve.
 607                         # Try a closest match solve to get a better
 608                         # error report for the user.
 609                         self.solve(root_interface, root_arch, command_name = command_name, closest_match = True)
 610                 else:
 611                         self.ready = ready and not closest_match
 612                         self.selections = selections.Selections(None)
 613                         self.selections.interface = root_interface
 614
 615                         sels = self.selections.selections
 616
 617                         for uri, group in group_clause_for.iteritems():
 618                                 if group.current is not None:
 619                                         lit_info = problem.get_varinfo_for_lit(group.current).obj
 620                                         if lit_info.is_dummy:
 621                                                 sels[lit_info.iface.uri] = None
 622                                         else:
 623                                                 impl = lit_info.impl
 624
 625                                                 deps = self.requires[lit_info.iface] = []
 626                                                 for dep in deps_in_use(lit_info.impl, lit_info.arch):
 627                                                         deps.append(dep)
 628
 629                                                 sels[lit_info.iface.uri] = selections.ImplSelection(lit_info.iface.uri, impl, deps)
 630
 631                         def add_command(iface, name):
 632                                 sel = sels.get(iface, None)
 633                                 if sel:
 634                                         command = sel.impl.commands[name]
 635                                         self.selections.commands.append(command)
 636                                         runner = command.get_runner()
 637                                         if runner:
 638                                                 add_command(runner.metadata['interface'], _get_command_name(runner))
 639
 640                         if command_name is not None:
 641                                 add_command(root_interface, command_name)
 642
 643         def get_failure_reason(self):
 644                 """Return an exception explaining why the solve failed."""
 645                 assert not self.ready
 646
 647                 if self._failure_reason:
 648                         return model.SafeException(self._failure_reason)
 649
 650                 return model.SafeException(_("Can't find all required implementations:") + '\n' +
 651                                 '\n'.join(["- %s -> %s" % (iface, self.selections[iface])
 652                                            for iface  in self.selections]))
 653
 654 DefaultSolver = SATSolver