Add --no-backtracking option for new resolver

Adds a new option `--no-backtracking` to the new resolver. Off by
default (no change to default behaviour), enabling this option prevents
us from running the backtracking behaviour when pinning fails and
instead skip directly to the error reporting.

This can be especially useful in a CI situation, where the developer is
interested in knowing about conflicts up front in order to manually
solve them, or is worried about allowing for a potentially large amount
of time spent in backtracking.

news/9258.feature.rst

@@ -0,0 +1 @@
+New resolver: Add ``--no-backtracking`` option to enable failfast debugging.

src/pip/_internal/cli/cmdoptions.py

@@ -927,6 +927,17 @@ def check_list_path_option(options):
     ),
 )  # type: Callable[..., Option]
 
+disable_backtracking = partial(
+    Option,
+    '--no-backtracking',
+    dest='disable_backtracking',
+    action='store_true',
+    default=False,
+    help='Do not attempt to backtrack to resolve package conflicts. This will '
+         'cause conflicts which may be otherwise automatically solveable to '
+         'fail fast and require manual intervention!',
+)  # type: Callable[..., Option]
+
 
 ##########
 # groups #

src/pip/_internal/commands/download.py

@@ -56,6 +56,7 @@ def add_options(self):
         self.cmd_opts.add_option(cmdoptions.no_build_isolation())
         self.cmd_opts.add_option(cmdoptions.use_pep517())
         self.cmd_opts.add_option(cmdoptions.no_use_pep517())
+        self.cmd_opts.add_option(cmdoptions.disable_backtracking())
 
         self.cmd_opts.add_option(
             '-d', '--dest', '--destination-dir', '--destination-directory',
@@ -128,7 +129,8 @@ def run(self, options, args):
         self.trace_basic_info(finder)
 
         requirement_set = resolver.resolve(
-            reqs, check_supported_wheels=True
+            reqs, check_supported_wheels=True,
+            should_backtrack=not options.disable_backtracking
         )
 
         downloaded = []  # type: List[str]

src/pip/_internal/commands/install.py

@@ -216,6 +216,7 @@ def add_options(self):
         self.cmd_opts.add_option(cmdoptions.prefer_binary())
         self.cmd_opts.add_option(cmdoptions.require_hashes())
         self.cmd_opts.add_option(cmdoptions.progress_bar())
+        self.cmd_opts.add_option(cmdoptions.disable_backtracking())
 
         index_opts = cmdoptions.make_option_group(
             cmdoptions.index_group,
@@ -318,7 +319,8 @@ def run(self, options, args):
             self.trace_basic_info(finder)
 
             requirement_set = resolver.resolve(
-                reqs, check_supported_wheels=not options.target_dir
+                reqs, check_supported_wheels=not options.target_dir,
+                should_backtrack=not options.disable_backtracking
             )
 
             try:

src/pip/_internal/commands/wheel.py

@@ -80,6 +80,7 @@ def add_options(self):
         self.cmd_opts.add_option(cmdoptions.no_deps())
         self.cmd_opts.add_option(cmdoptions.build_dir())
         self.cmd_opts.add_option(cmdoptions.progress_bar())
+        self.cmd_opts.add_option(cmdoptions.disable_backtracking())
 
         self.cmd_opts.add_option(
             '--global-option',
@@ -152,7 +153,8 @@ def run(self, options, args):
         self.trace_basic_info(finder)
 
         requirement_set = resolver.resolve(
-            reqs, check_supported_wheels=True
+            reqs, check_supported_wheels=True,
+            should_backtrack=not options.disable_backtracking
         )
 
         reqs_to_build = []  # type: List[InstallRequirement]

src/pip/_internal/resolution/base.py

@@ -12,8 +12,8 @@
 
 
 class BaseResolver(object):
-    def resolve(self, root_reqs, check_supported_wheels):
-        # type: (List[InstallRequirement], bool) -> RequirementSet
+    def resolve(self, root_reqs, check_supported_wheels, should_backtrack):
+        # type: (List[InstallRequirement], bool, bool) -> RequirementSet
         raise NotImplementedError()
 
     def get_installation_order(self, req_set):

src/pip/_internal/resolution/legacy/resolver.py

@@ -149,8 +149,8 @@ def __init__(
         self._discovered_dependencies = \
             defaultdict(list)  # type: DiscoveredDependencies
 
-    def resolve(self, root_reqs, check_supported_wheels):
-        # type: (List[InstallRequirement], bool) -> RequirementSet
+    def resolve(self, root_reqs, check_supported_wheels, _should_backtrack):
+        # type: (List[InstallRequirement], bool, bool) -> RequirementSet
         """Resolve what operations need to be done
 
         As a side-effect of this method, the packages (and their dependencies)

src/pip/_internal/resolution/resolvelib/resolver.py

@@ -75,8 +75,8 @@ def __init__(
         self.upgrade_strategy = upgrade_strategy
         self._result = None  # type: Optional[Result]
 
-    def resolve(self, root_reqs, check_supported_wheels):
-        # type: (List[InstallRequirement], bool) -> RequirementSet
+    def resolve(self, root_reqs, check_supported_wheels, should_backtrack):
+        # type: (List[InstallRequirement], bool, bool) -> RequirementSet
 
         constraints = {}  # type: Dict[str, Constraint]
         user_requested = set()  # type: Set[str]
@@ -120,6 +120,7 @@ def resolve(self, root_reqs, check_supported_wheels):
             try_to_avoid_resolution_too_deep = 2000000
             self._result = resolver.resolve(
                 requirements, max_rounds=try_to_avoid_resolution_too_deep,
+                should_backtrack=should_backtrack,
             )
 
         except ResolutionImpossible as e:

src/pip/_vendor/resolvelib/resolvers.py

@@ -297,7 +297,7 @@ def _backtrack(self):
         # No way to backtrack anymore.
         return False
 
-    def resolve(self, requirements, max_rounds):
+    def resolve(self, requirements, max_rounds, should_backtrack):
         if self._states:
             raise RuntimeError("already resolved")
 
@@ -341,7 +341,7 @@ def resolve(self, requirements, max_rounds):
             if failure_causes:
                 # Backtrack if pinning fails. The backtrack process puts us in
                 # an unpinned state, so we can work on it in the next round.
-                success = self._backtrack()
+                success = self._backtrack() if should_backtrack else False
 
                 # Dead ends everywhere. Give up.
                 if not success:
@@ -413,7 +413,7 @@ class Resolver(AbstractResolver):
 
     base_exception = ResolverException
 
-    def resolve(self, requirements, max_rounds=100):
+    def resolve(self, requirements, max_rounds=100, should_backtrack=True):
         """Take a collection of constraints, spit out the resolution result.
 
         The return value is a representation to the final resolution result. It
@@ -442,5 +442,6 @@ def resolve(self, requirements, max_rounds=100):
             `max_rounds` argument.
         """
         resolution = Resolution(self.provider, self.reporter)
-        state = resolution.resolve(requirements, max_rounds=max_rounds)
+        state = resolution.resolve(requirements, max_rounds=max_rounds,
+                                   should_backtrack=should_backtrack)
         return _build_result(state)