PythonToolPrepBase now sets up its interpreter.

Previously it was relying on the good graces of the underlying
interpreter running Pants in combination with the demands of resolving
the particular requirements needed by the tool in question.

Simplify the code by eliminating `_create_requirements` target injection
and just passing PythonRequirements directly to PexBuilderWrapper.

Prep work for pantsbuild#6927

src/python/pants/backend/python/tasks/python_tool_prep_base.py

@@ -6,19 +6,15 @@
 
 import os
 
-from pex.interpreter import PythonInterpreter
 from pex.pex import PEX
 from pex.pex_builder import PEXBuilder
 
+from pants.backend.python.interpreter_cache import PythonInterpreterCache
 from pants.backend.python.python_requirement import PythonRequirement
 from pants.backend.python.subsystems.pex_build_util import PexBuilderWrapper
-from pants.backend.python.targets.python_requirement_library import PythonRequirementLibrary
-from pants.base.build_environment import get_buildroot
-from pants.base.exceptions import TaskError
 from pants.base.workunit import WorkUnitLabel
-from pants.build_graph.address import Address
 from pants.task.task import Task
-from pants.util.dirutil import fast_relpath, safe_concurrent_creation
+from pants.util.dirutil import safe_concurrent_creation
 
 
 class PythonToolInstance(object):
@@ -54,58 +50,37 @@ def subsystem_dependencies(cls):
     return super(PythonToolPrepBase, cls).subsystem_dependencies() + (
       cls.tool_subsystem_cls.scoped(cls),
       PexBuilderWrapper.Factory,
+      PythonInterpreterCache,
     )
 
   @classmethod
   def product_types(cls):
     return [cls.tool_instance_cls]
 
-  @property
-  def create_target_dirs(self):
-    return True
-
-  def _tool_subsystem(self):
-    return self.tool_subsystem_cls.scoped_instance(self)
-
-  def _create_requirements(self, context, workdir):
-    tool_subsystem = self._tool_subsystem()
-    address = Address(spec_path=fast_relpath(workdir, get_buildroot()),
-                      target_name=tool_subsystem.options_scope)
-    context.build_graph.inject_synthetic_target(
-      address=address,
-      target_type=PythonRequirementLibrary,
-      requirements=[PythonRequirement(r) for r in tool_subsystem.get_requirement_specs()]
-    )
-    return context.build_graph.get_target(address=address)
-
-  def _build_tool_pex(self, context, interpreter, pex_path, requirements_lib):
+  def _build_tool_pex(self, tool_subsystem, interpreter, pex_path):
     with safe_concurrent_creation(pex_path) as chroot:
       pex_builder = PexBuilderWrapper.Factory.create(
         builder=PEXBuilder(path=chroot, interpreter=interpreter),
-        log=context.log)
-      pex_builder.add_requirement_libs_from(req_libs=[requirements_lib])
-      pex_builder.set_entry_point(self._tool_subsystem().get_entry_point())
+        log=self.context.log)
+      reqs = [PythonRequirement(r) for r in tool_subsystem.get_requirement_specs()]
+      pex_builder.add_resolved_requirements(reqs=reqs)
+      pex_builder.set_entry_point(tool_subsystem.get_entry_point())
       pex_builder.freeze()
 
   def execute(self):
-    tool_req_lib = self._create_requirements(self.context, self.workdir)
-
-    with self.invalidated(targets=[tool_req_lib]) as invalidation_check:
-      pex_name = self._tool_subsystem().options_scope
-      interpreter = PythonInterpreter.get()
-      if len(invalidation_check.all_vts) != 1:
-        raise TaskError('Expected exactly one versioned target found {}: {}'.format(
-          len(invalidation_check.all_vts), invalidation_check.all_vts))
-      vt = invalidation_check.all_vts[0]
-      pex_path = os.path.join(vt.results_dir, '{}.pex'.format(pex_name))
-
-      if invalidation_check.invalid_vts:
-        with self.context.new_workunit(name='create-{}-pex'.format(pex_name),
-                                       labels=[WorkUnitLabel.PREP]):
-          self._build_tool_pex(context=self.context,
-                               interpreter=interpreter,
-                               pex_path=pex_path,
-                               requirements_lib=tool_req_lib)
-
-      tool_instance = self.tool_instance_cls(pex_path, interpreter)
-      self.context.products.register_data(self.tool_instance_cls, tool_instance)
+    tool_subsystem = self.tool_subsystem_cls.scoped_instance(self)
+    pex_name = tool_subsystem.options_scope
+    pex_path = os.path.join(self.workdir, self.fingerprint, '{}.pex'.format(pex_name))
+
+    interpreter_cache = PythonInterpreterCache.global_instance()
+    interpreter = interpreter_cache.select_interpreter_for_targets([])
+
+    if not os.path.exists(pex_path):
+      with self.context.new_workunit(name='create-{}-pex'.format(pex_name),
+                                     labels=[WorkUnitLabel.PREP]):
+        self._build_tool_pex(tool_subsystem=tool_subsystem,
+                             interpreter=interpreter,
+                             pex_path=pex_path)
+
+    tool_instance = self.tool_instance_cls(pex_path, interpreter)
+    self.context.products.register_data(self.tool_instance_cls, tool_instance)