first implement v1 of robot impl (pantsbuild#8793)

### Problem

See pex-tool/pex#789 for a description of the issue, and https://docs.google.com/document/d/1B_g0Ofs8aQsJtrePPR1PCtSAKgBG1o59AhS_NwfFnbI/edit for a google doc with pros and cons of different approaches.

@jsirois was extremely helpful throughout the development of this feature, and pex-tool/pex#819 and pex-tool/pex#821 in pex `2.0.3` will help to optimize several other aspects of this process when we can unrevert pantsbuild#8787.

**Note:** `src/python/pants/backend/python/subsystems/pex_build_util.py` was removed in this PR, along with all floating references to it.

### Solution

With `--binary-py-generate-ipex`, a `.ipex` file will be created when `./pants binary` is run against a `python_binary()` target. This `.ipex` archive will create a `.pex` file and run it when first executed. The `.ipex` archive contains:
- in `IPEX-INFO`: the source files to inject into the resulting `.pex`, and pypi indices to resolve requirements from.
- in `BOOSTRAP-PEX-INFO`: the `PEX-INFO` of the pex file that *would* have been generated if `--generate-ipex` was False.
- in `ipex.py`: A bootstrap script which will generate a `.pex` file when the `.ipex` file is first executed.

### Result

For a `.ipex` file which hydrates the `tensorflow==1.14.0` dependency when it is first run, this translates to a >100x decrease in file size:
```bash
X> ls dist
total 145M
-rwxr-xr-x 1 dmcclanahan staff 267k Dec 10 21:11 dehydrated.ipex*
-rwxr-xr-x 1 dmcclanahan staff 134M Dec 10 21:11 dehydrated.pex*
```

contrib/python/src/python/pants/contrib/python/checks/tasks/python_eval.py

@@ -6,18 +6,18 @@
 import pkgutil
 
 from pants.backend.python.interpreter_cache import PythonInterpreterCache
-from pants.backend.python.subsystems.pex_build_util import (
-    has_python_requirements,
-    has_python_sources,
-)
 from pants.backend.python.targets.python_binary import PythonBinary
 from pants.backend.python.targets.python_library import PythonLibrary
 from pants.backend.python.targets.python_target import PythonTarget
 from pants.backend.python.tasks.resolve_requirements_task_base import ResolveRequirementsTaskBase
 from pants.base.exceptions import TaskError
 from pants.base.generator import Generator, TemplateData
 from pants.base.workunit import WorkUnit, WorkUnitLabel
-from pants.python.pex_build_util import PexBuilderWrapper
+from pants.python.pex_build_util import (
+    PexBuilderWrapper,
+    has_python_requirements,
+    has_python_sources,
+)
 from pants.task.lint_task_mixin import LintTaskMixin
 from pants.util.dirutil import safe_concurrent_creation, safe_mkdir
 from pants.util.memo import memoized_property

examples/3rdparty/python/BUILD

@@ -33,3 +33,8 @@ unpacked_whls(
   ],
   within_data_subdir='purelib/tensorflow',
 )
+
+files(
+  name='examples_python_3rdparty',
+  sources=['**/*'],
+)

examples/src/python/example/tensorflow_custom_op/BUILD

@@ -37,3 +37,21 @@ ctypes_compatible_cpp_library(
   ],
   ctypes_native_library=native_artifact(lib_name='tensorflow-zero-out-operator'),
 )
+
+
+python_binary(
+  name='show-tf-version',
+  source='show_tf_version.py',
+  dependencies=[
+    'examples/3rdparty/python:tensorflow',
+  ],
+  compatibility=['CPython>=3.6,<4'],
+)
+
+files(
+  name='show-tf-version-files',
+  sources=['**/*'],
+  dependencies=[
+    'examples/3rdparty/python:examples_python_3rdparty',
+  ],
+)

examples/src/python/example/tensorflow_custom_op/show_tf_version.py

@@ -0,0 +1,6 @@
+# Copyright 2019 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+import tensorflow as tf
+
+print(f"tf version: {tf.__version__}")

src/python/pants/backend/project_info/tasks/export.py

@@ -18,7 +18,6 @@
 from pants.backend.jvm.tasks.ivy_task_mixin import IvyTaskMixin
 from pants.backend.project_info.tasks.export_version import DEFAULT_EXPORT_VERSION
 from pants.backend.python.interpreter_cache import PythonInterpreterCache
-from pants.backend.python.subsystems.pex_build_util import has_python_requirements
 from pants.backend.python.targets.python_requirement_library import PythonRequirementLibrary
 from pants.backend.python.targets.python_target import PythonTarget
 from pants.backend.python.targets.python_tests import PythonTests
@@ -32,6 +31,7 @@
 from pants.java.distribution.distribution import DistributionLocator
 from pants.java.executor import SubprocessExecutor
 from pants.java.jar.jar_dependency_utils import M2Coordinate
+from pants.python.pex_build_util import has_python_requirements
 from pants.task.console_task import ConsoleTask
 from pants.util.memo import memoized_property
 from pants.util.ordered_set import OrderedSet

src/python/pants/backend/python/register.py

@@ -44,11 +44,16 @@
 from pants.build_graph.build_file_aliases import BuildFileAliases
 from pants.build_graph.resources import Resources
 from pants.goal.task_registrar import TaskRegistrar as task
+from pants.python.pex_build_util import PexBuilderWrapper
 from pants.python.python_requirement import PythonRequirement
 
 
 def global_subsystems():
-    return python_native_code.PythonNativeCode, subprocess_environment.SubprocessEnvironment
+    return {
+        python_native_code.PythonNativeCode,
+        subprocess_environment.SubprocessEnvironment,
+        PexBuilderWrapper.Factory,
+    }
 
 
 def build_file_aliases():

src/python/pants/backend/python/rules/BUILD

@@ -6,6 +6,7 @@ python_library(
     '3rdparty/python:dataclasses',
     '3rdparty/python:setuptools',
     'src/python/pants/backend/python/subsystems',
+    'src/python/pants/backend/python/subsystems/ipex',
     'src/python/pants/build_graph',
     'src/python/pants/engine/legacy:graph',
     'src/python/pants/engine:build_files',

src/python/pants/backend/python/subsystems/ipex/BUILD

@@ -0,0 +1,7 @@
+# Copyright 2019 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+# NB: This target is written into an .ipex file as the main script, and should not have any
+# dependencies on another python code! .ipex files should always contain pex and setuptools
+# requirements in order to run the main script!
+python_library()

src/python/pants/backend/python/subsystems/ipex/ipex_launcher.py

@@ -0,0 +1,129 @@
+# Copyright 2019 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+"""Entrypoint script for a "dehydrated" .ipex file generated with --generate-ipex.
+
+This script will "hydrate" a normal .pex file in the same directory, then execute it.
+"""
+
+import json
+import os
+import sys
+import tempfile
+
+from pex import resolver
+from pex.common import open_zip
+from pex.interpreter import PythonInterpreter
+from pex.pex_builder import PEXBuilder
+from pex.pex_info import PexInfo
+from pkg_resources import Requirement
+
+APP_CODE_PREFIX = "user_files/"
+
+
+def _strip_app_code_prefix(path):
+    if not path.startswith(APP_CODE_PREFIX):
+        raise ValueError(
+            "Path {path} in IPEX-INFO did not begin with '{APP_CODE_PREFIX}'.".format(
+                path=path, APP_CODE_PREFIX=APP_CODE_PREFIX
+            )
+        )
+    return path[len(APP_CODE_PREFIX) :]
+
+
+def _log(message):
+    sys.stderr.write(message + "\n")
+
+
+def _sanitize_requirements(requirements):
+    """Remove duplicate keys such as setuptools or pex which may be injected multiple times into the
+    resulting ipex when first executed."""
+    project_names = []
+    new_requirements = {}
+
+    for r in requirements:
+        r = Requirement(r)
+        if r.marker and not r.marker.evaluate():
+            continue
+        if r.name not in new_requirements:
+            project_names.append(r.name)
+            new_requirements[r.name] = str(r)
+    sanitized_requirements = [new_requirements[n] for n in project_names]
+
+    return sanitized_requirements
+
+
+def modify_pex_info(pex_info, **kwargs):
+    new_info = json.loads(pex_info.dump())
+    new_info.update(kwargs)
+    return PexInfo.from_json(json.dumps(new_info))
+
+
+def _hydrate_pex_file(self, hydrated_pex_file):
+    # We extract source files into a temporary directory before creating the pex.
+    td = tempfile.mkdtemp()
+
+    with open_zip(self) as zf:
+        # Populate the pex with the pinned requirements and distribution names & hashes.
+        bootstrap_info = PexInfo.from_json(zf.read("BOOTSTRAP-PEX-INFO"))
+        bootstrap_builder = PEXBuilder(pex_info=bootstrap_info, interpreter=PythonInterpreter.get())
+
+        # Populate the pex with the needed code.
+        try:
+            ipex_info = json.loads(zf.read("IPEX-INFO").decode("utf-8"))
+            for path in ipex_info["code"]:
+                unzipped_source = zf.extract(path, td)
+                bootstrap_builder.add_source(
+                    unzipped_source, env_filename=_strip_app_code_prefix(path)
+                )
+        except Exception as e:
+            raise ValueError(
+                "Error: {e}. The IPEX-INFO for this .ipex file was:\n{info}".format(
+                    e=e, info=json.dumps(ipex_info, indent=4)
+                )
+            )
+
+    # Perform a fully pinned intransitive resolve to hydrate the install cache.
+    resolver_settings = ipex_info["resolver_settings"]
+
+    sanitized_requirements = _sanitize_requirements(bootstrap_info.requirements)
+    bootstrap_info = modify_pex_info(bootstrap_info, requirements=sanitized_requirements)
+    bootstrap_builder.info = bootstrap_info
+
+    resolved_distributions = resolver.resolve(
+        requirements=bootstrap_info.requirements,
+        cache=bootstrap_info.pex_root,
+        platform="current",
+        transitive=False,
+        interpreter=bootstrap_builder.interpreter,
+        **resolver_settings
+    )
+    # TODO: this shouldn't be necessary, as we should be able to use the same 'distributions' from
+    # BOOTSTRAP-PEX-INFO. When the .ipex is executed, the normal pex bootstrap fails to see these
+    # requirements or recognize that they should be pulled from the cache for some reason.
+    for resolved_dist in resolved_distributions:
+        bootstrap_builder.add_distribution(resolved_dist.distribution)
+
+    bootstrap_builder.build(hydrated_pex_file, bytecode_compile=False)
+
+
+def main(self):
+    filename_base, ext = os.path.splitext(self)
+
+    # If the ipex (this pex) is already named '.pex', ensure the output filename doesn't collide by
+    # inserting an intermediate '.ipex'!
+    if ext == ".pex":
+        hydrated_pex_file = "{filename_base}.ipex.pex".format(filename_base=filename_base)
+    else:
+        hydrated_pex_file = "{filename_base}.pex".format(filename_base=filename_base)
+
+    if not os.path.exists(hydrated_pex_file):
+        _log("Hydrating {} to {}...".format(self, hydrated_pex_file))
+        _hydrate_pex_file(self, hydrated_pex_file)
+
+    os.execv(sys.executable, [sys.executable, hydrated_pex_file] + sys.argv[1:])
+
+
+if __name__ == "__main__":
+    self = sys.argv[0]
+    main(self)

src/python/pants/backend/python/subsystems/python_native_code.py

@@ -9,10 +9,10 @@
 
 from pants.backend.native.subsystems.native_toolchain import NativeToolchain
 from pants.backend.native.targets.native_library import NativeLibrary
-from pants.backend.python.subsystems import pex_build_util
 from pants.backend.python.targets.python_distribution import PythonDistribution
 from pants.base.exceptions import IncompatiblePlatformsError
 from pants.engine.rules import rule, subsystem_rule
+from pants.python import pex_build_util
 from pants.python.python_setup import PythonSetup
 from pants.subsystem.subsystem import Subsystem
 from pants.util.memo import memoized_property

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

@@ -12,13 +12,13 @@
 
 from pants.backend.native.targets.native_library import NativeLibrary
 from pants.backend.native.tasks.link_shared_libraries import SharedLibrary
-from pants.backend.python.subsystems.pex_build_util import is_local_python_dist
 from pants.backend.python.subsystems.python_native_code import PythonNativeCode
 from pants.backend.python.targets.python_requirement_library import PythonRequirementLibrary
 from pants.base.build_environment import get_buildroot
 from pants.base.exceptions import TargetDefinitionException, TaskError
 from pants.base.workunit import WorkUnitLabel
 from pants.build_graph.address import Address
+from pants.python.pex_build_util import is_local_python_dist
 from pants.python.python_requirement import PythonRequirement
 from pants.python.setup_py_runner import SetupPyRunner
 from pants.task.task import Task

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

@@ -7,14 +7,14 @@
 from pex.pex import PEX
 from pex.pex_builder import PEXBuilder
 
-from pants.backend.python.subsystems.pex_build_util import (
+from pants.base.exceptions import TaskError
+from pants.invalidation.cache_manager import VersionedTargetSet
+from pants.python.pex_build_util import (
+    PexBuilderWrapper,
     has_python_sources,
     has_resources,
     is_python_target,
 )
-from pants.base.exceptions import TaskError
-from pants.invalidation.cache_manager import VersionedTargetSet
-from pants.python.pex_build_util import PexBuilderWrapper
 from pants.task.task import Task
 from pants.util.dirutil import safe_concurrent_creation
 from pants.util.ordered_set import OrderedSet

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

@@ -3,8 +3,8 @@
 
 import os
 
-from pants.backend.python.subsystems.pex_build_util import is_local_python_dist
 from pants.base.build_environment import get_buildroot
+from pants.python.pex_build_util import is_local_python_dist
 from pants.task.task import Task
 from pants.util.dirutil import safe_mkdir
 from pants.util.fileutil import atomic_copy