Fix Pex emitting warnings about its Pip PEX venv. (#1351)

Previously we'd see:
```
rm -rf ~/.pex
$ pex pex -cpex -opex.pex
/home/jsirois/.venv/pex/lib/python3.9/site-packages/pex/tools/commands/venv.py:141: PEXWarning: Encountered collision building venv at /home/jsirois/.pex/venvs/short/4bd8ddd4 from /home/jsirois/.pex/pip.pex/46820cb5af0dcf9295a4e7f30184cc0e9fa063dc:
1. /home/jsirois/.pex/venvs/7fbdc9731d96d42f0de338b2dfbc6bbed1c938e7/832e95f85413646e8b7b056ab4e60414f83d7397.01034f69a38943809913ffdfd28dd5bf/lib/python3.9/site-packages/constraints.txt was provided by:
        /home/jsirois/.pex/pip.pex/46820cb5af0dcf9295a4e7f30184cc0e9fa063dc/.deps/setuptools/constraints.txt
        /home/jsirois/.pex/pip.pex/46820cb5af0dcf9295a4e7f30184cc0e9fa063dc/.deps/wheel/constraints.txt
  pex_warnings.warn(message)
```

pex/third_party/__init__.py

@@ -353,6 +353,13 @@ def isolated():
 
         module = "pex"
 
+        # These files are only used for running `tox -evendor` and should not pollute either the
+        # PEX_ROOT or built PEXs.
+        vendor_lockfiles = tuple(
+            os.path.join(os.path.relpath(vendor_spec.relpath, module), "constraints.txt")
+            for vendor_spec in vendor.iter_vendor_specs()
+        )
+
         # TODO(John Sirois): Unify with `pex.util.DistributionHelper.access_zipped_assets`.
         def recursive_copy(srcdir, dstdir):
             os.mkdir(dstdir)
@@ -370,7 +377,11 @@ def recursive_copy(srcdir, dstdir):
                     if os.path.basename(src_entry) == "__pycache__":
                         continue
                     recursive_copy(src_entry, dst_entry)
-                elif not entry_name.endswith(".pyc") and not is_pyc_temporary_file(entry_name):
+                elif (
+                    not entry_name.endswith(".pyc")
+                    and not is_pyc_temporary_file(entry_name)
+                    and src_entry not in vendor_lockfiles
+                ):
                     with open(dst_entry, "wb") as fp:
                         with closing(resource_stream(module, src_entry)) as resource:
                             shutil.copyfileobj(resource, fp)

pex/vendor/__init__.py

@@ -9,6 +9,10 @@
 from pex.common import filter_pyc_dirs, filter_pyc_files, touch
 from pex.compatibility import urlparse
 from pex.tracer import TRACER
+from pex.typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from typing import Iterator
 
 _PACKAGE_COMPONENTS = __name__.split(".")
 
@@ -100,10 +104,10 @@ def create_packages(self):
 
 
 def iter_vendor_specs():
+    # type: () -> Iterator[VendorSpec]
     """Iterate specifications for code vendored by pex.
 
     :return: An iterator over specs of all vendored code.
-    :rtype: :class:`collection.Iterator` of :class:`VendorSpec`
     """
     # We use this for a better @dataclass that is also Python2.7 and PyPy compatible.
     # N.B.: The `[testenv:typecheck]` section in `tox.ini` should have its deps list updated to

tests/test_pip.py

@@ -0,0 +1,35 @@
+# Copyright 2021 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+from __future__ import absolute_import
+
+import os
+import warnings
+
+from pex.interpreter import PythonInterpreter
+from pex.pip import Pip
+from pex.typing import TYPE_CHECKING
+from pex.variables import ENV
+
+if TYPE_CHECKING:
+    from typing import Any
+
+
+def test_no_duplicate_constraints_pex_warnings(tmpdir):
+    # type: (Any) -> None
+    pex_root = os.path.join(str(tmpdir), "pex_root")
+    pip_root = os.path.join(str(tmpdir), "pip_root")
+    interpreter = PythonInterpreter.get()
+    platform = interpreter.platform
+
+    with ENV.patch(PEX_ROOT=pex_root), warnings.catch_warnings(record=True) as events:
+        pip = Pip.create(path=pip_root, interpreter=interpreter)
+
+    pip.spawn_debug(
+        platform=platform.platform, impl=platform.impl, version=platform.version, abi=platform.abi
+    ).wait()
+
+    assert 0 == len([event for event in events if "constraints.txt" in str(event)]), (
+        "Expected no duplicate constraints warnings to be emitted when creating a Pip venv but "
+        "found\n{}".format("\n".join(map(str, events)))
+    )

tests/test_third_party.py

@@ -34,6 +34,18 @@ def test_isolated_pex_root():
         assert pex_root == os.path.commonprefix([pex_root, devendored_chroot])
 
 
+def test_isolated_vendoring_constraints_omitted():
+    # type: () -> None
+    with temporary_pex_root() as (pex_root, _):
+        devendored_chroot = os.path.realpath(third_party.isolated().chroot_path)
+        assert [] == [
+            os.path.join(root, file)
+            for root, _, files in os.walk(devendored_chroot)
+            for file in files
+            if file == "constraints.txt"
+        ]
+
+
 def test_isolated_idempotent_inprocess():
     # type: () -> None
     with temporary_pex_root():