Ensure Pip logs to stderr. (#1268)

Fixes #1267

pex/pip.py

@@ -323,8 +323,18 @@ def _spawn_pip_isolated(
  from pex.pex import PEX
 
  pip = PEX(pex=self._pip_pex_path, interpreter=python_interpreter)
+
+ # Pip has no discernable stdout / stderr discipline with its logging. Pex guarantees
+ # stdout will only contain useable (parseable) data and all logging will go to stderr.
+ # To uphold the Pex standard, force Pip to comply by re-directing stdout to stderr.
+ #
+ # See:
+ # + https:/pantsbuild/pex/issues/1267
+ # + https:/pypa/pip/issues/9420
+ stdout = popen_kwargs.pop("stdout", sys.stderr.fileno())
+
  return pip.cmdline(command), pip.run(
- args=command, env=env, blocking=False, **popen_kwargs
+ args=command, env=env, blocking=False, stdout=stdout, **popen_kwargs
  )
 
  def _spawn_pip_isolated_job(

tests/test_integration.py

@@ -2876,7 +2876,7 @@ def test_seed(
 ):
  # type: (...) -> None
  pex_file, args = isort_pex_args
- results = run_pex_command(args=args + mode_args + ["--seed"], quiet=True)
+ results = run_pex_command(args=args + mode_args + ["--seed"])
  results.assert_success()
 
  # Setting posix=False works around this issue under pypy: https://bugs.python.org/issue1170.