pip installer: fix installed script executable

poetry/utils/pip.py

+import os
+import sys
+
 from pathlib import Path
 from typing import Union
 
 from poetry.exceptions import PoetryException
 from poetry.utils.env import Env
+from poetry.utils.env import EnvCommandError
 from poetry.utils.env import ephemeral_environment
 
 
@@ -14,9 +18,13 @@ def pip_install(
     upgrade: bool = False,
 ) -> Union[int, str]:
     path = Path(path) if isinstance(path, str) else path
+    is_wheel = path.suffix == ".whl"
 
     args = ["install", "--prefix", str(environment.path)]
 
+    if not is_wheel:
+        args.insert(1, "--use-pep517")
+
     if upgrade:
         args.append("--upgrade")
 
@@ -32,16 +40,21 @@ def pip_install(
 
     args.append(str(path))
 
-    if path.is_file() and path.suffix == ".whl":
+    try:
         return environment.run_pip(*args)
-
-    with ephemeral_environment(
-        executable=environment.python, pip=True, setuptools=True
-    ) as env:
-        return env.run(
-            "pip",
-            *args,
-        )
+    except EnvCommandError as e:
+        if sys.version_info < (3, 7) and not is_wheel:
+            # Under certain Python3.6 installs vendored pip wheel does not contain zip-safe
+            # pep517 lib. In this cases we create an isolated ephemeral virtual environment.
+            with ephemeral_environment(
+                executable=environment.python, pip=True, setuptools=True
+            ) as env:
+                return environment.run(
+                    env._bin("pip"),
+                    *args,
+                    env={**os.environ, "PYTHONPATH": str(env.purelib)},
+                )
+        raise PoetryException(f"Failed to install {path.as_posix()}") from e
 
 
 def pip_editable_install(directory: Path, environment: Env) -> Union[int, str]:

tests/installation/test_executor.py

@@ -137,7 +137,7 @@ Package operations: 4 installs, 1 update, 1 removal
     expected = set(expected.splitlines())
     output = set(io.fetch_output().splitlines())
     assert expected == output
-    assert 4 == len(env.executed)
+    assert 5 == len(env.executed)
     assert 0 == return_code
     pip_editable_install.assert_called_once()