Provision lock (#2516)

Author: masenf

Diff for: CONTRIBUTORS

@@ -80,6 +80,7 @@ Marc Schlaich
 Marius Gedminas
 Mariusz Rusiniak
 Mark Hirota
+Masen Furer
 Matt Good
 Matt Jeffery
 Matthew Kenigsberg

Diff for: docs/changelog/2515.bugfix.rst

@@ -0,0 +1,2 @@
+Multiple tox instances no longer clobber the ``.tox`` directory when
+``provision_tox_env`` is used. - by :user:`masenf`

Diff for: docs/config.rst

@@ -76,6 +76,11 @@ Global settings are defined under the ``tox`` section as:
     When tox is invoked with the ``--no-provision`` flag,
     the provision won't be attempted, tox will fail instead.
 
+    .. versionchanged:: 3.27.0
+
+    When provisioning, tox will take a lock to ensure exclusive access to the
+    `provision_tox_env` and avoid clobbering by other tox instances.
+
     .. warning::
 
       The new virtual environment will only contain dependencies specified by the :conf:`requires` keyword.

Diff for: src/tox/session/commands/provision.py

@@ -4,6 +4,8 @@
 import os
 
 from tox.exception import InvocationError
+from tox.reporter import verbosity0
+from tox.util.lock import hold_lock
 
 
 def provision_tox(provision_venv, args):
@@ -21,5 +23,9 @@ def provision_tox(provision_venv, args):
 
 
 def ensure_meta_env_up_to_date(provision_venv):
-    if provision_venv.setupenv():
-        provision_venv.finishvenv()
+    config = provision_venv.envconfig.config
+    lock_file = config.toxworkdir.join("{}.lock".format(config.provision_tox_env))
+
+    with hold_lock(lock_file, verbosity0):
+        if provision_venv.setupenv():
+            provision_venv.finishvenv()

Diff for: tests/integration/test_provision_int.py

@@ -134,3 +134,34 @@ def test_provision_interrupt_child(initproj, monkeypatch, capfd, signal_type):
             out,
             "\n".join(repr(i) for i in all_process),
         )
+
+
+@pytest.mark.skipif("sys.platform == 'win32'", reason="pyenv does not exists on Windows")
+def test_provision_race(initproj, cmd, monkeypatch):
+    initproj(
+        "pkg123-0.7",
+        filedefs={
+            "tox.ini": """\
+                [tox]
+                skipsdist=True
+                minversion = 3.7.0
+                requires =
+                    setuptools == 40.6.3
+                [testenv]
+                commands=python -c "import sys; print(sys.executable); raise SystemExit(1)"
+                [testenv:x2]
+            """,
+        },
+    )
+
+    procs = [
+        subprocess.Popen(
+            [sys.executable, "-m", "tox", "-e", "py", "-vv"],
+            stdout=subprocess.PIPE,
+        )
+        for _ in range(2)
+    ]
+    for proc in procs:
+        stdout, stderr = proc.communicate()
+        assert proc.returncode != 0
+        assert b".tox/.tox/bin/python -m virtualenv" in stdout