Bug 1717051: Always populate Mach pths r=firefox-build-system-reviewers,glandium

The previous behaviour was to:
* Never add a `pthfile` to the Mach virtualenv, and
* Always add Mach's paths to the `sys.path` when Mach initializes

However, this meant that `pip` would needlessly install packages
that already exist in the vendored environment.

Tweak `pth` behaviour so that `pip` behaves more efficiently.

Differential Revision: https://phabricator.services.mozilla.com/D120402

build/mach_initialize.py

@@ -186,17 +186,28 @@ def _activate_python_environment(topsrcdir, state_dir):
         )
     ]
 
-    from mach.virtualenv import VirtualenvManager
-
-    mach_virtualenv = VirtualenvManager(
-        topsrcdir,
-        os.path.join(state_dir, "_virtualenvs"),
-        "mach",
-        populate_local_paths=False,
+    from mach.virtualenv import (
+        MozVirtualenvMetadata,
+        MozVirtualenvMetadataOutOfDateError,
+        VirtualenvManager,
     )
 
-    requirements = mach_virtualenv.requirements()
+    try:
+        mach_virtualenv = VirtualenvManager(
+            topsrcdir,
+            os.path.join(state_dir, "_virtualenvs"),
+            "mach",
+        )
+        active_metadata = MozVirtualenvMetadata.from_runtime()
+        is_mach_virtualenv = (
+            active_metadata and active_metadata.virtualenv_name == "mach"
+        )
+    except MozVirtualenvMetadataOutOfDateError as e:
+        print(e)
+        print('This should be resolved by running "./mach create-mach-environment".')
+        sys.exit(1)
 
+    requirements = mach_virtualenv.requirements()
     if os.environ.get("MACH_USE_SYSTEM_PYTHON") or os.environ.get("MOZ_AUTOMATION"):
         env_var = (
             "MOZ_AUTOMATION"
@@ -249,12 +260,12 @@ def _activate_python_environment(topsrcdir, state_dir):
             # (optional) dependencies are not installed.
             _scrub_system_site_packages()
 
-    elif sys.prefix == sys.base_prefix:
-        # We're in an environment where we normally use the Mach virtualenv,
-        # but we're running a "nativecmd" such as "create-mach-environment".
-        # Remove global site packages from sys.path to improve isolation accordingly.
-        _scrub_system_site_packages()
-    else:
+        sys.path[0:0] = [
+            os.path.join(topsrcdir, pth.path)
+            for pth in requirements.pth_requirements
+            + requirements.vendored_requirements
+        ]
+    elif is_mach_virtualenv:
         # We're running in the Mach virtualenv - check that it's up-to-date.
         # Note that the "pip package check" exists to ensure that a virtualenv isn't
         # corrupted by ad-hoc pip installs. Since the Mach virtualenv is unlikely
@@ -267,11 +278,17 @@ def _activate_python_environment(topsrcdir, state_dir):
                 '"./mach create-mach-environment"'
             )
             sys.exit(1)
+    else:
+        # We're in an environment where we normally *would* use the Mach virtualenv,
+        # but we're running a "nativecmd" such as "create-mach-environment".
+        # Remove global site packages from sys.path to improve isolation accordingly.
+        _scrub_system_site_packages()
 
-    sys.path[0:0] = [
-        os.path.join(topsrcdir, pth.path)
-        for pth in requirements.pth_requirements + requirements.vendored_requirements
-    ]
+        sys.path[0:0] = [
+            os.path.join(topsrcdir, pth.path)
+            for pth in requirements.pth_requirements
+            + requirements.vendored_requirements
+        ]
 
 
 def initialize(topsrcdir):