Bug 1712819: Avoid pip's "outdated" warning in virtualenvs r=ahal

Now, when running mach commands that invoke `pip`, it will no longer
inform the user that it needs an update.

We reach into `distutils` to determine the "site-packages" directory
pattern, then apply it to our virtualenv.

Differential Revision: https://phabricator.services.mozilla.com/D115940

python/mozbuild/mozbuild/virtualenv.py

@@ -282,6 +282,7 @@ class VirtualenvManager(VirtualenvHelper):
             )
 
         self.write_exe_info(python)
+        self._disable_pip_outdated_warning()
 
         return self.virtualenv_root
 
@@ -655,6 +656,55 @@ class VirtualenvManager(VirtualenvHelper):
 
         return self._run_pip(args)
 
+    def _disable_pip_outdated_warning(self):
+        """Disables the pip outdated warning by changing pip's 'installer'
+
+        "pip" has behaviour to ensure that it doesn't print it's "outdated"
+        warning if it's part of a Linux distro package. This is because
+        Linux distros generally have a slightly out-of-date pip package
+        that they know to be stable, and users aren't always able to
+        (or want to) update it.
+
+        This behaviour works by checking if the "pip" installer
+        (encoded in the dist-info/INSTALLER file) is "pip" itself,
+        or a different value (e.g.: a distro).
+
+        We can take advantage of this behaviour by telling pip
+        that it was installed by "mach", so it won't print the
+        warning.
+
+        https://github.com/pypa/pip/blob/5ee933aab81273da3691c97f2a6e7016ecbe0ef9/src/pip/_internal/self_outdated_check.py#L100-L101 # noqa F401
+        """
+
+        # Defer "distutils" import until this function is called so that
+        # "mach bootstrap" doesn't fail due to Linux distro python-distutils
+        # package not being installed.
+        # By the time this function is called, "distutils" must be installed
+        # because it's needed by the "virtualenv" package.
+        from distutils import dist
+
+        distribution = dist.Distribution({"script_args": "--no-user-cfg"})
+        installer = distribution.get_command_obj("install")
+        installer.prefix = self.virtualenv_root
+        installer.finalize_options()
+
+        # Path to virtualenv's "site-packages" directory
+        site_packages = installer.install_purelib
+
+        pip_dist_info = next(
+            (
+                file
+                for file in os.listdir(site_packages)
+                if file.startswith("pip-") and file.endswith(".dist-info")
+            ),
+            None,
+        )
+        if not pip_dist_info:
+            raise Exception("Failed to find pip dist-info in new virtualenv")
+
+        with open(os.path.join(site_packages, pip_dist_info, "INSTALLER"), "w") as file:
+            file.write("mach")
+
     def _run_pip(self, args):
         env = os.environ.copy()
         env.setdefault("ARCHFLAGS", get_archflags())