mach: adopt uv and avoid system python (#34632)

This allows us to use `uv` for:
1. Installing a pinned Python version
2. Installing the dependency packages using `uv`'s pip compatible interface.
4. Bootstrapping `mach` without a Python installion on the host, using `uv
   run`

This change also introduces a new 'composite' GitHub action to setup
python in the different CI workflows. There is no support for externally
managed python installations and virtual environments. These could be
added in the future.

Fixes #34095, #34547

Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>

python/mach_bootstrap.py

@@ -4,10 +4,9 @@
 
 import hashlib
 import os
-import platform
-import site
 import subprocess
 import sys
+import runpy
 
 SCRIPT_PATH = os.path.abspath(os.path.dirname(__file__))
 TOP_DIR = os.path.abspath(os.path.join(SCRIPT_PATH, ".."))
@@ -81,35 +80,16 @@ CATEGORIES = {
 }
 
 
-# venv calls its scripts folder "bin" on non-Windows and "Scripts" on Windows.
-def _get_virtualenv_script_dir():
-    if os.name == "nt" and os.sep != "/":
-        return "Scripts"
-    return "bin"
-
-
-# venv names its lib folder something like "lib/python3.11/site-packages" on
-# non-Windows and "Lib\site-packages" on Window.
-def _get_virtualenv_lib_dir():
-    if os.name == "nt" and os.sep != "/":
-        return os.path.join("Lib", "site-packages")
-    return os.path.join(
-        "lib",
-        f"python{sys.version_info[0]}.{sys.version_info[1]}",
-        "site-packages"
-    )
-
-
-def _process_exec(args):
+def _process_exec(args, cwd):
     try:
-        subprocess.check_output(args, stderr=subprocess.STDOUT)
+        subprocess.check_output(args, stderr=subprocess.STDOUT, cwd=cwd)
     except subprocess.CalledProcessError as exception:
         print(exception.output.decode(sys.stdout.encoding))
         print(f"Process failed with return code: {exception.returncode}")
         sys.exit(1)
 
 
-def install_virtual_env_requirements(project_path: str, python: str, virtualenv_path: str):
+def install_virtual_env_requirements(project_path: str, virtualenv_path: str):
     requirements_paths = [
         os.path.join(project_path, "python", "requirements.txt"),
         os.path.join(project_path, WPT_TOOLS_PATH, "requirements_tests.txt",),
@@ -131,45 +111,32 @@ def install_virtual_env_requirements(project_path: str, python: str, virtualenv_
     requirements_hash = requirements_hasher.hexdigest()
 
     if marker_hash != requirements_hash:
-        print(" * Upgrading pip...")
-        _process_exec([python, "-m", "pip", "install", "--upgrade", "pip"])
-
         print(" * Installing Python requirements...")
-        _process_exec([python, "-m", "pip", "install", "-I",
-                       "-r", requirements_paths[0],
-                       "-r", requirements_paths[1],
-                       "-r", requirements_paths[2]])
+        pip_install_command = ["uv", "pip", "install"]
+        for requirements in requirements_paths:
+            pip_install_command.extend(["-r", requirements])
+        _process_exec(pip_install_command, cwd=project_path)
         with open(marker_path, "w") as marker_file:
             marker_file.write(requirements_hash)
 
 
 def _activate_virtualenv(topdir):
-    virtualenv_path = os.path.join(topdir, "python", "_venv%d.%d" % (sys.version_info[0], sys.version_info[1]))
-    python = sys.executable
+    virtualenv_path = os.path.join(topdir, ".venv")
 
     if os.environ.get("VIRTUAL_ENV") != virtualenv_path:
-        venv_script_path = os.path.join(virtualenv_path, _get_virtualenv_script_dir())
         if not os.path.exists(virtualenv_path):
             print(" * Setting up virtual environment...")
-            _process_exec([python, "-m", "venv", "--system-site-packages", virtualenv_path])
+            _process_exec(["uv", "venv"], cwd=topdir)
 
-        # This general approach is taken from virtualenv's `activate_this.py`.
-        os.environ["PATH"] = os.pathsep.join([venv_script_path, *os.environ.get("PATH", "").split(os.pathsep)])
-        os.environ["VIRTUAL_ENV"] = virtualenv_path
+        script_dir = "Scripts" if _is_windows() else "bin"
+        runpy.run_path(os.path.join(virtualenv_path, script_dir, 'activate_this.py'))
 
-        prev_length = len(sys.path)
-        lib_path = os.path.realpath(os.path.join(virtualenv_path, _get_virtualenv_lib_dir()))
-        site.addsitedir(lib_path)
-        sys.path[:] = sys.path[prev_length:] + sys.path[0:prev_length]
+    install_virtual_env_requirements(topdir, virtualenv_path)
 
-        sys.real_prefix = sys.prefix
-        sys.prefix = virtualenv_path
-
-        # Use the python in our venv for subprocesses, not the python we were originally run with.
-        # Otherwise pip may still try to write to the wrong site-packages directory.
-        python = os.path.join(venv_script_path, "python")
-
-    install_virtual_env_requirements(topdir, python, virtualenv_path)
+    # Turn off warnings about deprecated syntax in our indirect dependencies.
+    # TODO: Find a better approach for doing this.
+    import warnings
+    warnings.filterwarnings('ignore', category=SyntaxWarning, module=r'.*.venv')
 
 
 def _ensure_case_insensitive_if_windows():
@@ -221,13 +188,6 @@ def bootstrap(topdir):
         print('Current path:', topdir)
         sys.exit(1)
 
-    # Ensure we are running Python 3.10+. We put this check here so we generate a
-    # user-friendly error message rather than a cryptic stack trace on module import.
-    if sys.version_info < (3, 10):
-        print('Python3 (>=3.10) is required to run mach.')
-        print('You are running Python', platform.python_version())
-        sys.exit(1)
-
     _activate_virtualenv(topdir)
 
     def populate_context(context, key=None):