Make create_pip_script work with uv actually this time (#185)

Weird hack to work around:
astral-sh/python-build-standalone#380
If we resolve the symlink all the way, the python-host interpreter works
but won't install into our pyodide venv. If we don't resolve the symlink,
sys.prefix is calculated incorrectly. To ensure that we get the right
sys.prefix, we explicitly set it with the PYTHONHOME environment variable
and then call the symlink.

Author: hoodmane

pyodide_build/out_of_tree/venv.py

@@ -222,26 +222,45 @@ def create_pip_script(venv_bin):
     # Python in the shebang. Use whichever Python was used to invoke
     # pyodide venv.
     host_python_path = venv_bin / f"python{get_pyversion()}-host"
+    host_python_path_no_version = venv_bin / "python-host"
     pip_path = venv_bin / "pip_patched"
+    python_host_link = venv_bin / "python-host-link"
 
     # To support the "--clear" and "--no-clear" args, we need to remove
     # the existing symlinks before creating new ones.
     host_python_path.unlink(missing_ok=True)
-    (venv_bin / "python-host").unlink(missing_ok=True)
+    host_python_path_no_version.unlink(missing_ok=True)
+    python_host_link.unlink(missing_ok=True)
     for pip in venv_bin.glob("pip*"):
         if pip == pip_path:
             continue
         pip.unlink(missing_ok=True)
         pip.symlink_to(pip_path)
 
-    host_python_path.symlink_to(sys.executable)
-    # in case someone needs a Python-version-agnostic way to refer to python-host
-    (venv_bin / "python-host").symlink_to(sys.executable)
+    # Weird hack to work around:
+    # https://github.com/astral-sh/python-build-standalone/issues/380
+    # If we resolve the symlink all the way, the python-host interpreter works
+    # but won't install into our pyodide venv. If we don't resolve the symlink,
+    # sys.prefix is calculated incorrectly. To ensure that we get the right
+    # sys.prefix, we explicitly set it with the PYTHONHOME environment variable
+    # and then call the symlink.
+    python_host_link.symlink_to(sys.executable)
+    pythonhome = Path(sys._base_executable).parents[1]
+    host_python_path.write_text(
+        dedent(
+            f"""\
+            #!/bin/sh
+            exec env PYTHONHOME={pythonhome} {python_host_link} $@
+            """
+        )
+    )
+    host_python_path.chmod(0o777)
+    host_python_path_no_version.symlink_to(host_python_path)
 
     pip_path.write_text(
         # Other than the shebang and the monkey patch, this is exactly what
         # normal pip looks like.
-        f"#!{host_python_path} -s\n"
+        f"#!/usr/bin/env -S {host_python_path} -s\n"
         + get_pip_monkeypatch(venv_bin)
         + dedent(
             """