Fix pywasmcross when pyodide_build is installed out of tree (#2824)

Symlinking cc to pywasmcross.py only works if pywasmcross.py has
execute permissions. When we install the package with pip, it will not
set execute permissions on pywasmcross.py. It does set execute flags
on entrypoints. Thus, define an entrypoint called _pywasmcross which
calls pywasmcross.main. If a script called _pywasmcross exists, we are
using an out-of-tree install so symlink cc to _pywasmcross. Otherwise,
we should be in tree and pywasmcross.py should have the execute flag
set, so symlink cc to pywasmcross.py.

On the other side, if __main__.__file__ is in a folder named pyodide_build
or bin we are being invoked normally, otherwise we are being invoked via
a symlink.

pyodide-build/pyodide_build/pywasmcross.py

@@ -12,33 +12,45 @@ import json
 import os
 import sys
 from pathlib import Path, PurePosixPath
+from typing import Any
 
-IS_MAIN = __name__ == "__main__"
-if IS_MAIN:
+from __main__ import __file__ as INVOKED_PATH_STR
+
+INVOKED_PATH = Path(INVOKED_PATH_STR)
+
+SYMLINKS = {"cc", "c++", "ld", "ar", "gcc", "gfortran", "cargo"}
+IS_COMPILER_INVOCATION = INVOKED_PATH.name in SYMLINKS
+
+if IS_COMPILER_INVOCATION:
     # If possible load from environment variable, if necessary load from disk.
     if "PYWASMCROSS_ARGS" in os.environ:
         PYWASMCROSS_ARGS = json.loads(os.environ["PYWASMCROSS_ARGS"])
-    else:
-        with open(Path(__file__).parent / "pywasmcross_env.json") as f:
+    try:
+        with open(INVOKED_PATH.parent / "pywasmcross_env.json") as f:
             PYWASMCROSS_ARGS = json.load(f)
+    except FileNotFoundError:
+        raise RuntimeError(
+            "Invalid invocation: can't find PYWASMCROSS_ARGS."
+            f" Invoked from {INVOKED_PATH}."
+        )
 
-    # restore __name__ so that relative imports work as we expect
-    __name__ = PYWASMCROSS_ARGS.pop("orig__name__")
     sys.path = PYWASMCROSS_ARGS.pop("PYTHONPATH")
     os.environ["PATH"] = PYWASMCROSS_ARGS.pop("PATH")
+    # restore __name__ so that relative imports work as we expect
+    __name__ = PYWASMCROSS_ARGS.pop("orig__name__")
+
 
 import re
+import shutil
 import subprocess
 from collections import namedtuple
 from contextlib import contextmanager
 from tempfile import TemporaryDirectory
-from typing import Any, Iterator, Literal, MutableMapping, NoReturn
+from typing import Iterator, Literal, MutableMapping, NoReturn
 
 from pyodide_build import common
 from pyodide_build._f2c_fixes import fix_f2c_input, fix_f2c_output, scipy_fixes
 
-symlinks = {"cc", "c++", "ld", "ar", "gcc", "gfortran", "cargo"}
-
 
 def symlink_dir() -> Path:
     return Path(common.get_make_flag("TOOLSDIR")) / "symlinks"
@@ -68,13 +80,17 @@ def make_command_wrapper_symlinks(
     exist.
     """
     exec_path = Path(__file__).resolve()
-    for symlink in symlinks:
+    for symlink in SYMLINKS:
         symlink_path = symlink_dir / symlink
         if os.path.lexists(symlink_path) and not symlink_path.exists():
             # remove broken symlink so it can be re-created
             symlink_path.unlink()
         try:
-            symlink_path.symlink_to(exec_path)
+            pywasmcross_exe = shutil.which("_pywasmcross")
+            if pywasmcross_exe:
+                symlink_path.symlink_to(pywasmcross_exe)
+            else:
+                symlink_path.symlink_to(exec_path)
         except FileExistsError:
             pass
         if symlink == "c++":
@@ -609,13 +625,13 @@ def environment_substitute_args(
     return subbed_args
 
 
-if IS_MAIN:
-    REPLAY_ARGS = ReplayArgs(**PYWASMCROSS_ARGS)
-
+def compiler_main():
+    replay_args = ReplayArgs(**PYWASMCROSS_ARGS)
     basename = Path(sys.argv[0]).name
     args = list(sys.argv)
     args[0] = basename
-    if basename in symlinks:
-        sys.exit(handle_command(args, REPLAY_ARGS))
-    else:
-        raise Exception(f"Unexpected invocation '{basename}'")
+    sys.exit(handle_command(args, replay_args))
+
+
+if IS_COMPILER_INVOCATION:
+    compiler_main()

pyodide-build/setup.py

@@ -2,4 +2,10 @@
 import setuptools
 
 if __name__ == "__main__":
-    setuptools.setup()
+    setuptools.setup(
+        entry_points={
+            "console_scripts": [
+                "_pywasmcross = pyodide_build.pywasmcross:compiler_main",
+            ]
+        }
+    )