make_tree_writable: handle junctions and add tests

As found out here, os.walk() by default follows junctions, which we don't
want and can even lead to loops:
https://github.com/msys2/msys2-autobuild/issues/101#issuecomment-2583121845

Integrate the workaround mentioned in the CPython bug report:
https://github.com/python/cpython/issues/67596#issuecomment-1918112817
Since this is Python 3.12+ only and we still support 3.10 make
it optional though.

This also adds tests, which uncovered some other minor issues:
It was not chmoding top-down, which meant that os.walk would
skip things if there were no read permissions. So chmod before
os.walk() lists the dir.

msys2_autobuild/build.py

@@ -122,6 +122,29 @@ def run_cmd(msys2_root: PathLike, args: Sequence[PathLike], **kwargs: Any) -> No
     check_call([executable, '-lc'] + [shlex_join([str(a) for a in args])], env=env, **kwargs)
 
 
+def make_tree_writable(topdir: PathLike) -> None:
+    # Ensure all files and directories under topdir are writable
+    # (and readable) by owner.
+    # Taken from meson, and adjusted
+
+    def chmod(p: PathLike) -> None:
+        print(p)
+        os.chmod(p, os.stat(p).st_mode | stat.S_IWRITE | stat.S_IREAD)
+
+    chmod(topdir)
+    for root, dirs, files in os.walk(topdir):
+        for d in dirs:
+            chmod(os.path.join(root, d))
+        # Work around Python bug following junctions
+        # https://github.com/python/cpython/issues/67596#issuecomment-1918112817
+        if hasattr(os.path, 'isjunction'):  # Python 3.12 only
+            dirs[:] = [d for d in dirs if not os.path.isjunction(os.path.join(root, d))]
+        for fname in files:
+            fpath = os.path.join(root, fname)
+            if os.path.isfile(fpath):
+                chmod(fpath)
+
+
 def reset_git_repo(path: PathLike):
 
     def clean():
@@ -129,17 +152,6 @@ def reset_git_repo(path: PathLike):
         check_call(["git", "clean", "-xfdf"], cwd=path)
         check_call(["git", "reset", "--hard", "HEAD"], cwd=path)
 
-    def make_tree_writable(topdir: PathLike) -> None:
-        # Ensure all files and directories under topdir are writable
-        # (and readable) by owner.
-        # Taken from meson
-        for d, _, files in os.walk(topdir):
-            os.chmod(d, os.stat(d).st_mode | stat.S_IWRITE | stat.S_IREAD)
-            for fname in files:
-                fpath = os.path.join(d, fname)
-                if os.path.isfile(fpath):
-                    os.chmod(fpath, os.stat(fpath).st_mode | stat.S_IWRITE | stat.S_IREAD)
-
     made_writable = False
     for i in range(10):
         try:

tests/main_test.py

@@ -1,7 +1,40 @@
 # type: ignore
 
+import os
+import stat
+import tempfile
+from pathlib import Path
+
 from msys2_autobuild.utils import parse_optional_deps
 from msys2_autobuild.queue import parse_buildqueue, get_cycles
+from msys2_autobuild.build import make_tree_writable
+
+
+def test_make_tree_writable():
+    with tempfile.TemporaryDirectory() as tempdir:
+        nested_dir = Path(tempdir) / "nested"
+        nested_junction = nested_dir / "junction"
+        nested_dir.mkdir()
+        file_path = nested_dir / "test_file.txt"
+        file_path.write_text("content")
+
+        # Create a junction loop if possible, to make sure we ignore it
+        if hasattr(os.path, 'isjunction') and os.name == 'nt':
+            import _winapi
+            _winapi.CreateJunction(str(nested_dir), str(nested_junction))
+        else:
+            nested_junction.mkdir()
+
+        # Remove permissions
+        for p in [tempdir, nested_dir, file_path, nested_junction]:
+            os.chmod(p, os.stat(p).st_mode & ~stat.S_IWRITE & ~stat.S_IREAD)
+
+        make_tree_writable(tempdir)
+
+        assert os.access(tempdir, os.W_OK) and os.access(tempdir, os.R_OK)
+        assert os.access(nested_dir, os.W_OK) and os.access(nested_dir, os.R_OK)
+        assert os.access(file_path, os.W_OK) and os.access(file_path, os.R_OK)
+        assert os.access(nested_junction, os.W_OK) and os.access(nested_junction, os.R_OK)
 
 
 def test_parse_optional_deps():