Fix sysroot installation when emscripten itself is read-only

The resulting header tree need to be at least user-writable so that
we can incrementally copy files into it, adding new files and also
updating previously copied ones.

Sadly, python's shutil doesn't seem to have any options for disabling
the copying of mode bits.

Fixes: #24404

Author: sbc100

tools/system_libs.py

@@ -13,6 +13,7 @@
 import shutil
 import textwrap
 import shlex
+import stat
 from enum import IntEnum, auto
 from glob import iglob
 from typing import List, Optional
@@ -2453,9 +2454,29 @@ def calculate(options):
   return ret
 
 
-def copytree_exist_ok(src, dst):
+def ensure_writable(filename):
+  if not os.access(filename, os.W_OK):
+    current_mode = stat.S_IMODE(os.stat(filename).st_mode)
+    os.chmod(filename, current_mode | stat.S_IWUSR)
+
+
+def copy_header_tree(src, dst):
   shutil.copytree(src, dst, dirs_exist_ok=True)
 
+  # Python's `shutil.copytree` will unconditonally copy the mode bits.
+  # However, because we install header incrementally (including overwriting
+  # earlier ones) we need the tree to at least be user-writable.  The use
+  # case for this is OS distributions where the entire emscripten tree is
+  # completely read-only (e.g. NixOs apparently).
+  ensure_writable(dst)
+  for root, dirs, files in os.walk(dst):
+    for d in dirs:
+      fullname = os.path.join(dst, root, d)
+      ensure_writable(fullname)
+    for f in files:
+      fullname = os.path.join(dst, root, f)
+      ensure_writable(fullname)
+
 
 def install_system_headers(stamp):
   install_dirs = {
@@ -2477,15 +2498,15 @@ def install_system_headers(stamp):
   for src, dest in install_dirs.items():
     src = utils.path_from_root('system', *src)
     dest = os.path.join(target_include_dir, dest)
-    copytree_exist_ok(src, dest)
+    copy_header_tree(src, dest)
 
   pkgconfig_src = utils.path_from_root('system/lib/pkgconfig')
   pkgconfig_dest = cache.get_sysroot_dir('lib/pkgconfig')
-  copytree_exist_ok(pkgconfig_src, pkgconfig_dest)
+  copy_header_tree(pkgconfig_src, pkgconfig_dest)
 
   bin_src = utils.path_from_root('system/bin')
   bin_dest = cache.get_sysroot_dir('bin')
-  copytree_exist_ok(bin_src, bin_dest)
+  copy_header_tree(bin_src, bin_dest)
 
   # Create a version header based on the emscripten-version.txt
   version_file = cache.get_include_dir('emscripten/version.h')