Fix sysroot installation when emscripten itself is read-only (#24405)

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.

This was originally fixed in #15386 but then regressed in #23417.

Fixes: #24404

Author: sbc100

test/test_sanity.py

@@ -7,6 +7,7 @@
 import os
 import platform
 import shutil
+import stat
 import time
 import re
 import tempfile
@@ -35,6 +36,14 @@ def restore():
   shutil.copyfile(EM_CONFIG + '_backup', EM_CONFIG)
 
 
+def for_all_files(dir, callback):
+  for root, dirs, files in os.walk(dir):
+    for d in dirs:
+      callback(os.path.join(dir, root, d))
+    for f in files:
+      callback(os.path.join(dir, root, f))
+
+
 # restore the config file and set it up for our uses
 def restore_and_set_up():
   restore()
@@ -460,6 +469,23 @@ def test_emcc_multiprocess_cache_access(self):
     # Exactly one child process should have triggered libc build!
     self.assertEqual(num_times_libc_was_built, 1)
 
+  # Test that sysroot headers can be installed from a read-only
+  # emscripten tree.
+  def test_readonly_sysroot_install(self):
+    restore_and_set_up()
+
+    def make_readonly(filename):
+      old_mode = stat.S_IMODE(os.stat(filename).st_mode)
+      os.chmod(filename, old_mode & ~(stat.S_IWUSR | stat.S_IWGRP | stat.S_IWOTH))
+
+    try:
+      for_all_files(path_from_root('system/include'), make_readonly)
+
+      with env_modify({'EM_CACHE': self.in_dir('test_cache')}):
+        self.run_process([EMCC, test_file('hello_world.c'), '-c'])
+    finally:
+      for_all_files(path_from_root('system/include'), shared.make_writable)
+
   @parameterized({
     '': [False, False],
     'response_files': [True, False],

tools/shared.py

@@ -694,7 +694,7 @@ def get_file_suffix(filename):
 
 
 def make_writable(filename):
-  assert os.path.isfile(filename)
+  assert os.path.exists(filename)
   old_mode = stat.S_IMODE(os.stat(filename).st_mode)
   os.chmod(filename, old_mode | stat.S_IWUSR)
 

tools/system_libs.py

@@ -2453,8 +2453,21 @@ def calculate(options):
   return ret
 
 
-def copytree_exist_ok(src, dst):
-  shutil.copytree(src, dst, dirs_exist_ok=True)
+def safe_copytree(src, dst):
+  # We cannot use `shutil.copytree` there because we need to ensure the
+  # output tree is writable, and in some cases the emscripten tree
+  # itself is readonly (e.g. NixOS).
+  # Even if we pass copy_function=safe_copy python's `shutil.copytree`
+  # will use its internal logic for copying directories and it will
+  # unconditionally copy the source directory's mode bits.
+  os.makedirs(dst, exist_ok=True)
+  for entry in os.scandir(src):
+    srcname = os.path.join(src, entry.name)
+    dstname = os.path.join(dst, entry.name)
+    if entry.is_dir():
+      safe_copytree(srcname, dstname)
+    else:
+      shared.safe_copy(srcname, dstname)
 
 
 def install_system_headers(stamp):
@@ -2477,15 +2490,15 @@ 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)
+    safe_copytree(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)
+  safe_copytree(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)
+  safe_copytree(bin_src, bin_dest)
 
   # Create a version header based on the emscripten-version.txt
   version_file = cache.get_include_dir('emscripten/version.h')