Fix false positive writability check on cache directory

We were incorrectly reporting non-existent parent directories as
non-writable.

This was broken in emscripten-core#22801.

Author: sbc100

test/test_other.py

@@ -15400,3 +15400,18 @@ def test_rust_integration_basics(self):
        return 0;
     }''')
     self.do_runf('main.cpp', 'Hello from rust!', emcc_args=[lib])
+
+  @crossplatform
+  def test_create_cache_directory(self):
+    # Test that cache directory (including parents directories) is
+    # created on demand.
+    with env_modify({'EM_CACHE': os.path.abspath('foo/bar')}):
+      self.run_process([EMCC, '-c', test_file('hello_world.c')])
+      self.assertExists('foo/bar/sysroot_install.stamp')
+
+    # Test that we generate a nice error when we cannot create the cache
+    # because it is in a read-only location.
+    self.assertFalse(os.access('/', os.W_OK))
+    with env_modify({'EM_CACHE': '/foo'}):
+      err = self.expect_fail([EMCC, '-c', test_file('hello_world.c')])
+      self.assertContained('emcc: error: unable to create cache directory "/foo"', err)

tools/cache.py

@@ -75,10 +75,10 @@ def lock(reason):
 def ensure():
   ensure_setup()
   if not os.path.isdir(cachedir):
-    parent_dir = os.path.dirname(cachedir)
-    if not is_writable(parent_dir):
-      utils.exit_with_error(f'unable to create cache directory "{cachedir}": parent directory not writable (see https://emscripten.org/docs/tools_reference/emcc.html for info on setting the cache directory)')
-    utils.safe_ensure_dirs(cachedir)
+    try:
+      utils.safe_ensure_dirs(cachedir)
+    except Exception as e:
+      utils.exit_with_error(f'unable to create cache directory "{cachedir}": {e} (see https://emscripten.org/docs/tools_reference/emcc.html for info on setting the cache directory)')
 
 
 def erase():