Avoid keeping module file open in tools/webassembly.py API (#17271)

These was a bug in my implementation of `@cache` that I added in #17255
where it was keeping the underlying wasm file open and never actually
closing it.  This is because I was using the incoming arguments as the
hash key instead of their hash value.

The bug only manifested on windows where open files are locked in some
way prevented further actions from modifying them and resulting is a
strange error:

```llvm-objcopy.exe: error: permission denied```

.circleci/config.yml

@@ -485,6 +485,7 @@ jobs:
             core2.test_em_asm_unicode
             core2.test_em_js
             core2.test_em_js_pthreads
+            other.test_c_preprocessor
             other.test_prejs_unicode
             other.test_em_js_side_module
             other.test_es5_transpile

tools/webassembly.py

@@ -8,6 +8,7 @@
 
 from collections import namedtuple
 from enum import IntEnum
+from functools import wraps
 import logging
 import os
 import sys
@@ -54,28 +55,26 @@ def read_sleb(iobuf):
   return leb128.i.decode_reader(iobuf)[0]
 
 
-# TODO(sbc): Use the builtin functools.cache once we update to python 3.9
-def cache(f):
-  results = {}
+def cache(method):
 
-  def helper(*args, **kwargs):
+  @wraps(method)
+  def wrapper(self, *args, **kwargs):
     assert not kwargs
-    key = args
-    if key not in results:
-      results[key] = f(*args, **kwargs)
-    return results[key]
+    key = method
+    if key not in self._cache:
+      self._cache[key] = method(self, *args, **kwargs)
+    return self._cache[key]
 
-  return helper
+  return wrapper
 
 
-def once(f):
-  done = False
+def once(method):
 
-  def helper(*args, **kwargs):
-    nonlocal done
-    if not done:
-      done = True
-      f(*args, **kwargs)
+  @wraps(method)
+  def helper(self, *args, **kwargs):
+    key = method
+    if key not in self._cache:
+      self._cache[key] = method(self, *args, **kwargs)
 
   return helper
 
@@ -167,7 +166,7 @@ def __init__(self, filename):
     version = self.buf.read(4)
     if magic != MAGIC or version != VERSION:
       raise InvalidWasmError(f'{filename} is not a valid wasm file')
-    self._done_calc_indexes = False
+    self._cache = {}
 
   def __del__(self):
     if self.buf: