gh-123431: Harmonize extension code checks in pickle (GH-123434)

This checks are redundant in normal circumstances and can only work if
the extension registry was intentionally broken.

* The Python implementation now raises exception for the extension code
  with false boolean value.
* Simplify the C code. RuntimeError is now raised in explicit checks.
* Add many tests.

1 files changed, 11 insertions, 7 deletions

diff --git a/Lib/pickle.py b/Lib/pickle.py
index b8e114a..ade0491 100644
--- a/Lib/pickle.py
+++ b/Lib/pickle.py
@@ -1086,11 +1086,16 @@ class _Pickler:
 
         module_name = whichmodule(obj, name)
         if self.proto >= 2:
-            code = _extension_registry.get((module_name, name))
-            if code:
-                assert code > 0
+            code = _extension_registry.get((module_name, name), _NoValue)
+            if code is not _NoValue:
                 if code <= 0xff:
-                    write(EXT1 + pack("<B", code))
+                    data = pack("<B", code)
+                    if data == b'\0':
+                        # Should never happen in normal circumstances,
+                        # since the type and the value of the code are
+                        # checked in copyreg.add_extension().
+                        raise RuntimeError("extension code 0 is out of range")
+                    write(EXT1 + data)
                 elif code <= 0xffff:
                     write(EXT2 + pack("<H", code))
                 else:
@@ -1581,9 +1586,8 @@ class _Unpickler:
     dispatch[EXT4[0]] = load_ext4
 
     def get_extension(self, code):
-        nil = []
-        obj = _extension_cache.get(code, nil)
-        if obj is not nil:
+        obj = _extension_cache.get(code, _NoValue)
+        if obj is not _NoValue:
             self.append(obj)
             return
         key = _inverted_registry.get(code)