Harden Unpacker.__init__ re-entry cleanup to prevent buffer/context leaks

msgpack/_unpacker.pyx

@@ -317,9 +317,6 @@ cdef class Unpacker:
     cdef Py_ssize_t max_buffer_size
     cdef uint64_t stream_offset
 
-    def __cinit__(self):
-        self.buf = NULL
-
     def __dealloc__(self):
         unpack_clear(&self.ctx)
         PyMem_Free(self.buf)
@@ -338,6 +335,12 @@ cdef class Unpacker:
                  Py_ssize_t max_ext_len=-1):
         cdef const char *cerr=NULL
 
+        unpack_clear(&self.ctx)
+        unpack_init(&self.ctx)
+        if self.buf != NULL:
+            PyMem_Free(self.buf)
+            self.buf = NULL
+
         self.object_hook = object_hook
         self.object_pairs_hook = object_pairs_hook
         self.list_hook = list_hook

test/test_unpack.py

@@ -1,4 +1,6 @@
+import gc
 import sys
+import weakref
 from io import BytesIO
 
 from pytest import mark, raises
@@ -87,3 +89,37 @@ def test_unpacker_tell_read_bytes():
         assert obj == unp
         assert pos == unpacker.tell()
         assert unpacker.read_bytes(n) == raw
+
+
+@mark.skipif(
+    Unpacker.__module__ == "msgpack.fallback",
+    reason="specific to C extension reinit leak",
+)
+def test_unpacker_reinit_clears_partial_state():
+    refs = []
+
+    class Marker:
+        pass
+
+    def hook(code, data):
+        obj = Marker()
+        refs.append(weakref.ref(obj))
+        return obj
+
+    unpacker = Unpacker(ext_hook=hook, strict_map_key=False)
+    # Keep parser state mid-map with a live key object from ext_hook.
+    # Encodes: [ {ExtType(1, b"a"): <missing value>} ].
+    unpacker.feed(b"\x91\x81\xd4\x01a")
+    with raises(OutOfData):
+        unpacker.unpack()
+    assert len(refs) == 1
+    assert refs[0]() is not None
+
+    unpacker.__init__()
+    gc.collect()
+    assert refs[0]() is None
+    with raises(OutOfData):
+        unpacker.unpack()
+
+    unpacker.feed(packb({"a": 1}))
+    assert unpacker.unpack() == {"a": 1}