fix: add reentrant guard to Unpacker.feed() (#704)

fix ##695

msgpack/_unpacker.pyx

@@ -305,6 +305,8 @@ cdef class Unpacker:
     Raises ``OutOfData`` when *packed* is incomplete.
     Raises ``FormatError`` when *packed* is not valid msgpack.
     Raises ``StackError`` when *packed* contains too nested.
+    Raises ``RuntimeError`` when ``feed()`` is called while unpacking
+    is in progress (e.g. from a hook).
     Other exceptions can be raised during unpacking.
     """
     cdef unpack_context ctx
@@ -318,6 +320,7 @@ cdef class Unpacker:
     cdef object unicode_errors
     cdef Py_ssize_t max_buffer_size
     cdef uint64_t stream_offset
+    cdef bint _unpacking
 
     def __dealloc__(self):
         unpack_clear(&self.ctx)
@@ -381,6 +384,7 @@ cdef class Unpacker:
         self.buf_head = 0
         self.buf_tail = 0
         self.stream_offset = 0
+        self._unpacking = False
 
         if unicode_errors is not None:
             self.unicode_errors = unicode_errors
@@ -398,6 +402,11 @@ cdef class Unpacker:
         cdef char* buf
         cdef Py_ssize_t buf_len
 
+        if self._unpacking:
+            raise RuntimeError(
+                "Unpacker.feed() cannot be called while unpacking is in progress"
+            )
+
         if self.file_like is not None:
             raise AssertionError(
                     "unpacker.feed() is not be able to use with `file_like`.")
@@ -465,36 +474,40 @@ cdef class Unpacker:
         cdef object obj
         cdef Py_ssize_t prev_head
 
-        while 1:
-            prev_head = self.buf_head
-            if prev_head < self.buf_tail:
-                ret = execute(&self.ctx, self.buf, self.buf_tail, &self.buf_head)
-                self.stream_offset += self.buf_head - prev_head
-            else:
-                ret = 0
-
-            if ret == 1:
-                obj = unpack_data(&self.ctx)
-                unpack_init(&self.ctx)
-                return obj
-            if ret == 0:
-                if self.file_like is not None:
-                    self.read_from_file()
-                    continue
-                if iter:
-                    raise StopIteration("No more data to unpack.")
+        self._unpacking = True
+        try:
+            while 1:
+                prev_head = self.buf_head
+                if prev_head < self.buf_tail:
+                    ret = execute(&self.ctx, self.buf, self.buf_tail, &self.buf_head)
+                    self.stream_offset += self.buf_head - prev_head
                 else:
-                    raise OutOfData("No more data to unpack.")
+                    ret = 0
 
-            unpack_clear(&self.ctx)
-            if ret == -2:
-                raise FormatError
-            elif ret == -3:
-                raise StackError
-            elif PyErr_Occurred():
-                raise
-            else:
-                raise ValueError("Unpack failed: error = %d" % (ret,))
+                if ret == 1:
+                    obj = unpack_data(&self.ctx)
+                    unpack_init(&self.ctx)
+                    return obj
+                if ret == 0:
+                    if self.file_like is not None:
+                        self.read_from_file()
+                        continue
+                    if iter:
+                        raise StopIteration("No more data to unpack.")
+                    else:
+                        raise OutOfData("No more data to unpack.")
+
+                unpack_clear(&self.ctx)
+                if ret == -2:
+                    raise FormatError
+                elif ret == -3:
+                    raise StackError
+                elif PyErr_Occurred():
+                    raise
+                else:
+                    raise ValueError("Unpack failed: error = %d" % (ret,))
+        finally:
+            self._unpacking = False
 
     @cython.critical_section
     def read_bytes(self, Py_ssize_t nbytes):

test/test_unpack.py

@@ -123,3 +123,22 @@ def test_unpacker_reinit_clears_partial_state():
 
     unpacker.feed(packb({"a": 1}))
     assert unpacker.unpack() == {"a": 1}
+
+
+@mark.skipif(
+    Unpacker.__module__ == "msgpack.fallback",
+    reason="reentrant guard is implemented in C extension only",
+)
+def test_unpacker_reentrant_feed():
+    import struct
+
+    def ext_hook(code, data):
+        # re-entrant feed on the SAME unpacker, large enough to force a buffer realloc
+        up.feed(b"\xc0" * 100)
+        return 0
+
+    up = Unpacker(ext_hook=ext_hook, max_buffer_size=64 * 1024 * 1024)
+    # array(11): [ ExtType(code=5, data=b'A') (fires the re-entrant hook), then 10 more elements ]
+    up.feed(b"\xdc" + struct.pack(">H", 11) + b"\xd4\x05A" + b"\x2a" * 10)
+    with raises(RuntimeError):
+        up.unpack()