Fix pack_map_pairs recovery after packing errors

msgpack/_packer.pyx

@@ -332,13 +332,17 @@ cdef class Packer:
         (`len(pairs)` and `for k, v in pairs:` should be supported.)
         """
         self._check_exports()
-        size = len(pairs)
-        if size > ITEM_LIMIT:
-            raise ValueError("map too large")
-        msgpack_pack_map(&self.pk, size)
-        for k, v in pairs:
-            self._pack(k)
-            self._pack(v)
+        try:
+            size = len(pairs)
+            if size > ITEM_LIMIT:
+                raise ValueError("map too large")
+            msgpack_pack_map(&self.pk, size)
+            for k, v in pairs:
+                self._pack(k)
+                self._pack(v)
+        except:
+            self.pk.length = 0
+            raise
         if self.autoreset:
             buf = PyBytes_FromStringAndSize(self.pk.buf, self.pk.length)
             self.pk.length = 0

msgpack/fallback.py

@@ -830,7 +830,11 @@ def pack(self, obj):
             return ret
 
     def pack_map_pairs(self, pairs):
-        self._pack_map_pairs(len(pairs), pairs)
+        try:
+            self._pack_map_pairs(len(pairs), pairs)
+        except:
+            self._buffer = BytesIO()
+            raise
         if self._autoreset:
             ret = self._buffer.getvalue()
             self._buffer = BytesIO()

test/test_pack.py

@@ -190,6 +190,37 @@ def test_pairlist():
     assert pairlist == unpacked
 
 
+@pytest.mark.parametrize("autoreset", [True, False])
+@pytest.mark.parametrize("method", ["pack", "pack_map_pairs"])
+def test_packer_resets_after_default_error(autoreset, method):
+    class Invoice:
+        def __init__(self, ready):
+            self.ready = ready
+
+    def default(invoice):
+        if not invoice.ready:
+            raise ValueError("invoice not ready")
+        return {"amount": 15}
+
+    packer = Packer(default=default, autoreset=autoreset)
+    packer.pack({"previous": 1})
+    pack = getattr(packer, method)
+    failed = [("id", 1), ("invoice", Invoice(False))]
+    with pytest.raises(ValueError, match="invoice not ready"):
+        pack(dict(failed) if method == "pack" else failed)
+    assert packer.bytes() == b""
+
+    valid = [("invoice", Invoice(True))]
+    packed = pack(dict(valid) if method == "pack" else valid)
+    if autoreset:
+        assert unpackb(packed) == {"invoice": {"amount": 15}}
+    else:
+        packer.pack({"next": 2})
+        unpacker = Unpacker()
+        unpacker.feed(packer.bytes())
+        assert list(unpacker) == [{"invoice": {"amount": 15}}, {"next": 2}]
+
+
 def test_get_buffer():
     packer = Packer(autoreset=0, use_bin_type=True)
     packer.pack([1, 2])