Update event stream tests (boto#3254)

This fixes a pair of event stream tests. Implementations of
`application/vnd.amazon.eventstream` are expected to check the CRC
before attempting to semantically parse the prelude. The tests meant
to enforce that were present, but the wrong error was being asserted.

This change fixes the ordering of checks, fixes the tests, and adds
two new test cases to make assertions about the error types that were
previously being applied to the old tests.

botocore/eventstream.py

@@ -466,9 +466,9 @@ def _parse_prelude(self):
         prelude_bytes = self._data[:_PRELUDE_LENGTH]
         raw_prelude, _ = DecodeUtils.unpack_prelude(prelude_bytes)
         prelude = MessagePrelude(*raw_prelude)
-        self._validate_prelude(prelude)
         # The minus 4 removes the prelude crc from the bytes to be checked
         _validate_checksum(prelude_bytes[: _PRELUDE_LENGTH - 4], prelude.crc)
+        self._validate_prelude(prelude)
         return prelude
 
     def _parse_headers(self):

tests/unit/test_eventstream.py

@@ -218,7 +218,7 @@
         b"\x00\x00\x00=\xff\x00\x01\x02\x07\xfd\x83\x96\x0ccontent-type\x07\x00"
         b"\x10application/json{'foo':'bar'}\x8d\x9c\x08\xb1"
     ),
-    InvalidHeadersLength,
+    ChecksumMismatch,
 )
 
 CORRUPTED_HEADERS = (
@@ -231,7 +231,7 @@
 
 CORRUPTED_LENGTH = (
     b"\x01\x00\x00\x1d\x00\x00\x00\x00\xfdR\x8cZ{'foo':'bar'}\xc3e96",
-    InvalidPayloadLength,
+    ChecksumMismatch,
 )
 
 CORRUPTED_PAYLOAD = (
@@ -247,13 +247,40 @@
     DuplicateHeader,
 )
 
+# In contrast to the CORRUPTED_HEADERS case, this message is otherwise
+# well-formed - the checksums match.
+INVALID_HEADERS_LENGTH = (
+    (
+        b"\x00\x00\x00\x3d"  # total length
+        b"\xff\x00\x01\x02"  # headers length
+        b"\x15\x83\xf5\xc2"  # prelude crc
+        b"\x0ccontent-type\x07\x00\x10application/json"  # headers
+        b"{'foo':'bar'}"  # payload
+        b"\x2f\x37\x7f\x5d"  # message crc
+    ),
+    InvalidHeadersLength,
+)
+
+# In contrast to the CORRUPTED_PAYLOAD case, this message is otherwise
+# well-formed - the checksums match.
+INVALID_PAYLOAD_LENGTH = (
+    b"\x01\x00\x00\x11"  # total length
+    + b"\x00\x00\x00\x00"  # headers length
+    + b"\xf4\x08\x61\xc5"  # prelude crc
+    + b"0" * (16 * 1024**2 + 1)  # payload
+    + b"\x2a\xb4\xc5\xa5",  # message crc
+    InvalidPayloadLength,
+)
+
 # Tuples of encoded messages and their expected exception
 NEGATIVE_CASES = [
     CORRUPTED_LENGTH,
     CORRUPTED_PAYLOAD,
     CORRUPTED_HEADERS,
     CORRUPTED_HEADER_LENGTH,
     DUPLICATE_HEADER,
+    INVALID_HEADERS_LENGTH,
+    INVALID_PAYLOAD_LENGTH,
 ]
 
 
@@ -311,7 +338,19 @@ def test_all_positive_cases():
         assert_message_equal(expected, decoded)
 
 
-@pytest.mark.parametrize("encoded, exception", NEGATIVE_CASES)
+@pytest.mark.parametrize(
+    "encoded, exception",
+    NEGATIVE_CASES,
+    ids=[
+        "corrupted-length",
+        "corrupted-payload",
+        "corrupted-headers",
+        "corrupted-headers-length",
+        "duplicate-headers",
+        "invalid-headers-length",
+        "invalid-payload-length",
+    ],
+)
 def test_negative_cases(encoded, exception):
     """Test that all negative cases raise the expected exception."""
     with pytest.raises(exception):