Repository: twisted/twisted

Issue:
Twisted web client doesn't support trailer Server-Timing
**Describe the incorrect behavior you saw**
Twisted web client doesn't support trailer Server-Timing.
Example: https://w3c.github.io/server-timing/#the-server-timing-header-field

**Describe how to cause this behavior**
Use console program (simple http client) from this gist: https://gist.github.com/taroved/06922ceeb872094304cc889dacae099f 

Always you run the code:
```shell
python httpclient.py https://www.esquire.com/
```
You see such exception:
```
Traceback (most recent call last):
Failure: twisted.web._newclient.ResponseFailed: [<twisted.python.failure.Failure twisted.web.http._MalformedChunkedDataError: Chunk did not end with CRLF>, <twisted.python.failure.Failure twisted.web.http._DataLoss: Chunked decoder in 'TRAILER' state, still expecting more data to get to 'FINISHED' state.>]
```

**Describe the correct behavior you'd like to see**
Such pages as https://www.esquire.com/ should be downloaded without exceptions.

**Testing environment**
 - Linux 939ff8bae3fe 5.15.49-linuxkit-pr
 - Twisted version: 23.8.0



Discussion:

Resolving patch:
diff --git a/src/twisted/web/http.py b/src/twisted/web/http.py
--- a/src/twisted/web/http.py
+++ b/src/twisted/web/http.py
@@ -1805,7 +1805,6 @@ def noMoreData(self):
 
 maxChunkSizeLineLength = 1024
 
-
 _chunkExtChars = (
     b"\t !\"#$%&'()*+,-./0123456789:;<=>?@"
     b"ABCDEFGHIJKLMNOPQRSTUVWXYZ[]^_`"
@@ -1889,12 +1888,20 @@ class _ChunkedTransferDecoder:
         state transition this is truncated at the front so that index 0 is
         where the next state shall begin.
 
-    @ivar _start: While in the C{'CHUNK_LENGTH'} state, tracks the index into
-        the buffer at which search for CRLF should resume. Resuming the search
-        at this position avoids doing quadratic work if the chunk length line
-        arrives over many calls to C{dataReceived}.
+    @ivar _start: While in the C{'CHUNK_LENGTH'} and C{'TRAILER'} states,
+        tracks the index into the buffer at which search for CRLF should resume.
+        Resuming the search at this position avoids doing quadratic work if the
+        chunk length line arrives over many calls to C{dataReceived}.
+
+    @ivar _trailerHeaders: Accumulates raw/unparsed trailer headers.
+        See https://github.com/twisted/twisted/issues/12014
+
+    @ivar _maxTrailerHeadersSize: Maximum bytes for trailer header from the
+        response.
+    @type _maxTrailerHeadersSize: C{int}
 
-        Not used in any other state.
+    @ivar _receivedTrailerHeadersSize: Bytes received so far for the tailer headers.
+    @type _receivedTrailerHeadersSize: C{int}
     """
 
     state = "CHUNK_LENGTH"
@@ -1908,6 +1915,9 @@ def __init__(
         self.finishCallback = finishCallback
         self._buffer = bytearray()
         self._start = 0
+        self._trailerHeaders: List[bytearray] = []
+        self._maxTrailerHeadersSize = 2**16
+        self._receivedTrailerHeadersSize = 0
 
     def _dataReceived_CHUNK_LENGTH(self) -> bool:
         """
@@ -1984,23 +1994,37 @@ def _dataReceived_CRLF(self) -> bool:
 
     def _dataReceived_TRAILER(self) -> bool:
         """
-        Await the carriage return and line feed characters that follow the
-        terminal zero-length chunk. Then invoke C{finishCallback} and switch to
-        state C{'FINISHED'}.
+        Collect trailer headers if received and finish at the terminal zero-length
+        chunk. Then invoke C{finishCallback} and switch to state C{'FINISHED'}.
 
         @returns: C{False}, as there is either insufficient data to continue,
             or no data remains.
-
-        @raises _MalformedChunkedDataError: when anything other than CRLF is
-            received.
         """
-        if len(self._buffer) < 2:
+        if (
+            self._receivedTrailerHeadersSize + len(self._buffer)
+            > self._maxTrailerHeadersSize
+        ):
+            raise _MalformedChunkedDataError("Trailer headers data is too long.")
+
+        eolIndex = self._buffer.find(b"\r\n", self._start)
+
+        if eolIndex == -1:
+            # Still no end of network line marker found.
+            # Continue processing more data.
             return False
 
-        if not self._buffer.startswith(b"\r\n"):
-            raise _MalformedChunkedDataError("Chunk did not end with CRLF")
+        if eolIndex > 0:
+            # A trailer header was detected.
+            self._trailerHeaders.append(self._buffer[0:eolIndex])
+            del self._buffer[0 : eolIndex + 2]
+            self._start = 0
+            self._receivedTrailerHeadersSize += eolIndex + 2
+            return True
+
+        # eolIndex in this part of code is equal to 0
 
         data = memoryview(self._buffer)[2:].tobytes()
+
         del self._buffer[:]
         self.state = "FINISHED"
         self.finishCallback(data)


Test patch:
diff --git a/src/twisted/web/test/test_http.py b/src/twisted/web/test/test_http.py
--- a/src/twisted/web/test/test_http.py
+++ b/src/twisted/web/test/test_http.py
@@ -1224,6 +1224,7 @@ def test_short(self):
             p.dataReceived(s)
         self.assertEqual(L, [b"a", b"b", b"c", b"1", b"2", b"3", b"4", b"5"])
         self.assertEqual(finished, [b""])
+        self.assertEqual(p._trailerHeaders, [])
 
     def test_long(self):
         """
@@ -1381,20 +1382,6 @@ def test_malformedChunkEnd(self):
             http._MalformedChunkedDataError, p.dataReceived, b"3\r\nabc!!!!"
         )
 
-    def test_malformedChunkEndFinal(self):
-        r"""
-        L{_ChunkedTransferDecoder.dataReceived} raises
-        L{_MalformedChunkedDataError} when the terminal zero-length chunk is
-        followed by characters other than C{\r\n}.
-        """
-        p = http._ChunkedTransferDecoder(
-            lambda b: None,
-            lambda b: None,  # pragma: nocov
-        )
-        self.assertRaises(
-            http._MalformedChunkedDataError, p.dataReceived, b"3\r\nabc\r\n0\r\n!!"
-        )
-
     def test_finish(self):
         """
         L{_ChunkedTransferDecoder.dataReceived} interprets a zero-length
@@ -1469,6 +1456,79 @@ def finished(extra):
         self.assertEqual(errors, [])
         self.assertEqual(successes, [True])
 
+    def test_trailerHeaders(self):
+        """
+        L{_ChunkedTransferDecoder.dataReceived} decodes chunked-encoded data
+        and ignores trailer headers which come after the terminating zero-length
+        chunk.
+        """
+        L = []
+        finished = []
+        p = http._ChunkedTransferDecoder(L.append, finished.append)
+        p.dataReceived(b"3\r\nabc\r\n5\r\n12345\r\n")
+        p.dataReceived(
+            b"a\r\n0123456789\r\n0\r\nServer-Timing: total;dur=123.4\r\nExpires: Wed, 21 Oct 2015 07:28:00 GMT\r\n\r\n"
+        )
+        self.assertEqual(L, [b"abc", b"12345", b"0123456789"])
+        self.assertEqual(finished, [b""])
+        self.assertEqual(
+            p._trailerHeaders,
+            [
+                b"Server-Timing: total;dur=123.4",
+                b"Expires: Wed, 21 Oct 2015 07:28:00 GMT",
+            ],
+        )
+
+    def test_shortTrailerHeader(self):
+        """
+        L{_ChunkedTransferDecoder.dataReceived} decodes chunks of input with
+        tailer header broken up and delivered in multiple calls.
+        """
+        L = []
+        finished = []
+        p = http._ChunkedTransferDecoder(L.append, finished.append)
+        for s in iterbytes(
+            b"3\r\nabc\r\n5\r\n12345\r\n0\r\nServer-Timing: total;dur=123.4\r\n\r\n"
+        ):
+            p.dataReceived(s)
+        self.assertEqual(L, [b"a", b"b", b"c", b"1", b"2", b"3", b"4", b"5"])
+        self.assertEqual(finished, [b""])
+        self.assertEqual(p._trailerHeaders, [b"Server-Timing: total;dur=123.4"])
+
+    def test_tooLongTrailerHeader(self):
+        r"""
+        L{_ChunkedTransferDecoder.dataReceived} raises
+        L{_MalformedChunkedDataError} when the trailing headers data is too long.
+        """
+        p = http._ChunkedTransferDecoder(
+            lambda b: None,
+            lambda b: None,  # pragma: nocov
+        )
+        p._maxTrailerHeadersSize = 10
+        self.assertRaises(
+            http._MalformedChunkedDataError,
+            p.dataReceived,
+            b"3\r\nabc\r\n0\r\nTotal-Trailer: header;greater-then=10\r\n\r\n",
+        )
+
+    def test_unfinishedTrailerHeader(self):
+        r"""
+        L{_ChunkedTransferDecoder.dataReceived} raises
+        L{_MalformedChunkedDataError} when the trailing headers data is too long
+        and doesn't have final CRLF characters.
+        """
+        p = http._ChunkedTransferDecoder(
+            lambda b: None,
+            lambda b: None,  # pragma: nocov
+        )
+        p._maxTrailerHeadersSize = 10
+        p.dataReceived(b"3\r\nabc\r\n0\r\n0123456789")
+        self.assertRaises(
+            http._MalformedChunkedDataError,
+            p.dataReceived,
+            b"A",
+        )
+
 
 class ChunkingTests(unittest.TestCase, ResponseTestMixin):
     strings = [b"abcv", b"", b"fdfsd423", b"Ffasfas\r\n", b"523523\n\rfsdf", b"4234"]