[3.7] Form data processed (#4351) (#4352)

(cherry picked from commit 0273535)

Co-authored-by: Adam Bannister <adam.p.bannister@gmail.com>

CHANGES/4345.bugfix

@@ -0,0 +1 @@
+Raise ClientPayloadError if FormData re-processed.

aiohttp/formdata.py

@@ -22,6 +22,7 @@ def __init__(self, fields:
         self._writer = multipart.MultipartWriter('form-data')
         self._fields = []  # type: List[Any]
         self._is_multipart = False
+        self._is_processed = False
         self._quote_fields = quote_fields
         self._charset = charset
 
@@ -115,6 +116,8 @@ def _gen_form_urlencoded(self) -> payload.BytesPayload:
 
     def _gen_form_data(self) -> multipart.MultipartWriter:
         """Encode a list of fields using the multipart/form-data MIME format"""
+        if self._is_processed:
+            raise RuntimeError('Form data has been processed already')
         for dispparams, headers, value in self._fields:
             try:
                 if hdrs.CONTENT_TYPE in headers:
@@ -141,6 +144,7 @@ def _gen_form_data(self) -> multipart.MultipartWriter:
 
             self._writer.append_payload(part)
 
+        self._is_processed = True
         return self._writer
 
     def __call__(self) -> Payload:

tests/test_formdata.py

@@ -2,7 +2,7 @@
 
 import pytest
 
-from aiohttp.formdata import FormData
+from aiohttp import ClientSession, FormData
 
 
 @pytest.fixture
@@ -86,3 +86,16 @@ async def test_formdata_field_name_is_not_quoted(buf, writer) -> None:
     payload = form()
     await payload.write(writer)
     assert b'name="emails[]"' in buf
+
+
+async def test_mark_formdata_as_processed() -> None:
+    async with ClientSession() as session:
+        url = "http://httpbin.org/anything"
+        data = FormData()
+        data.add_field("test", "test_value", content_type="application/json")
+
+        await session.post(url, data=data)
+        assert len(data._writer._parts) == 1
+
+        with pytest.raises(RuntimeError):
+            await session.post(url, data=data)

tests/test_web_functional.py

@@ -38,6 +38,13 @@ def fname(here):
     return here / 'conftest.py'
 
 
+def new_dummy_form():
+    form = FormData()
+    form.add_field('name', b'123',
+                   content_transfer_encoding='base64')
+    return form
+
+
 async def test_simple_get(aiohttp_client) -> None:
 
     async def handler(request):
@@ -514,15 +521,11 @@ async def expect_handler(request):
         if request.version == HttpVersion11:
             await request.writer.write(b"HTTP/1.1 100 Continue\r\n\r\n")
 
-    form = FormData()
-    form.add_field('name', b'123',
-                   content_transfer_encoding='base64')
-
     app = web.Application()
     app.router.add_post('/', handler, expect_handler=expect_handler)
     client = await aiohttp_client(app)
 
-    resp = await client.post('/', data=form, expect100=True)
+    resp = await client.post('/', data=new_dummy_form(), expect100=True)
     assert 200 == resp.status
     assert expect_received
 
@@ -541,20 +544,16 @@ async def expect_handler(request):
 
             await request.writer.write(b"HTTP/1.1 100 Continue\r\n\r\n")
 
-    form = FormData()
-    form.add_field('name', b'123',
-                   content_transfer_encoding='base64')
-
     app = web.Application()
     app.router.add_post('/', handler, expect_handler=expect_handler)
     client = await aiohttp_client(app)
 
     auth_err = False
-    resp = await client.post('/', data=form, expect100=True)
+    resp = await client.post('/', data=new_dummy_form(), expect100=True)
     assert 200 == resp.status
 
     auth_err = True
-    resp = await client.post('/', data=form, expect100=True)
+    resp = await client.post('/', data=new_dummy_form(), expect100=True)
     assert 403 == resp.status