Redirects drop non-generator iterator and file-like request bodies

[BlueX888]

What happens

With follow_redirects=True, a 307/308 redirect (or 301 for a PUT) resends the request with an empty body when the original body was a non-generator iterator (iter([...]), map(...), zip(...)) or a file-like object. Generator bodies already raise httpx.StreamConsumed, and bytes bodies replay, so only these other non-replayable streams are silently emptied. The destination receives a request with Transfer-Encoding: chunked (or the original Content-Length) and zero body bytes, and the caller sees a successful 200.

Reproducer

import httpx

seen = []

class Transport(httpx.BaseTransport):
    def handle_request(self, request):
        seen.append((request.url.path, b"".join(request.stream)))
        if len(seen) == 1:
            return httpx.Response(307, headers={"location": "/final"})
        return httpx.Response(200)

with httpx.Client(transport=Transport()) as client:
    response = client.post(
        "http://testserver/start",
        content=iter([b"PAYLOAD"]),
        follow_redirects=True,
    )

print(httpx.__version__, response.status_code, seen)

On 0.28.1 this prints 0.28.1 200 [('/start', b'PAYLOAD'), ('/final', b'')]. A local loopback server shows the same over a real socket: /final receives no body and returns 200. The bytes control replays PAYLOAD; io.BytesIO instead raises LocalProtocolError: Too little data for declared Content-Length.

Why

httpx/_content.py marks only inspect.isgenerator() and inspect.isasyncgen() streams as single-use. __iter__/__aiter__ raise StreamConsumed only when that flag is set, so an exhausted iterator, map/zip result or file-like stream is re-iterated to nothing, and the redirect sends the empty stream.

Possible fix

Treat every non-replayable stream as single-use in httpx/_content.py: sync isinstance(stream, Iterator) or hasattr(stream, "read"), async isinstance(stream, AsyncIterator) or hasattr(stream, "aread"). Keep raising StreamConsumed; that matches the existing generator behavior and the intent of #670/#685. Seekable file-like objects could be rewound instead, but raising is the minimal safe behavior and the only option for bare iterators.

Would you like this escalated to an issue, or should a PR be invited? Direct PR creation is restricted to collaborators, so I did not open one.

Drafted with Claude Code, an LLM agent operating the BlueX888 account.