PUT requests for zero-byte files get 'Transfer-Encoding': 'chunked' header

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
48/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
api

Research direction

Start by reproducing the issue through the requests.put or session.put entry points with an empty file, then inspect the prepared request headers and transfer-encoding path. Done means an empty PUT sends Content-Length: 0 without Transfer-Encoding: chunked, including when Content-Length is supplied explicitly, with regression coverage for the reported cases.

Written by the indexing model from the issue text.

Description

Context: uploading files to AWS S3 via requests. Files may be empty. When attempting to upload an empty file, a response of 501: Not implemented is received. This seems to be because, when content length is 0, a Transfer-Encoding: chunked header is automatically added. S3 does not support the encoding.

Expected Result

When an empty file PUT is attempted via requests.put, a Content-Length: 0 header should be set on the request. A Transfer-Encoding: chunked header should not be set automatically.

Actual Result

When an empty file PUT is attempted via requests.put, a Transfer-Encoding: chunked header is set. No Content-Length header is set. If a Content-Length: 0 header is set explicitly, the Transfer-Encoding: chunked header is still set.

Reproduction Steps

If AWS account, bucket, credentials for principal with PutObject permissions aren't available:

import requests

file = open(<path_to_empty_file>, "rb")
session = requests.session()

link = "http://httpbin.org/put"

# using requests.put yields the same results. Using session.put here to examine request
# with S3 instead of httpbin.org, the call below results in a 501. Note the request headers
res_no_CL = session.put(link, data=file, headers={"Content-Type": "text/plain"})
print(res_no_CL.request.headers)
# with S3 instead of httpbin.org, the call below results in a 501. With httpbin.org it returns
# ConnectionError: ('Connection aborted.', RemoteDisconnected('Remote end closed connection without response'))
# so can't examine headers, but I assume behaviour is consistent
res_explicit_CL = session.put(link, data=file, headers={"Content-Type": "text/plain", "Content-Length": "0"})
print(res_explicit_CL.request.headers)

If AWS prerequisites above are available:

import requests
import boto3

s3 = boto3.client('s3')
file = open(<path_to_empty_file>, "rb")
session = requests.session()

link = s3.generate_presigned_url(ClientMethod="put_object", Params={"Bucket": "<your_bucket>", "Key": "<path_in_bucket>", "ContentType": "text/plain"})

# using requests.put yields the same results. Using session.put here to examine request
# the call below results in a 501. Note the request headers
res_no_CL = session.put(link, data=file, headers={"Content-Type": "text/plain"})
print(res_no_CL.request.headers)
# the call below results in a 501. Note the request headers
res_explicit_CL = session.put(link, data=file, headers={"Content-Type": "text/plain", "Content-Length": "0"})
print(res_explicit_CL.request.headers)

System Information

$ python -m requests.help

{
  "chardet": {
    "version": null
  },
  "charset_normalizer": {
    "version": "2.1.1"
  },
  "cryptography": {
    "version": ""
  },
  "idna": {
    "version": "3.4"
  },
  "implementation": {
    "name": "CPython",
    "version": "3.9.5"
  },
  "platform": {
    "release": "5.10.102.1-microsoft-standard-WSL2",
    "system": "Linux"
  },
  "pyOpenSSL": {
    "openssl_version": "",
    "version": null
  },
  "requests": {
    "version": "2.28.1"
  },
  "system_ssl": {
    "version": "1010106f"
  },
  "urllib3": {
    "version": "1.26.12"
  },
  "using_charset_normalizer": true,
  "using_pyopenssl": false
}
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