Cheroot ignores the Content-Length header when the connection is half-closed.

[kenballus]

❓ I'm submitting a ...

• 🐞 bug report
• 🐣 feature request
• ❓ question about the decisions made in the repository

🐞 Describe the bug. What is the current behavior?
From RFC 9112:

If a valid Content-Length header field is present without Transfer-Encoding, its decimal value defines the expected message body length in octets. If the sender closes the connection or the recipient times out before the indicated number of octets are received, the recipient MUST consider the message to be incomplete and close the connection.

Cheroot does not enforce this rule. When it receives a request, and the sender half-closes the connection, Cheroot responds regardless of whether the request's body has been fully received.

❓ What is the motivation / use case for changing the behavior?
RFC compliance and hardening against potential framing-related attacks.

💡 To Reproduce

1. Start a Cheroot-based HTTP server that echos the message body. (e.g., this one)
2. Send it a request with an incomplete message body, followed by half-closing the socket, and observe that it still responds:

printf 'GET / HTTP/1.1\r\nHost: a\r\nContent-Length: 10\r\n\r\nA' | nc localhost 80

HTTP/1.1 200 OK
Content-type: application/json
Content-Length: 133
Date: Thu, 04 Jul 2024 16:12:55 GMT
Server: Cheroot/10.0.2.dev71+g1ff20b18

{"headers":[["SE9TVA==","YQ=="],["Q09OVEVOVF9MRU5HVEg=","MTA="]],"body":"QQ==","version":"SFRUUC8xLjE=","uri":"Lw==","method":"R0VU"}

5. Decode the response to see that it accepted the incomplete message body:

printf '{"headers":[["SE9TVA==","YQ=="],["Q09OVEVOVF9MRU5HVEg=","MTA="]],"body":"QQ==","version":"SFRUUC8xLjE=","uri":"Lw==","method":"R0VU"}' \
    | jq '.["body"]' \
    | xargs echo \
    | base64 -d \
    | xxd

00000000: 41                                       A

💡 Expected behavior
Cheroot should close the connection, as the RFC requires.

📋 Environment

• Cheroot version: 10.0.2.dev71+g1ff20b18
• Python version: 3.11.9
• OS: Linux d8087152685c 6.9.7-arch1-1 #1 SMP PREEMPT_DYNAMIC Fri, 28 Jun 2024 04:32:50 +0000 x86_64 GNU/Linux