fix(http): consume Content-Length bodies exactly + honor Expect: 100-continue

[jrjones]

Fixes #46

Summary

Two bugs in the HTTP request-body path (src/http/request.rs), both affecting uploads (POST /empty):

1. Fixed-length bodies are read in rigid 1024-byte read_exact chunks, counting buffer.len() (always 1024) per iteration. A Content-Length that is not a multiple of 1024 therefore (a) over-reads the final chunk, consuming bytes of the next pipelined request, and (b) against a peer that keeps the connection open, blocks forever in read_exact waiting for the missing tail of the final chunk — hanging the upload until the client times out / disconnects. Clients pad upload payloads to a 1024 boundary to avoid this; the fix removes that need.
2. Expect: 100-continue is never answered. Clients that use it (curl and many HTTP libraries) wait out their continue-timeout (~1s for curl) before sending the body on every upload.

Root cause

// BodyType::Fixed (before)
let mut buffer = [0; 1024];
let mut len: usize = 0;
loop {
    let bytes_read = buf_reader.read_exact(&mut buffer).await; // demands a FULL 1024 bytes
    match bytes_read {
        Ok(_) => { len += buffer.len();                        // always +1024, not actual
                   if len >= body_size as usize { break; } }
        Err(_) => break,
        // ...
    }
}

read_exact completes only once the whole 1024-byte buffer is filled, so the last partial chunk of a non-aligned body never completes against a still-open connection. And len += buffer.len() counts 1024 regardless of what was actually read, so the loop can stop 1–1023 bytes past the body. The Expect header is simply never inspected before the body read.

Fix

• Read the body with read (not read_exact) into a 64 KiB buffer, decrementing by the actual bytes returned and stopping exactly at Content-Length. Safe for any length; the larger buffer also cuts per-1 KiB read-syscall overhead.
• Before reading the body, if the request carries Expect: 100-continue, write the interim HTTP/1.1 100 Continue\r\n\r\n (RFC 9110 §10.1.1).

Validation

New regression tests (src/http/request.rs) that fail on master and pass with the fix:

test	before	after
non-1024-multiple body not over-read into a pipelined request	FAIL — 1 response instead of 2 (the pipelined GET is swallowed)	ok
Expect: 100-continue receives an interim 100	FAIL — no 100 Continue emitted	ok

End-to-end (release build, vs unpatched master on a second port):

scenario	unpatched	patched
non-1024-multiple upload, client keeps socket open	hangs — no response in 8s	200 OK, immediate
upload with Expect: 100-continue (curl default)	~1.07 s	~0.03 s
upload throughput (Expect disabled, 50 MB)	~8 Gbps	~14 Gbps
GET /empty ping · 100 MB garbage download	—	unchanged

cargo build --release and cargo clippy are clean.

[jrjones]

Have been running some private speedtest servers for my own client, ran into a couple relatively edge-casey issues in production that were easy behavior-preserving fixes under normal circumstances. LMK if questions/concerns. Thanks to the maintainers, we've gotten a lot of utility out of this repo. Once we are out of beta I will open at least one of our servers and add it to the list of public servers.)

--- JRJ