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writeonce/docs/plan/08-sendfile-static-assets.md

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08 — sendfile Zero-Copy Static Serving

Context sources: ./03-hand-rolled-http.md, ./linux/00-linux.md § Efficient File Serving, ../02-recovery.md.

Goal

Serve static file bytes — eventually ##ui-emitted HTML + CSS + JS bundle, today anything put under a project's static/ directory — via sendfile(sock_fd, file_fd, NULL, count). Zero userspace copy on the payload path: the kernel moves bytes from the page cache directly to the socket's send buffer. Completes the v1 kernel-primitive port started in phase 02.

Design decisions (locked)

  1. sendfile(2) only. Not splice, not vmsplice. sendfile handles "file fd → socket fd" exactly, which is 100% of the use case here. splice-through-pipe is ~30% more code for cases we don't have (non-regular-file sources).
  2. GET /static/... is the only route mounted. Hard-coded for Stage 3. When ##ui arrives in phase 6+ it'll emit bundles into this path; when typed SDK codegen arrives in phase 5 the generated JS client goes here too.
  3. No path traversal. Canonicalise the requested path; reject anything that escapes the configured static root. Standard directory-traversal defence — .. segments already stripped by the HTTP request parser from phase 03, but the static resolver double-checks with a realpath comparison.
  4. open + fstat + sendfile chain. No mmap. mmap wins for repeated reads of the same file (where the page cache warming pays off), but sendfile is strictly faster for one-shot delivery since the kernel manages the page cache itself. 00-linux.md lists both; the runtime's static-asset pattern is one-shot, pick sendfile.
  5. EAGAIN backoff through the event loop. If sendfile returns partial bytes (send buffer full), re-arm EPOLLOUT for the socket and resume when the kernel signals writable. Matches v1 wo-serve's flow.
  6. MIME by extension table. Compact match on .html/.css/.js/.json/.svg/.png/.jpg/.woff2/.wasm covers every asset the SSR layer will emit. Unknown extensions default to application/octet-stream.

Scope

New files inside crates/rt/src/static_files/

File Responsibility Port source
mod.rs Re-exports StaticHandler, resolve —
sendfile.rs Raw sendfile(2) wrapper + non-blocking send_all that co-operates with EPOLLOUT reference/crates/wo-serve/src/sendfile.rs (109 LOC)
resolve.rs Path canonicalisation + traversal defence + file existence check reference/crates/wo-serve/src/resolve.rs (80 LOC)
mime.rs Extension → Content-Type table reference/crates/wo-serve/src/mime.rs (44 LOC)
handler.rs StaticHandler — integrates the three with phase-03's Response builder; returns 404 / 403 / 200 as appropriate ~120 new LOC

Total: ~350 LOC (233 ported + ~120 new).

Cargo.toml change

None.

Router change in crates/rt/src/server.rs

One new route per project:

let static_root = project_dir.join("static");
let handler     = StaticHandler::new(static_root);
router.route(Method::GET, "/static/*path", move |req, _| handler.serve(req));

/static/*path is a new wildcard pattern in the phase-03 router — add it to route.rs if not already supported.

API shape (target)

use rt::static_files::StaticHandler;

let handler = StaticHandler::new("/app/static");
handler.serve(&request)?;   // returns a Response that streams via sendfile()

The Response returned by handler.serve() owns the open File fd. The phase-03 connection writer notices it's a sendfile-backed response and uses the non-blocking send_all path instead of write.

Exit criteria

  1. cargo build green. No new deps.
  2. Unit test: place a 10 MB file under a temp static root, StaticHandler::serve on a mock request, assert the Response reports 200 / correct Content-Length / correct Content-Type. A separate integration test validates the actual sendfile path using an accepted socket.
  3. strace validation: 
    cargo run --bin wo -- run docs/examples/blog &
PID=$!
# ship a 10 MB file into docs/examples/blog/static/big.bin
strace -p $PID -f -e sendfile,read,write 2>&1 | tee /tmp/strace.log &
curl -s -o /dev/null http://127.0.0.1:8080/static/big.bin
# assert /tmp/strace.log shows sendfile(...) calls and zero read/write
# of the file content
  4. Path traversal attempts fail closed. curl :8080/static/../Cargo.toml returns 403. curl :8080/static/nonexistent.png returns 404.
  5. EAGAIN handling. A test that rate-limits the socket sendbuf to force a partial write exercises the EPOLLOUT re-arm path; the full payload still arrives.
  6. All 14 rt tests + phase-02/03/04/05/06/07 additions pass. reference/rest/blog.rest 20 assertions still green (no regressions on the JSON endpoints).

Non-scope

  • No sendfile64. Modern glibc aliases sendfile to sendfile64 transparently; explicit 64-bit selection isn't needed.
  • No range requests. GET /static/big.bin with Range: bytes=... returns 200 + full payload in Stage 3; proper range-request handling is a follow-on. Nothing in the blog or ecommerce samples uses ranges.
  • No in-memory cache. The kernel page cache is the only cache. Re-opening the file on every request is cheap; if future profiling says otherwise, add an LRU fd cache — but not pre-emptively.
  • No TLS. sendfile over TLS requires kTLS (setsockopt(TCP_ULP, "tls") + kernel 4.13+ and the right cipher suites). Worth doing when TLS lands as its own phase; out of scope here.
  • No compression. The HTTP response writer in phase 03 doesn't gzip; sendfile can't gzip on the fly either. Pre-compress (.br / .gz sibling files) is a future phase — for now the table lists .br/.gz extensions with the correct Content-Encoding but the caller has to produce the pre-compressed file itself.

Verification

cargo build
cargo test --lib static_files
cargo test --lib                               # all existing tests green

# strace-backed zero-copy proof
cargo run --bin wo -- run docs/examples/blog &
# ... (full script from exit criterion 3)

# .rest smoke unchanged
# full 20-assertion battery against reference/rest/blog.rest

cd reference/crates && cargo build && cargo test   # v1 untouched

After this phase

The runtime covers every kernel primitive listed in 00-linux.md except io_uring, mmap, fallocate, and memfd_create — which all belong to the storage engine (phase 3 of the database series), not the runtime per se.

Next natural phase: 09-native-subscriptions.md — the register! { #{blog-title} => notify(fd) } model from 00-linux.md. Takes the inotify watcher from phase 07 and wires it into a subscription table that dispatches delta writes directly to subscriber sockets over the phase-03 HTTP connection. That replaces the Stage-3 501 stub the /api/<type>/live endpoint currently returns.

After that phase, crates/rt/ is feature-complete for Stages 1–3 of the runtime, with exactly one external dependency.