Deep Dives
Language-specific excursions — embedding Lua as a policy engine in all three languages, Rust macros for systems code, and the Go runtime from a systems programmer's seat.
Embedding Lua as a policy engine: one policy.lua, three host runtimes
lua-policy moves pmon's restart/backoff decision out of the compiled binary and into a single sandboxed policy.lua, embedded three structurally different ways -- C++23 via sol2, Go via gopher-lua (Lua 5.1), Rust via mlua (Lua 5.4) -- and reloaded live on a real SIGHUP with no process restart. Verified on the Fedora 44 host: byte-identical decisions across all three languages (diff-confirmed), PASS 19/FAIL 0 per language, and three genuinely different sandbox-rejection diagnostics for the same malicious policy.
Rust macros for systems code: proc-macros, syn/quote, and what miri proves about unsafe
sysmacros is chapter 43's proc-macro crate, powering two Rust-only techniques in the app binary -- #[derive(SysError)], a miniature thiserror built with syn 2.0's AST and quote's token emission, and #[instrument], a compile-time tracing attribute that times a function with Instant -- plus a sound unsafe unaligned read that cargo +nightly miri validates clean and, under the deliberately-named --oob-read path, catches as undefined behaviour. The book's first single-language deep dive, verified on the Fedora 44 host with verify.lua PASS 8/FAIL 0, the exact six-line stable stdout, and miri's own out-of-bounds diagnostic reproduced byte for byte.
The Go runtime from a systems programmer's seat: the scheduler, the collector, and the netpoller
go-runtime-for-systems is a single Go binary that observes its own runtime across four themes -- the GMP scheduler and signal-based async preemption, GC pacing under GOGC/GOMEMLIMIT with Go 1.26's Green Tea collector on by default, the epoll-backed netpoller that turns 2,000 blocked goroutines into roughly a dozen OS threads, and the effective knobs a process is actually running under -- each theme printing a self-computed ok/FAIL verdict. Verified on the Fedora 44 host on go1.26.5 with verify.lua PASS 16/FAIL 0, and the same preemption check flipped to FAIL on demand by GODEBUG=asyncpreemptoff=1.