wado-run-pjrt

The wado run-pjrt subcommand: it compiles a Wado program with wado and runs the component on a wasmtime host that serves core:pjrt from a PJRT plugin.

Rules

• A plugin is loaded at run time, from --plugin or WADO_PJRT_PLUGIN, so no build depends on one. mise run test-pjrt fetches the pinned CPU plugin and libtpu.so (scripts/fetch-pjrt-plugin.sh) and runs the #[ignore]d tests on them.
• xla/pjrt/c/pjrt_c_api.h is the one part of XLA that may be read. src/ffi.rs follows it. An argument struct must be at least the plugin's size, so each struct is declared in full, whatever a call uses of it.
• A program behaves the same here as under wado run, which is the only reason to have both. wado_host::runner, guest and options hold what makes that so, and a divergence is a defect, not a variant.
• A plugin's fault ends the process, so crashes.md records each SIGSEGV and SIGABRT, fixed or not, with the top of its backtrace.
• A plugin reads as far as a guest's values say, so the host checks what reaches it through a pointer before the call. crashes.md lists what a plugin refuses on its own.
• Every plugin call goes through Plugin::enter, on the plugin's thread, never on the fiber a guest runs on.