xtc_proc.h

/*-
 * Copyright (c) 2026, The XTC Project
 * Use of this source code is governed by the ISC License.
 *
 * src/inc/xtc_proc.h
 *      Lightweight processes with mailboxes, selective receive,
 *      links, monitors, and explicit exit.  M8 ships the core; the
 *      `xtc_orc` supervisor (M10) sits on top of these primitives.
 *
 *      A process is a coroutine with identity (xtc_pid_t) plus a
 *      mailbox.  Send is fire-and-forget; the message is copied into
 *      an envelope owned by the mailbox.  Receive is selective: the
 *      caller supplies a match function, and envelopes that don't
 *      match are kept in arrival order in a save queue and re-tested
 *      on the next receive.
 */
 
#ifndef XTC_PROC_H
#define XTC_PROC_H
 
#include <stddef.h>
#include <stdint.h>
 
#include "xtc.h"
#include "xtc_loop.h"
#include "xtc_async.h"
 
/*
 * Process identifier.  Encodes the loop ID, a per-loop slot index,
 * and a generation counter so a stale pid (after a process exits and
 * its slot is reused) is recognisably stale on lookup.
 */
typedef struct xtc_pid {
        uint16_t loop_id;
        uint16_t local_id;
        uint32_t gen;
} xtc_pid_t;
 
#define XTC_PID_NONE ((xtc_pid_t){0, 0, 0})
 
static inline int
xtc_pid_eq(xtc_pid_t a, xtc_pid_t b)
{
        return a.loop_id == b.loop_id &&
               a.local_id == b.local_id &&
               a.gen == b.gen;
}
 
static inline int
xtc_pid_is_none(xtc_pid_t p)
{
        return p.loop_id == 0 && p.local_id == 0 && p.gen == 0;
}
 
/*
 * Match callback for selective receive.  Inspects an envelope's
 * data + size and returns:
 *   1  -> consume this envelope (the receive call returns it)
 *   0  -> skip; envelope stays in the save queue for the next receive
 */
typedef int (*xtc_match_fn)(const void *data, size_t size, void *user_data);
 
/*
 * Process entry function.  The proc runs as a coroutine and the
 * function returns nothing; exit happens by returning from the entry,
 * by calling xtc_exit, or by being killed via a link.
 */
typedef void (*xtc_proc_fn)(void *arg);
 
typedef struct xtc_proc_opts {
        const char *name;          /* optional, for debug */
        size_t      mailbox_cap;   /* 0 = default */
        int         link_to;       /* if != 0, this is a pid index to link to */
        /* Mailbox watermark: when an accepted message brings the depth to
         * this percent of mailbox_cap (1..100; 0 = disabled), the callback
         * fires once on the rising edge, so the app can shed load before
         * the hard cap rejects with XTC_E_AGAIN.  The callback runs on the
         * sender's thread, outside the mailbox lock; keep it cheap and do
         * not block. */
        int         mailbox_watermark_pct;
        void      (*mailbox_watermark_fn)(xtc_pid_t self, size_t depth,
                                          size_t cap, void *user);
        void       *mailbox_watermark_user;
} xtc_proc_opts_t;
 
/* Mailbox statistics snapshot (see xtc_proc_mailbox_stats). */
typedef struct xtc_mailbox_stats {
        size_t   depth;        /* messages currently in the mailbox */
        size_t   saved;        /* messages held in the selective-receive
                                * save queue (inspected, not yet matched) */
        size_t   peak;         /* high-water mailbox depth ever reached */
        size_t   cap;          /* capacity bound on depth + saved (0 = none) */
        uint64_t recv_total;   /* messages accepted over the proc's life */
        uint64_t drop_total;   /* messages rejected (full / dead) */
} xtc_mailbox_stats_t;
 
/*
 * PUBLIC: int       xtc_proc_spawn __P((xtc_loop_t *, xtc_proc_fn, void *, const xtc_proc_opts_t *, xtc_pid_t *));
 * PUBLIC: int       xtc_proc_spawn_link __P((xtc_loop_t *, xtc_proc_fn, void *, const xtc_proc_opts_t *, xtc_pid_t *));
 * PUBLIC: int       xtc_proc_spawn_monitor __P((xtc_loop_t *, xtc_proc_fn, void *, const xtc_proc_opts_t *, xtc_pid_t *, uint64_t *));
 * PUBLIC: xtc_pid_t xtc_self __P((void));
 * PUBLIC: int       xtc_send __P((xtc_pid_t, const void *, size_t));
 * PUBLIC: int       xtc_recv __P((void **, size_t *, int64_t));
 * PUBLIC: int       xtc_recv_match __P((xtc_match_fn, void *, void **, size_t *, int64_t));
 * PUBLIC: int       xtc_recv_correlate __P((const void *, size_t, int, xtc_msg_t *, int *, int64_t));
 * PUBLIC: int       xtc_proc_wait_fd __P((int, uint32_t, int64_t, uint32_t *));
 * PUBLIC: int       xtc_proc_sleep __P((int64_t));
 * PUBLIC: int       xtc_exit_self __P((int));
 * PUBLIC: int       xtc_exit_pid __P((xtc_pid_t, int));
 * PUBLIC: int       xtc_link __P((xtc_pid_t));
 * PUBLIC: int       xtc_unlink __P((xtc_pid_t));
 * PUBLIC: int       xtc_monitor __P((xtc_pid_t, uint64_t *));
 */
 
int       xtc_proc_spawn(xtc_loop_t *loop, xtc_proc_fn fn, void *arg,
                          const xtc_proc_opts_t *opts, xtc_pid_t *out_pid);
 
/*
 * Atomic spawn + link / spawn + monitor (Erlang spawn_link /
 * spawn_monitor).  Identical to xtc_proc_spawn, but the parent<->child
 * relationship is established BEFORE the child is made runnable, so
 * there is no window in which the child exists but is not yet
 * linked/monitored -- even if the child runs and exits immediately,
 * its EXIT/DOWN is delivered (no XTC_DOWN_NOPROC race that a
 * spawn-then-link/monitor idiom can hit).  The CALLER MUST be a
 * process (xtc_self() != NONE); returns XTC_E_INVAL otherwise.
 *
 * _link:    bidirectional fate, exactly like calling xtc_link(child)
 *           the instant the child is born -- an abnormal exit on either
 *           side raises an EXIT on the other.
 * _monitor: unidirectional; the caller receives a DOWN (with *out_ref
 *           as the monitor reference) when the child exits, exactly
 *           like xtc_monitor(child).
 */
int       xtc_proc_spawn_link(xtc_loop_t *loop, xtc_proc_fn fn, void *arg,
                          const xtc_proc_opts_t *opts, xtc_pid_t *out_pid);
int       xtc_proc_spawn_monitor(xtc_loop_t *loop, xtc_proc_fn fn,
                          void *arg, const xtc_proc_opts_t *opts,
                          xtc_pid_t *out_pid, uint64_t *out_ref);
 
/* From inside a process, return its pid; from outside, returns NONE. */
/*
 * Asynchronous cross-process exit signal.  Sets a kill flag on the
 * target proc; the target raises the exit at its next yield/recv
 * point with the supplied reason.  Idempotent (first call wins).
 * Returns XTC_E_INVAL if the target is unknown or already dead.
 */
int xtc_exit_pid(xtc_pid_t target, int reason);
 
xtc_pid_t xtc_self(void);
 
/*
 * Send a message.  Copies `size` bytes from `data` into a mailbox
 * envelope; the caller retains ownership of `data`.  Returns:
 *   XTC_OK            queued successfully
 *   XTC_E_INVAL       NULL data with non-zero size, or stale/unknown pid
 *   XTC_E_AGAIN       target mailbox at capacity
 *   XTC_E_RESOURCE    global slot cap (XTC_RES_CHAN_SLOTS) hit
 *
 * BACKPRESSURE CONTRACT -- read this.  Mailboxes are bounded (the
 * cap is xtc_proc_opts_t.mailbox_cap, default 4096).  This is
 * deliberate: an unbounded mailbox is how an actor system OOMs when
 * a fast sender outruns a slow receiver (the classic unbounded-mailbox failure).
 * The price is that send can fail with XTC_E_AGAIN when the target
 * is full, and a dropped XTC_E_AGAIN is a SILENT MESSAGE LOSS.
 *
 * Senders MUST check the return value and decide a policy:
 *   - retry later (re-arm on a timer, or yield and resend),
 *   - shed load (drop the message and account it),
 *   - apply end-to-end flow control (e.g. stop reading the upstream
 *     socket until the target drains -- see examples/07_kaka for a
 *     credit-based scheme), or
 *   - treat it as fatal for a must-deliver path.
 * Ignoring the return is a bug, not a shortcut.
 */
int       xtc_send(xtc_pid_t to, const void *data, size_t size);
 
/*
 * Receive the next envelope from this process's mailbox.  Allocates
 * a new buffer for the caller (via the library allocator); the caller
 * frees it with xtc_free.  Blocks (yields the coroutine) up to
 * timeout_ns; -1 is
 * indefinite, 0 is non-blocking.
 *
 * Returns:
 *   XTC_OK            *out / *size set
 *   XTC_E_AGAIN       timeout fired with no message
 *   XTC_E_INVAL       called outside a process
 */
int       xtc_recv(void **out, size_t *out_size, int64_t timeout_ns);
 
/*
 * Selective receive.  match_fn is called for each envelope in
 * arrival order; the first one for which match_fn returns 1 is
 * delivered.  Non-matching envelopes are kept in the save queue.
 */
int       xtc_recv_match(xtc_match_fn match_fn, void *user_data,
                          void **out, size_t *out_size,
                          int64_t timeout_ns);
 
/*
 * Receive `n_expected` messages whose leading `corr_size` bytes
 * match `corr_value`.  The first `n_expected` matching messages
 * are delivered as an array via `out_msgs[]`; non-matching
 * messages stay in the mailbox / save queue for subsequent
 * receives.  Returns XTC_OK on full collection; XTC_E_AGAIN if
 * the timeout fires before n_expected matches arrive (in which
 * case `*out_n` is the number actually collected; out_msgs[0..*out_n]
 * are still owned by the caller and must be freed).
 *
 * This is the canonical helper for fork-join and request-reply
 * patterns: pick a correlation id, send N children a request
 * containing that id, wait for N replies whose first corr_size
 * bytes equal the id.  Avoids manual save-queue management.
 *
 * Each delivered message conforms to the same ownership contract
 * as xtc_recv: the caller owns the buffer and must free() it.
 */
typedef struct xtc_msg {
        void   *data;
        size_t  size;
} xtc_msg_t;
 
int       xtc_recv_correlate(const void *corr_value, size_t corr_size,
                              int n_expected,
                              xtc_msg_t *out_msgs,
                              int *out_n,
                              int64_t timeout_ns);
 
/*
 * Wait until ANY of the following becomes true:
 *   - the given fd has any of `interest` bits set
 *     (XTC_IO_READABLE / WRITABLE / ERR / HUP),
 *   - a message arrives in the calling proc's mailbox,
 *   - the timeout elapses (only if `timeout_ns >= 0`),
 *   - the proc is killed (xtc_exit_pid raises the exit as usual).
 *
 * Returns:
 *   XTC_OK       on a non-timeout wakeup.  *out_revents has the
 *                XTC_IO_* bits that fired plus XTC_WAIT_MAILBOX if
 *                a message is queued.  Multiple bits can be set if
 *                more than one source raced to wake.
 *   XTC_E_AGAIN  timeout fired with nothing else.  *out_revents has
 *                XTC_WAIT_TIMEOUT.
 *   XTC_E_INVAL  bad args (NULL out_revents, fd<0, etc.) or called
 *                from outside a process.
 *
 * The fd is auto-unregistered before return; the mailbox is left
 * untouched (caller still calls xtc_recv to actually drain).
 */
#define XTC_WAIT_MAILBOX  0x10000u   /* in out_revents only */
#define XTC_WAIT_TIMEOUT  0x20000u   /* in out_revents only */
 
int       xtc_proc_wait_fd(int fd, uint32_t interest, int64_t timeout_ns,
                            uint32_t *out_revents);
 
/* Sleep the calling process for at least ns nanoseconds by parking it
 * on a timer (the loop runs other work meanwhile -- it does not block
 * the thread).  Unlike a timed xtc_recv it does not touch the mailbox.
 * Returns XTC_E_INVAL if not called from a process. */
int       xtc_proc_sleep(int64_t ns);
 
/* Explicit exit from inside a process; reason is delivered via
 * EXIT/DOWN signals to linked / monitoring procs. */
int       xtc_exit_self(int reason);
 
/* Link / unlink: bidirectional fate. */
int       xtc_link(xtc_pid_t other);
int       xtc_unlink(xtc_pid_t other);
 
/* ---- Monitor DOWN reasons ------------------------------------------
 *
 * A monitor's DOWN carries an int `reason` describing how the target
 * ended.  The reason space is:
 *
 *   0            -- clean exit (the target returned, or called
 *                   xtc_exit_self(0)).
 *   > 0          -- an application exit code the target passed to
 *                   xtc_exit_self(code) (including a contained fault
 *                   that ran xtc_exit_self(sig), which passes the
 *                   POSITIVE signal number, e.g. 11 for SIGSEGV).
 *   XTC_DOWN_NOPROC -- the monitor was registered on a target that had
 *                   ALREADY exited (the monitor raced the target's
 *                   exit), so its real exit reason was already reaped
 *                   and is unknown.  This is NOT a crash: a short-lived
 *                   target that a supervisor monitors just after it
 *                   finished delivers this, and it is expected.  It is a
 *                   distinct value (not XTC_E_NOTFOUND, and outside the
 *                   1..255 signal-number range) precisely so a
 *                   supervisor can tell "already gone" apart from a real
 *                   fault exit -- a DOWN reason of, say, 11 is a
 *                   contained SIGSEGV, whereas XTC_DOWN_NOPROC is not.
 *
 * xtc_down_is_noproc(reason) is the readable test for the last case.
 */
#define XTC_DOWN_NOPROC  (-100000)
 
static inline int
xtc_down_is_noproc(int reason)
{
        return reason == XTC_DOWN_NOPROC;
}
 
/* Monitor: unidirectional notification.  out_ref is filled with the
 * monitor reference; the watcher receives a DOWN message of shape
 * { uint8_t kind = 'D'; uint64_t ref; xtc_pid_t pid; int reason; }
 * when the monitored process exits (reason per the DOWN-reason space
 * above).  Registering a monitor on an ALREADY-dead target is not an
 * error: it delivers an immediate DOWN with reason XTC_DOWN_NOPROC. */
int       xtc_monitor(xtc_pid_t target, uint64_t *out_ref);
 
/* Snapshot a process's mailbox statistics into *out.  Returns XTC_OK,
 * or XTC_E_INVAL if the pid is dead / unknown.  Safe to call from any
 * thread. */
int       xtc_proc_mailbox_stats(xtc_pid_t pid, xtc_mailbox_stats_t *out);
 
/* ---- R1: per-fiber fault containment ----
 *
 * Turns a real synchronous fault (SIGSEGV / SIGBUS / SIGFPE / SIGILL
 * on POSIX; the equivalent EXCEPTION_* on Windows) inside one
 * coroutine into an unwind of only that process, leaving siblings on
 * the same loop untouched -- the runtime support PG's "let it crash"
 * session containment needs.
 *
 * POSIX: sigaltstack + sigaction + siglongjmp.  Windows: a Vectored
 * Exception Handler restores the CONTEXT captured at
 * xtc_proc_recovery_arm() (no stack unwind).  Both paths are
 * runtime-verified on their hosts: a contained fault outside a
 * critical section unwinds the one proc and delivers DOWN to its
 * monitors; a fault inside a critical section escalates to process
 * abort (POSIX re-raise; Windows EXCEPTION_CONTINUE_SEARCH ->
 * 0xC0000005), preserving PG's critical-section PANIC semantics.
 */
#if (defined(__sun) || defined(__illumos__)) && !defined(__EXTENSIONS__)
/* illumos/Solaris gate sigjmp_buf / sigsetjmp / siglongjmp behind a
 * feature-test macro; under -std=c11 (strict ISO C) <setjmp.h> hides
 * them.  A consumer that includes this PUBLIC header need not know to
 * set the macro itself, so expose the POSIX setjmp surface here before
 * the include.  No effect on other platforms. */
#define __EXTENSIONS__ 1
#endif
#include <setjmp.h>
 
#if defined(_WIN32)
#include <windows.h>
/*
 * Windows recovery uses CONTEXT capture/restore rather than
 * setjmp/longjmp.  The fault is caught by a Vectored Exception Handler
 * that restores this saved CONTEXT via EXCEPTION_CONTINUE_EXECUTION --
 * the OS reloads the thread's registers and resumes at the capture
 * point.  longjmp out of (or via) a VEH is unsafe: on a fiber stack it
 * walks unwind tables that no longer match and corrupts the CRT; a
 * context restore does no unwinding at all.
 */
typedef struct xtc_recovery_buf { CONTEXT ctx; } xtc_recovery_buf_t;
#else
typedef sigjmp_buf xtc_recovery_buf_t;
#endif
 
/* Install the process-wide fault handler.  On POSIX it registers a
 * SIGSEGV/SIGBUS/SIGFPE/SIGILL handler on an alternate signal stack
 * (call once per loop thread for the alt stack; the handler is
 * installed once).  On Windows it registers a Vectored Exception
 * Handler.  Returns XTC_OK on success. */
int       xtc_fault_guard_install(void);
 
/* Internal arm-slot for the xtc_proc_recovery_arm() macro (POSIX). */
xtc_recovery_buf_t *__xtc_proc_recovery_slot(void);
 
#if defined(_WIN32)
/* Windows recovery-arm helpers (used by the macro below).
 * __xtc_recovery_prep arms the frame and clears the fired flag;
 * __xtc_recovery_ctx returns the CONTEXT to capture into;
 * __xtc_recovery_result returns 0 on the arming pass and the fault
 * code when the VEH has restored the context. */
void      __xtc_recovery_prep(void);
CONTEXT  *__xtc_recovery_ctx(void);
int       __xtc_recovery_result(void);
#endif
 
/*
 * Arm a recovery frame for the calling process, exactly like
 * sigsetjmp: returns 0 on the normal path and the fault signal number
 * (POSIX) or exception code (Windows) when control returns here via a
 * contained fault.  Use it as:
 *
 *     int sig = xtc_proc_recovery_arm();
 *     if (sig != 0) {                 // recovered from a contained fault
 *         ... release locks, reset the memory context, close fds ...
 *         xtc_exit_self(reason);       // delivers DOWN to the supervisor
 *     }
 *     ... session work ...
 *
 * The frame is disarmed automatically when a fault fires it (so a
 * fault during recovery escalates to process abort); re-arm or
 * xtc_proc_recovery_disarm() as needed.
 *
 * IMPORTANT: containment only unwinds the fiber's CALL STACK.  Any
 * resources the proc held at fault time -- locks, fds, allocations,
 * buffer pins -- are the recovery block's responsibility to release:
 * abort any in-progress transaction and release every held lock and
 * resource before returning.  Hold those under an xtc_mctx you
 * can reset, and release lock-manager locks with the lock manager's
 * release-all; otherwise a contained fault leaks or, worse, leaves a
 * lock held and wedges peers.
 */
#if defined(_WIN32)
/* Capture the proc fn's own frame inline (like setjmp), so the VEH can
 * restore it; the comma expression returns 0 while arming and the
 * fault code after a contained fault resumes execution here. */
#define xtc_proc_recovery_arm() \
        (__xtc_recovery_prep(), \
         RtlCaptureContext(__xtc_recovery_ctx()), \
         __xtc_recovery_result())
#else
#define xtc_proc_recovery_arm() (sigsetjmp(*__xtc_proc_recovery_slot(), 1))
#endif
 
/* Disarm the calling process's recovery frame. */
void      xtc_proc_recovery_disarm(void);
 
/* Critical section: while crit_depth > 0, a fault is NOT contained --
 * it escalates to process abort, because shared state may be torn.
 * Nestable.  Mirrors PG's START_CRIT_SECTION / END_CRIT_SECTION. */
void      xtc_proc_critical_enter(void);
void      xtc_proc_critical_leave(void);
 
/* Register a callback to run when the calling process exits -- on a
 * normal return OR a contained-fault recovery (after
 * xtc_proc_recovery_arm -> xtc_exit_self).  Callbacks run LIFO,
 * outside signal context, with the proc still current, BEFORE its
 * monitors observe DOWN.  This is where an embedder guarantees a
 * faulted session releases what it held -- e.g. register
 * xtc_lock_release_all so no lock-manager lock outlives the proc, or
 * a memory-context reset.  Up to a small fixed number per proc;
 * returns XTC_E_RESOURCE past the limit, XTC_E_INVAL off a proc. */
int       xtc_proc_at_exit(void (*fn)(void *), void *arg);
 
/* A memory context scoped to the calling process: created lazily on
 * first call, destroyed automatically on proc exit (a backstop so a
 * faulted session's allocations are reclaimed even if its recovery
 * block itself faults).  Returns NULL off a proc.  See xtc_mctx.h. */
struct xtc_mctx *xtc_proc_mctx(void);
 
/*
 * Recovery resource registry.
 *
 * A contained fault unwinds the faulting fiber's call stack but frees
 * NONE of the resources the proc held -- locks stay locked, fds stay
 * open, memory arenas stay live -- and a leaked lock can wedge every
 * peer.  Register the resources a proc acquires so the runtime can
 * release them automatically:
 *
 *   xtc_proc_recovery_track_fd(fd);          // close(fd) on cleanup
 *   xtc_proc_recovery_track_mctx(mctx);      // xtc_mctx_reset(mctx)
 *   xtc_proc_recovery_track_locks(mgr, locker, release_all);
 *   xtc_proc_recovery_track(fn, arg);        // generic fn(arg)
 *
 * xtc_proc_recovery_cleanup() releases everything registered (LIFO).
 * It is the DEFAULT recovery action and is ALSO callable from a custom
 * recovery block to finish the standard bits after the block's own
 * application-specific unwinding:
 *
 *   int sig = xtc_proc_recovery_arm();
 *   if (sig != 0) {                 // recovered from a contained fault
 *       my_app_abort_txn();          // custom unwinding first
 *       xtc_proc_recovery_cleanup();  // then the registered standard bits
 *       xtc_exit_self(sig);
 *   }
 *
 * Or use xtc_proc_recovery_arm_clean(), which performs the cleanup and
 * xtc_exit_self automatically on the recovered branch (no custom block).
 *
 * Registered resources are ALSO released automatically on a NORMAL
 * proc exit (before the at-exit hooks), so a proc that simply returns
 * without explicitly releasing still cleans up.  Use
 * xtc_proc_recovery_untrack_fd() when the proc releases an fd itself,
 * to avoid a double close on a later recovery.
 *
 * Each registration returns XTC_OK, XTC_E_RESOURCE past the per-proc
 * limit, or XTC_E_INVAL off a proc / on a bad argument.
 */
int  xtc_proc_recovery_track_fd(int fd);
int  xtc_proc_recovery_track_mctx(struct xtc_mctx *mctx);
int  xtc_proc_recovery_track_locks(void *mgr, uint64_t locker,
                                   void (*release_all)(void *, uint64_t));
int  xtc_proc_recovery_track(void (*fn)(void *), void *arg);
int  xtc_proc_recovery_untrack_fd(int fd);
void xtc_proc_recovery_cleanup(void);
 
/*
 * Arm a recovery frame whose default recovered action is to release
 * all tracked resources and exit the proc with the fault code.  On the
 * normal (arming) pass it returns 0 and execution continues into the
 * session work; on a contained fault it does NOT return -- it runs
 * xtc_proc_recovery_cleanup() then xtc_exit_self(sig).  Use this when
 * the registered resources are the whole cleanup story; use the bare
 * xtc_proc_recovery_arm() + a custom block when they are not.
 */
#define xtc_proc_recovery_arm_clean()                       \
        do {                                                \
                int __xtc_rsig = xtc_proc_recovery_arm();   \
                if (__xtc_rsig != 0) {                      \
                        xtc_proc_recovery_cleanup();        \
                        (void)xtc_exit_self(__xtc_rsig);    \
                }                                           \
        } while (0)
 
/* Decode a DOWN signal (delivered to a monitor when its target exits)
 * into the target pid and exit reason, without hand-rolling the
 * on-wire layout.  The DOWN/EXIT signals are sent packed; a mismatched
 * (unpacked) mirror struct misreads `reason`.  Returns XTC_OK if msg
 * is a DOWN, XTC_E_INVAL otherwise.  out_pid / out_reason may be NULL. */
int       xtc_down_decode(const void *msg, size_t len,
                          xtc_pid_t *out_pid, int *out_reason);
 
/*
 * Self-describing DOWN classification (requested by embedders whose
 * app exit codes and signal numbers would otherwise share the single
 * `reason` integer: a bare xtc_exit_self(1) was indistinguishable from
 * a signal-1 (SIGHUP) contained fault).  xtc_down_decode_ex fills an
 * xtc_down_info_t whose `kind` says HOW the target ended, with the
 * signal number and the app exit code in SEPARATE fields that never
 * collide.  The legacy single-integer xtc_down_decode still works and
 * returns the same `reason` it always did.
 */
typedef enum {
        XTC_DOWN_KIND_CLEAN  = 0,  /* target returned or xtc_exit_self(0) */
        XTC_DOWN_KIND_EXIT   = 1,  /* xtc_exit_self(code), code in .exit_code */
        XTC_DOWN_KIND_SIGNAL = 2,  /* R1 contained fault, signal in .signal */
        XTC_DOWN_KIND_NOPROC = 3   /* monitor raced a dead target (benign) */
} xtc_down_kind_t;
 
typedef struct {
        xtc_pid_t       pid;        /* the target that went DOWN */
        xtc_down_kind_t kind;       /* how it ended (never ambiguous) */
        int             signal;     /* signal number iff kind == SIGNAL, else 0 */
        int             exit_code;  /* app code iff kind == EXIT, else 0 */
        int             reason;     /* the legacy xtc_down_decode reason value */
        uint64_t        ref;        /* the monitor reference (0 for a link EXIT) */
} xtc_down_info_t;
 
/*
 * Decode a DOWN or EXIT signal into a fully-classified xtc_down_info_t.
 * Accepts both the monitor DOWN ('D') and the link EXIT ('E') signal
 * shapes.  Returns XTC_OK on either, XTC_E_INVAL for any other message.
 * A monitor need no longer know the producer's encoding convention:
 * kind + signal + exit_code are unambiguous by construction.
 *
 * PUBLIC: int xtc_down_decode_ex __P((const void *, size_t, xtc_down_info_t *));
 */
int       xtc_down_decode_ex(const void *msg, size_t len,
                             xtc_down_info_t *out);
 
/* Predicate + accessor helpers over a legacy `reason` integer, for
 * callers that keep using xtc_down_decode.  A signal-N fault and a
 * bare xtc_exit_self(N) are STILL not distinguishable from `reason`
 * alone (that is the whole reason to prefer xtc_down_decode_ex); these
 * helpers only classify the two unambiguous sentinels. */
static inline int xtc_down_is_signal_reason(int reason)
{
        return reason >= 1 && reason <= 255;
}
static inline int xtc_down_is_exit_reason(int reason)
{
        return reason == 0 || reason >= 256;
}
 
/* Internal: save / restore the current-proc context across a yield
 * done by a lower-level primitive (e.g. xtc_amutex parking the
 * fiber), so the proc still sees itself on resume.  Opaque to the
 * caller. */
void     *__xtc_proc_ctx_save(void);
void      __xtc_proc_ctx_restore(void *ctx);
 
#endif /* XTC_PROC_H */