// SPDX-License-Identifier: GPL-2.0-or-later
//
// schinitd -- the supervisor. PID 2, and restartable.
//
// Everything that used to make PID 1 complicated lives here: reading
// /etc/schinit, parsing manifests, deciding what runs, respawn policy and
// throttling. docs/architecture/init.md calls this "where the bugs will be", which is
// exactly why it is a separate program -- a bug here is a restart, not a panic.
//
// The one rule that makes the split worth having:
//
//   **This program never forks a job.** It decides, and asks PID 1 to act. So
//   every job on the machine is a child of PID 1 at all times, and this
//   process dying orphans nothing, reparents nothing and restarts nothing. It
//   comes back, reads /proc/schinit, and finds the world exactly as it left it.
//
// A supervisor that forked its own children could not be restarted without
// either losing them to reparenting or racing the new copy to adopt them --
// the split-brain failure docs/architecture/init.md rejects the two-init scheme for.

#include "core/types.h"
#include "uapi/user.h"
#include "uapi/fcntl.h"
#include "uapi/posix.h"
#include "net/socket.h"
#include "uapi/schinit.h"

#define JOBDIR "/etc/schinit"

// Sizes. These mirror the protocol's, deliberately: a job that parses here but
// will not fit in a spawn message is a job that fails at the last moment, and
// keeping the two the same makes that impossible rather than unlikely.
#define MAXJOBS   32
#define MAXARGS   SCHINIT_MAXARGS
#define MAXENVV   SCHINIT_MAXENVV
#define MAXLABEL  SCHINIT_MAXLABEL
#define MAXPATH   SCHINIT_MAXPATH
#define MAXLINE   256

static int
slen(const char *s)
{
    int n = 0;
    while (s[n]) n++;
    return n;
}

static void
scopy(char *dst, const char *src, int cap)
{
    int i = 0;
    while (src[i] && i < cap - 1) { dst[i] = src[i]; i++; }
    dst[i] = '\0';
}

static int
seq(const char *a, const char *b)
{
    while (*a && *a == *b) { a++; b++; }
    return *a == *b;
}

// Case-insensitive, so a manifest may say KeepAlive, keepalive or KEEPALIVE.
// Keys are for humans and this is not a language.
static int
sieq(const char *a, const char *b)
{
    for (;;) {
        char ca = *a, cb = *b;
        if (ca >= 'A' && ca <= 'Z') ca += 32;
        if (cb >= 'A' && cb <= 'Z') cb += 32;
        if (ca != cb) return 0;
        if (!ca) return 1;
        a++; b++;
    }
}

static int
ends_with(const char *s, const char *suffix)
{
    int ls = slen(s), lf = slen(suffix);
    if (lf > ls) return 0;
    return seq(s + ls - lf, suffix);
}

// Whole lines, in one write() each.
//
// Every message here is assembled from pieces -- a prefix, a job's label, a
// number -- and each piece used to be its own write(). The console keeps one
// write() together, but nothing keeps two of them together, so a job starting
// on another CPU landed in the middle of the line: "schinitd: starting vgpud:
// os.schos.resolvconfno virtio-gpu on this machine" was this process and
// vgpud taking turns a word at a time. The pieces now collect here and go out
// at the newline.
static char lbuf[512];
static int  lbn;

static void
lflush(void)
{
    if (lbn)
        write(1, lbuf, lbn);
    lbn = 0;
}

static void
emit(const char *s, int n)
{
    for (int i = 0; i < n; i++) {
        lbuf[lbn++] = s[i];
        if (s[i] == '\n' || lbn == (int)sizeof lbuf)
            lflush();
    }
}

static void
out(const char *s)
{
    emit(s, slen(s));
}

// How the machine was booted, read once.
//
// The routine progress lines below -- one per job started, plus the summary --
// are the userspace half of the same problem the kernel has: worth reading on
// a boot you are watching, and noise on the several thousand after it. They
// are not deleted, they are held back, and `make qemu` with F8 or any verbose
// boot brings all of them back.
//
// Failures are never held back. A job that could not start, a channel that
// closed, a pid 1 that stopped answering: those print either way, because a
// silent supervisor is worse than a chatty one.
static int verbose_boot;

// Whether the boot menu was asked for a network debugger. See
// SCHBOOT_DEBUG_FLAG in include/arch/schboot.h -- the kernel only publishes it, and
// this is what acts on it, because what it turns on is a job.
static int debug_boot;

static void
read_bootmode(void)
{
    char b[16];
    int fd = open("/proc/bootmode", O_RDONLY);
    int n;

    if (fd >= 0) {
        n = read(fd, b, sizeof(b) - 1);
        close(fd);
        if (n > 0) {
            b[n] = 0;
            verbose_boot = (b[0] == 'v');
        }
    }

    fd = open("/proc/bootdebug", O_RDONLY);
    if (fd >= 0) {
        n = read(fd, b, sizeof(b) - 1);
        close(fd);
        if (n > 0) {
            b[n] = 0;
            debug_boot = (b[0] == 'o' && b[1] == 'n');
        }
    }
}

// Progress, as opposed to news.
static void
vout(const char *s)
{
    if (verbose_boot)
        emit(s, slen(s));
}

static void
outn(int v)
{
    char buf[12];
    int i = 11;
    int neg = v < 0;
    if (neg) v = -v;
    buf[i--] = '\0';
    if (v == 0) buf[i--] = '0';
    while (v > 0) { buf[i--] = (char)('0' + v % 10); v /= 10; }
    if (neg) buf[i--] = '-';
    out(&buf[i + 1]);
}

// ---------------------------------------------------------------------------
// Jobs
// ---------------------------------------------------------------------------

struct job {
    char label[MAXLABEL];
    char program[MAXPATH];
    char *argv[MAXARGS + 1];
    char argstore[MAXLINE];

    // Environment, one "KEY=VALUE" per Environment line, repeatable. Stored
    // as pointers into envstore so the job owns its own strings -- the parse
    // buffer is gone by the time anything execs.
    char *envp[MAXENVV + 1];
    char  envstore[MAXLINE * 2];
    int   envlen;
    int   envn;

    char stdin_path[MAXPATH];
    char stdout_path[MAXPATH];
    char stderr_path[MAXPATH];

    int  run_at_load;
    int  keep_alive;
    int  disabled;
    int  throttle;          // seconds between respawns
    int  own_tty;           // claim the console as controlling terminal

    // A socket the job wants schinit to hold. Parsed now, honoured in
    // milestone 2 -- recorded here so the manifest format does not have to
    // change when it is.
    char sock_path[MAXPATH];

    int  pid;               // 0 when not running
    unsigned int cookie;    // PID 1's handle for this job while it runs
    int  last_start;        // uptime() ticks, for throttling
    int  starts;
};

static struct job jobs[MAXJOBS];
static int njobs;

static void
job_defaults(struct job *j)
{
    j->label[0] = j->program[0] = '\0';
    j->stdin_path[0] = j->stdout_path[0] = j->stderr_path[0] = '\0';
    j->sock_path[0] = '\0';
    j->argstore[0] = '\0';
    j->argv[0] = 0;
    j->run_at_load = 0;
    j->keep_alive  = 0;
    j->disabled    = 0;
    j->throttle    = 1;
    j->own_tty     = 0;
    j->pid = 0;
    j->last_start = 0;
    j->starts = 0;
}

static int
truthy(const char *v)
{
    return sieq(v, "yes") || sieq(v, "true") || sieq(v, "1") || sieq(v, "on");
}

static int
parse_int(const char *s)
{
    int v = 0;
    while (*s >= '0' && *s <= '9') { v = v * 10 + (*s - '0'); s++; }
    return v;
}

// Split Args into argv, in place within the job's own storage. argv[0] is the
// program's name as it should see it, which is conventionally the basename and
// not the full path -- so a manifest says `Args = sh` for `/bin/sh`.
static void
split_args(struct job *j, const char *value)
{
    scopy(j->argstore, value, MAXLINE);
    int n = 0;
    char *p = j->argstore;
    while (*p && n < MAXARGS) {
        while (*p == ' ' || *p == '\t') p++;
        if (!*p) break;
        j->argv[n++] = p;
        while (*p && *p != ' ' && *p != '\t') p++;
        if (*p) *p++ = '\0';
    }
    j->argv[n] = 0;
}

// ---------------------------------------------------------------------------
// Manifest parsing
//
// Sectioned key = value. Comments start with #. A [Socket name] section marks
// everything after it as belonging to that socket, which is why Path is read
// into sock_path rather than treated as a job key.
// ---------------------------------------------------------------------------

// One "KEY=VALUE" onto the job's environment. Repeatable, so a manifest reads
//
//     Environment = TERM=linux
//     Environment = PATH=/bin:/usr/bin
//
// rather than inventing a separator that a value might contain.
static void
add_env(struct job *j, const char *kv)
{
    if (j->envn >= MAXENVV)
        return;
    int n = slen(kv);
    if (j->envlen + n + 1 > (int)sizeof(j->envstore))
        return;
    char *dst = j->envstore + j->envlen;
    scopy(dst, kv, n + 1);
    j->envp[j->envn++] = dst;
    j->envp[j->envn] = 0;
    j->envlen += n + 1;
}

static char *
trim(char *s)
{
    while (*s == ' ' || *s == '\t') s++;
    int n = slen(s);
    while (n > 0 && (s[n-1] == ' ' || s[n-1] == '\t' ||
                     s[n-1] == '\r' || s[n-1] == '\n'))
        s[--n] = '\0';
    return s;
}

static int
load_job(const char *path)
{
    if (njobs >= MAXJOBS) {
        out("schinit: too many jobs, ignoring "); out(path); out("\n");
        return -1;
    }

    int fd = open((char *)path, O_RDONLY);
    if (fd < 0) return -1;

    // Whole file into one static buffer. A job manifest that does not fit in
    // 4 KB is a manifest doing something it should not.
    static char buf[4096];
    int n = read(fd, buf, sizeof(buf) - 1);
    close(fd);
    if (n <= 0) return -1;
    buf[n] = '\0';

    struct job *j = &jobs[njobs];
    job_defaults(j);

    int in_socket = 0;
    char *p = buf;
    while (*p) {
        char *line = p;
        while (*p && *p != '\n') p++;
        if (*p) *p++ = '\0';

        // Strip comments before trimming, so trailing comments work too.
        for (char *c = line; *c; c++)
            if (*c == '#') { *c = '\0'; break; }

        line = trim(line);
        if (!*line) continue;

        if (line[0] == '[') {
            // [Socket name] -- the only section kind for now. Anything else is
            // ignored rather than fatal: an unknown section from a newer
            // schinit should not stop this one booting the machine.
            in_socket = (line[1] == 'S' || line[1] == 's');
            continue;
        }

        char *eq = line;
        while (*eq && *eq != '=') eq++;
        if (!*eq) continue;                 // not key = value; skip
        *eq = '\0';
        char *key = trim(line);
        char *val = trim(eq + 1);

        if (in_socket) {
            if (sieq(key, "Path")) scopy(j->sock_path, val, MAXPATH);
            continue;
        }

        if      (sieq(key, "Label"))          scopy(j->label, val, MAXLABEL);
        else if (sieq(key, "Program"))        scopy(j->program, val, MAXPATH);
        else if (sieq(key, "Args"))           split_args(j, val);
        else if (sieq(key, "RunAtLoad"))      j->run_at_load = truthy(val);
        else if (sieq(key, "KeepAlive"))      j->keep_alive = truthy(val);
        else if (sieq(key, "Disabled"))       j->disabled = truthy(val);
        else if (sieq(key, "ThrottleInterval")) j->throttle = parse_int(val);
        else if (sieq(key, "ControllingTerminal")) j->own_tty = truthy(val);
        else if (sieq(key, "Environment"))    add_env(j, val);
        else if (sieq(key, "StandardIn"))     scopy(j->stdin_path, val, MAXPATH);
        else if (sieq(key, "StandardOut"))    scopy(j->stdout_path, val, MAXPATH);
        else if (sieq(key, "StandardError"))  scopy(j->stderr_path, val, MAXPATH);
        // Unknown keys are ignored on purpose, same reasoning as sections.
    }

    if (!j->program[0]) {
        out("schinit: "); out(path); out(": no Program, ignoring\n");
        return -1;
    }
    if (!j->label[0])
        scopy(j->label, j->program, MAXLABEL);
    if (!j->argv[0]) {
        // No Args given: pass the program path as argv[0], which is what a
        // program expects when nothing better is said.
        scopy(j->argstore, j->program, MAXLINE);
        j->argv[0] = j->argstore;
        j->argv[1] = 0;
    }
    if (j->throttle <= 0)
        j->throttle = 1;

    njobs++;
    return 0;
}

// Read every *.job in JOBDIR. getdents64 rather than a hardcoded list, so
// dropping a file in the directory is all it takes to add a job.
static void
load_jobs(void)
{
    int fd = open(JOBDIR, O_RDONLY);
    if (fd < 0) {
        out("schinit: no " JOBDIR "\n");
        return;
    }

    static char dbuf[4096];
    for (;;) {
        int n = getdents64(fd, dbuf, sizeof(dbuf));
        if (n <= 0) break;

        int off = 0;
        while (off < n) {
            struct dirent64 *d = (struct dirent64 *)(dbuf + off);
            // Step by d_reclen: records are packed and variable length.
            if (d->d_reclen == 0) break;
            off += d->d_reclen;

            if (d->d_name[0] == '.') continue;
            if (!ends_with(d->d_name, ".job")) continue;

            char path[MAXPATH];
            scopy(path, JOBDIR "/", MAXPATH);
            int at = slen(path);
            scopy(path + at, d->d_name, MAXPATH - at);
            load_job(path);
        }
    }
    close(fd);
}

// ---------------------------------------------------------------------------
// Talking to PID 1
// ---------------------------------------------------------------------------

static int sock = SCHINIT_SOCK_FD;

// Flatten a job into a spawn request. Strings are copied into one buffer and
// referred to by offset, because pointers do not survive the trip.
static int
pack(struct schinit_spawn *s, struct job *j, unsigned int cookie)
{
    int used = 0;

    for (unsigned int z = 0; z < sizeof(*s); z++) ((char *)s)[z] = 0;
    s->op     = SCHINIT_SPAWN;
    s->cookie = cookie;
    scopy(s->label,       j->label,       SCHINIT_MAXLABEL);
    scopy(s->program,     j->program,     SCHINIT_MAXPATH);
    scopy(s->stdin_path,  j->stdin_path,  SCHINIT_MAXPATH);
    scopy(s->stdout_path, j->stdout_path, SCHINIT_MAXPATH);
    scopy(s->stderr_path, j->stderr_path, SCHINIT_MAXPATH);
    s->own_tty = j->own_tty;

    for (int i = 0; i < MAXARGS && j->argv[i]; i++) {
        int n = slen(j->argv[i]) + 1;
        if (used + n > SCHINIT_STRBUF)
            return -1;                 // too much text; refuse rather than trim
        s->args[s->nargs++] = (unsigned short)used;
        scopy(&s->strbuf[used], j->argv[i], n);
        used += n;
    }
    for (int i = 0; i < j->envn; i++) {
        int n = slen(j->envp[i]) + 1;
        if (used + n > SCHINIT_STRBUF)
            return -1;
        s->envs[s->nenvs++] = (unsigned short)used;
        scopy(&s->strbuf[used], j->envp[i], n);
        used += n;
    }
    return 0;
}

// Ask PID 1 to start a job. The reply may be preceded by exit notifications
// for other jobs, so this reads until it sees the answer to *this* cookie and
// handles anything else on the way past.
static void note_exit(unsigned int cookie, int pid, int status);

static unsigned int next_cookie = 1;

static int
ask_spawn(struct job *j)
{
    struct schinit_spawn s;
    unsigned int cookie = next_cookie++;

    if (pack(&s, j, cookie) < 0) {
        out("schinitd: "); out(j->label); out(": job too large to send\n");
        return -1;
    }
    if (write(sock, &s, sizeof(s)) != sizeof(s))
        return -1;

    for (;;) {
        struct schinit_reply r;
        if (read(sock, &r, sizeof(r)) != sizeof(r))
            return -1;                 // PID 1 is gone, which cannot happen
        if (r.op == SCHINIT_SPAWNED && r.cookie == cookie) {
            if (r.pid > 0) {
                j->pid = r.pid;
                j->cookie = cookie;
                j->last_start = uptime();
                j->starts++;
            }
            return r.pid;
        }
        if (r.op == SCHINIT_EXITED)
            note_exit(r.cookie, r.pid, r.status);
    }
}

static struct job *
job_by_cookie(unsigned int cookie)
{
    for (int i = 0; i < njobs; i++)
        if (jobs[i].cookie == cookie && jobs[i].pid)
            return &jobs[i];
    return 0;
}

static void
note_exit(unsigned int cookie, int pid, int status)
{
    struct job *j = job_by_cookie(cookie);
    if (!j)
        return;                        // not ours, or already noticed
    j->pid = 0;
    j->cookie = 0;

    if (!j->keep_alive)
        return;

    // A job that says it cannot run on this machine is believed. The status is
    // wait(2)-encoded: the low seven bits are the signal that killed it, so a
    // job that was killed or faulted never matches this and is restarted the
    // way KeepAlive promises. See SCHINIT_EXIT_NORESTART.
    if ((status & 0x7f) == 0 && ((status >> 8) & 0xff) == SCHINIT_EXIT_NORESTART) {
        // Not a failure: the job looked, found the machine did not have what
        // it drives, and said so properly. Worth reading once and not every
        // boot on a machine that will never have one.
        vout("schinitd: "); vout(j->label);
        vout(" has nothing to do on this machine; not restarting\n");
        return;
    }

    // Throttle. A job that exits immediately and forever would otherwise spin
    // the machine; launchd's ThrottleInterval exists for exactly this.
    int ticks = j->throttle * 100;
    int since = uptime() - j->last_start;
    if (since < ticks)
        sleep(ticks - since);

    out("schinitd: restarting "); out(j->label); out("\n");
    ask_spawn(j);
}

// ---------------------------------------------------------------------------
// Adopting what is already running
// ---------------------------------------------------------------------------

// Read /proc/schinit and claim the jobs PID 1 is already holding.
//
// This is the half of "State outside the process" that makes the other half worth
// anything. PID 1 publishes the table; without this, a restarted supervisor
// reads its manifests, sees nothing running *as far as it knows*, and starts
// everything a second time. The console shell then exists twice, both reading
// the same terminal, and every keystroke goes to whichever woke first -- the
// split-brain failure the plan rejects the two-init scheme for, arrived at by
// a different route.
//
// The state is outside the process precisely so that coming back is a *read*
// rather than a guess.
static unsigned int
adopt(void)
{
    char buf[2048];
    int  fd = open("/proc/schinit", O_RDONLY);
    unsigned int highest = 0;

    if (fd < 0)
        return 0;
    int n = read(fd, buf, sizeof(buf) - 1);
    close(fd);
    if (n <= 0)
        return 0;
    buf[n] = 0;

    // Lines are "cookie pid started label". The two header lines do not match
    // that shape and fall out of the parse on their own.
    char *p = buf;
    while (*p) {
        char *line = p;
        while (*p && *p != '\n') p++;
        if (*p == '\n') *p++ = 0;

        unsigned int cookie = 0;
        int pid = 0, started = 0, f = 0;
        char *q = line;

        // cookie
        if (*q < '0' || *q > '9') continue;
        while (*q >= '0' && *q <= '9') { cookie = cookie * 10 + (*q++ - '0'); f++; }
        while (*q == ' ') q++;
        while (*q >= '0' && *q <= '9') { pid = pid * 10 + (*q++ - '0'); }
        while (*q == ' ') q++;
        while (*q >= '0' && *q <= '9') { started = started * 10 + (*q++ - '0'); }
        while (*q == ' ') q++;
        if (!f || !pid || !*q) continue;

        for (int i = 0; i < njobs; i++) {
            if (!seq(jobs[i].label, q))
                continue;
            jobs[i].pid        = pid;
            jobs[i].cookie     = cookie;
            jobs[i].last_start = started;
            out("schinitd: adopting "); out(jobs[i].label); out("\n");
            break;
        }
        if (cookie > highest)
            highest = cookie;
    }
    return highest;
}

// ---------------------------------------------------------------------------

int
main(void)
{
    struct schinit_hello h;

    read_bootmode();

    // Version handshake before anything else. A mismatch is a refusal rather
    // than a best-effort interpretation -- see the header for why.
    h.op = SCHINIT_HELLO;
    h.proto = SCHINIT_PROTO;
    h.generation = 0;
    h.reserved = 0;
    if (write(sock, &h, sizeof(h)) != sizeof(h)) {
        out("schinitd: no channel to pid 1\n");
        lflush();
        exit();
    }
    if (read(sock, &h, sizeof(h)) != sizeof(h) || h.op != SCHINIT_WELCOME) {
        out("schinitd: pid 1 did not answer\n");
        lflush();
        exit();
    }
    if (h.proto != SCHINIT_PROTO) {
        out("schinitd: protocol mismatch with pid 1\n");
        lflush();
        exit();
    }

    load_jobs();
    vout("schinitd: "); if (verbose_boot) outn(njobs);
    vout(" job(s) loaded, generation ");
    if (verbose_boot) { outn((int)h.generation); out("\n"); }

    // Anything PID 1 is already holding is ours again, not something to start.
    unsigned int highest = adopt();
    next_cookie = highest + 1;

    // Everything except whatever owns the console. That one waits for the
    // greeting below -- see the comment on it.
    for (int i = 0; i < njobs; i++) {
        struct job *j = &jobs[i];
        // The one job a boot-menu choice can un-disable.
        //
        // Deliberately by label rather than by a manifest key like
        // `EnableIfDebug`: a key would be a general mechanism, and the general
        // version of "the boot menu can start things that ship switched off"
        // is a worse idea than the specific one. One job, named here, where
        // anybody reading this file can see which.
        if (debug_boot && j->disabled && seq(j->label, "os.schos.schdbgd")) {
            out("schinitd: remote debugging was chosen at boot; "
                "starting schdbgd\n");
            j->disabled = 0;
        }

        if (j->disabled || !j->run_at_load || j->own_tty)
            continue;
        if (j->pid)
            continue;                  // adopted; it never stopped running
        vout("schinitd: starting "); vout(j->label); vout("\n");
        ask_spawn(j);
    }

    vout("schinitd: ready\n");

    // The one line a working boot prints, and the last one.
    //
    // Here rather than in the kernel because "the machine is up" is not true
    // until the jobs that make it usable have been started, and because
    // anything printed before this point would come after it on screen. The
    // kernel's whole boot is a kinfo() and is in the klog; this is the part a
    // person is meant to read.
    // Only the first supervisor of a boot says it. A replacement adopts
    // running jobs rather than starting them, so greeting the user again would
    // be announcing a boot that already happened -- and it printed twice for
    // real, the day schinitd was replaced during startup.
    if (!verbose_boot && h.generation <= 1)
        out("\nWelcome to SchOS!"
            "  (F8 at boot for the full log, or dmesg once you are in)\n\n");

    // *Now* the shell on the console.
    //
    // It used to be started with everything else, and then this greeting was
    // printed -- which made the two a race that the shell won about half the
    // time. A boot that sometimes reads
    //
    //     Welcome to SchOS!  (F8 at boot for the full log, ...)
    //     root@schos / »
    //
    // and sometimes reads the same two lines the other way round is not two
    // orderings of one boot, it is a greeting that looks like it was printed
    // by mistake.
    //
    // Holding it back until after the write is enough to settle it: this
    // process writes the line, and only then asks PID 1 to spawn the shell, so
    // the fork cannot even have happened yet. Nothing else waits on it -- the
    // console shell is the last job to start either way, and starting it a
    // few microseconds later costs nothing a person can see.
    //
    // `own_tty` rather than a name: the job that claims the console as its
    // controlling terminal is exactly the job whose output lands here.
    for (int i = 0; i < njobs; i++) {
        struct job *j = &jobs[i];
        if (j->disabled || !j->run_at_load || !j->own_tty || j->pid)
            continue;
        vout("schinitd: starting "); vout(j->label); vout("\n");
        ask_spawn(j);
    }

    // From here on this program does two things: answer PID 1's heartbeat, and
    // act on the exits it reports. There is no wait() in this program and
    // there cannot be -- the children are not ours.
    //
    // The read is polled rather than blocking, so that a PID 1 which has
    // stopped talking is noticed. A blocking read would sit there forever
    // looking exactly like a quiet machine, which is the failure mode this
    // whole heartbeat exists to make visible.
    int last_ping = uptime();

    for (;;) {
        struct pollfd pf;
        pf.fd = sock;
        pf.events = POLLIN;
        pf.revents = 0;

        int pr = poll(&pf, 1, 1000);

        // The channel is gone. This happens to the *old* supervisor when PID 1
        // has replaced it: PID 1 closes its end, so poll reports a hangup and
        // never reports readable again. Without this the replaced supervisor
        // sits in the loop for ever, complaining that PID 1 has stopped
        // pinging -- which it has, because it is no longer talking to us.
        if (pr > 0 && (pf.revents & (POLLHUP | POLLERR | POLLNVAL))) {
            out("schinitd: channel closed; this supervisor has been replaced\n");
            lflush();
        exit();
        }

        if (pr > 0 && (pf.revents & POLLIN)) {
            union schinit_msg m;
            int n = read(sock, &m, sizeof(m));
            if (n <= 0) {
                // PID 1 closed the channel. It is the one process that cannot
                // have gone away, so this means we are being replaced.
                out("schinitd: channel closed\n");
                lflush();
        exit();
            }
            if (m.op == SCHINIT_PING) {
                struct schinit_hello p;
                p.op = SCHINIT_PONG;
                p.proto = SCHINIT_PROTO;
                p.generation = 0;
                p.reserved = 0;
                write(sock, &p, sizeof(p));
                last_ping = uptime();
            } else if (m.op == SCHINIT_EXITED) {
                note_exit(m.reply.cookie, m.reply.pid, m.reply.status);
            }
        }

        // Silence from PID 1.
        //
        // There is nothing this program can do about it -- PID 1 cannot be
        // killed and cannot be replaced from here -- so it says so, loudly,
        // once. docs/architecture/init.md is explicit that this is the limit: a hang
        // in PID 1 is only caught by noticing, and if both ends are wedged at
        // once nothing catches it. There is no hardware watchdog.
        //
        // Saying so is still worth doing. The alternative is a machine that
        // looks idle while its init is stuck, which is how this class of bug
        // stays invisible for months.
        static int complained;
        if (!complained && uptime() - last_ping >= SCHINIT_DEADLINE_SECS * 100) {
            out("schinitd: pid 1 has not pinged for "
                "the deadline -- it may be wedged\n");
            complained = 1;
        }
    }
}