fsqlite-vfs 0.4.6

//! Cross-platform process-liveness probe for crash cleanup (bd-4dr7g).
//!
//! `fsqlite-vfs` is the designated FFI boundary (it already performs `libc` /
//! Win32 calls and does not `forbid(unsafe_code)`), so the OS-specific liveness
//! probes live here. `fsqlite-mvcc` stays `unsafe`-free by consuming these
//! through its existing function-pointer injection into
//! `check_serialized_writer_exclusion`.
//!
//! A liveness answer is `Alive`, `Dead`, or `Unknown`. Callers treat `Unknown`
//! (an ambiguous OS error such as `EACCES`/`ERROR_ACCESS_DENIED`) as alive:
//! never reclaim a possibly-live writer's lease on the strength of a probe we
//! could not complete. A missing Linux procfs entry is not proof of death:
//! `hidepid` mounts can conceal live processes, so an unavailable start time
//! requires an independent signal-zero probe before reporting absence.
//!
//! ## PID-reuse safety
//!
//! A raw PID can be recycled by the OS. Each probe pairs the PID with a
//! *birth token* — a reuse-safe snapshot of the process's start time — so a
//! recycled PID (same number, different start time) reads as `Dead`. The token
//! is platform-tagged in its top bits, so a token minted on one platform is
//! never mis-decoded on another (and, on Linux, keeps bit 63 for on-disk
//! compatibility with lock tables written before this change).

/// Liveness verdict for a `(pid, birth)` pair.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum ProcessLiveness {
    /// The process exists and its start time matches the birth token.
    Alive,
    /// The process does not exist, or the PID was recycled (start-time mismatch).
    Dead,
    /// The probe could not complete (ambiguous OS error); treat as alive.
    Unknown,
}

/// Top-bit tag for a Linux procfs start-ticks birth token (unchanged for
/// on-disk compatibility with pre-bd-4dr7g lock tables).
pub const PID_BIRTH_PROCFS_TAG: u64 = 1_u64 << 63;
/// Top-bit tag for a macOS process start-time (microseconds) birth token, read
/// via `proc_pidinfo(PROC_PIDTBSDINFO)`.
pub const PID_BIRTH_SYSCTL_TAG: u64 = 1_u64 << 62;
/// Top-bit tag for a Windows process-creation `FILETIME` (100 ns) birth token.
pub const PID_BIRTH_FILETIME_TAG: u64 = 1_u64 << 61;

/// Mask isolating the payload bits (all three platform tags cleared).
///
/// Gate native-only helpers so unsupported targets retain their conservative
/// fallback without dead-code warnings.
#[cfg(any(target_os = "linux", target_os = "macos", windows, test))]
const PAYLOAD_MASK: u64 = !(PID_BIRTH_PROCFS_TAG | PID_BIRTH_SYSCTL_TAG | PID_BIRTH_FILETIME_TAG);

#[cfg(any(target_os = "linux", target_os = "macos", windows, test))]
const fn has_exact_platform_tag(token: u64, expected_tag: u64) -> bool {
    token & !PAYLOAD_MASK == expected_tag
}

/// Classification shared by the native probes after an OS call fails.
///
/// Only an error code that unambiguously means "no such process" may release
/// shared ownership. Every other error remains ambiguous and therefore maps to
/// [`ProcessLiveness::Unknown`] at the platform boundary.
#[cfg(any(target_os = "linux", target_os = "macos", windows, test))]
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum ProbeFailure {
    Absent,
    Ambiguous,
}

#[cfg(any(target_os = "linux", target_os = "macos", windows, test))]
const fn classify_probe_failure(
    error_code: Option<i64>,
    definitive_absence_code: i64,
) -> ProbeFailure {
    if matches!(error_code, Some(code) if code == definitive_absence_code) {
        ProbeFailure::Absent
    } else {
        ProbeFailure::Ambiguous
    }
}

/// The platform-tagged birth token for the current process, or `None` when the
/// OS start time is unavailable (the caller then falls back to a time token).
#[must_use]
pub fn current_process_birth_token() -> Option<u64> {
    // ubs:ignore - the "birth token" is a process start time used only to
    // distinguish a recycled PID; it is not a security token / secret / nonce.
    let pid = std::process::id();
    #[cfg(target_os = "linux")]
    {
        linux::birth_token(pid)
    }
    #[cfg(target_os = "macos")]
    {
        macos::birth_token(pid)
    }
    #[cfg(windows)]
    {
        windows_impl::birth_token(pid)
    }
    #[cfg(not(any(target_os = "linux", target_os = "macos", windows)))]
    {
        let _ = pid;
        None
    }
}

/// Probe whether `pid` (with the reuse-safe `pid_birth` token) is still the same
/// live process.
///
/// Linux, macOS and Windows are implemented here. Unsupported targets return
/// `Unknown`. An untagged or foreign-platform birth token cannot prove PID
/// reuse. Linux callers sharing owner records must use the same PID namespace
/// and a procfs mount corresponding to that namespace.
#[must_use]
pub fn process_alive(pid: u32, pid_birth: u64) -> ProcessLiveness {
    if pid == 0 {
        return ProcessLiveness::Dead;
    }
    #[cfg(target_os = "linux")]
    {
        linux::alive(pid, pid_birth)
    }
    #[cfg(target_os = "macos")]
    {
        macos::alive(pid, pid_birth)
    }
    #[cfg(windows)]
    {
        windows_impl::alive(pid, pid_birth)
    }
    #[cfg(not(any(target_os = "linux", target_os = "macos", windows)))]
    {
        let _ = pid_birth;
        ProcessLiveness::Unknown
    }
}

#[cfg(target_os = "linux")]
mod linux {
    use std::io;

    use super::{
        PID_BIRTH_PROCFS_TAG, ProbeFailure, ProcessLiveness, classify_probe_failure,
        has_exact_platform_tag,
    };

    fn read_stat(pid: u32) -> io::Result<Vec<u8>> {
        let path = std::path::Path::new("/proc")
            .join(pid.to_string())
            .join("stat");
        // comm is an arbitrary byte string, not necessarily UTF-8. Restrict
        // text decoding to the numeric fields rather than the whole record.
        std::fs::read(path)
    }

    fn start_ticks(stat: &[u8], expected_pid: u32) -> Option<u64> {
        let comm_start = stat.iter().position(|byte| *byte == b'(')?;
        let recorded_pid = std::str::from_utf8(&stat[..comm_start])
            .ok()?
            .trim()
            .parse::<u32>()
            .ok()?;
        if recorded_pid != expected_pid {
            return None;
        }
        // comm may contain spaces, newlines and ')'. The fields following its
        // final ')' are ASCII; field 22 is the twentieth token starting at state.
        let comm_end = stat.iter().rposition(|byte| *byte == b')')?;
        if comm_end < comm_start {
            return None;
        }
        std::str::from_utf8(stat.get(comm_end + 1..)?)
            .ok()?
            .split_ascii_whitespace()
            .nth(19)?
            .parse::<u64>()
            .ok()
    }

    pub(super) fn birth_token(pid: u32) -> Option<u64> {
        let stat = read_stat(pid).ok()?;
        let ticks = start_ticks(&stat, pid)?;
        // Preserve the existing Linux token encoding used by MVCC mappings.
        Some(PID_BIRTH_PROCFS_TAG | (ticks & !PID_BIRTH_PROCFS_TAG))
    }

    fn presence(pid: u32) -> ProcessLiveness {
        let Ok(native_pid) = libc::pid_t::try_from(pid) else {
            return ProcessLiveness::Unknown;
        };
        if native_pid <= 0 {
            // Never turn an invalid u32 PID into a process-group probe.
            return ProcessLiveness::Unknown;
        }
        // SAFETY: native_pid is strictly positive and representable in pid_t;
        // signal zero only probes existence/permission and sends no signal.
        if unsafe { libc::kill(native_pid, 0) } == 0 {
            return ProcessLiveness::Alive;
        }
        // Capture errno immediately. EPERM, seccomp refusal and every other
        // ambiguous failure must not authorize releasing another writer.
        let error_code = io::Error::last_os_error().raw_os_error().map(i64::from);
        match classify_probe_failure(error_code, i64::from(libc::ESRCH)) {
            ProbeFailure::Absent => ProcessLiveness::Dead,
            ProbeFailure::Ambiguous => ProcessLiveness::Unknown,
        }
    }

    fn alive_with(
        pid: u32,
        pid_birth: u64,
        read: impl FnOnce(u32) -> io::Result<Vec<u8>>,
        probe_presence: impl FnOnce(u32) -> ProcessLiveness,
    ) -> ProcessLiveness {
        if let Some(ticks) = read(pid).ok().and_then(|stat| start_ticks(&stat, pid)) {
            if !has_exact_platform_tag(pid_birth, PID_BIRTH_PROCFS_TAG) {
                return ProcessLiveness::Alive;
            }
            return if (ticks & !PID_BIRTH_PROCFS_TAG) == (pid_birth & !PID_BIRTH_PROCFS_TAG) {
                ProcessLiveness::Alive
            } else {
                ProcessLiveness::Dead
            };
        }
        // ENOENT may be hidepid=2, an unmounted procfs, or actual process death.
        // Even a successful presence probe cannot establish the birth token,
        // so preserve Unknown unless the kernel independently proves absence.
        match probe_presence(pid) {
            ProcessLiveness::Dead => ProcessLiveness::Dead,
            ProcessLiveness::Alive | ProcessLiveness::Unknown => ProcessLiveness::Unknown,
        }
    }

    pub(super) fn alive(pid: u32, pid_birth: u64) -> ProcessLiveness {
        alive_with(pid, pid_birth, read_stat, presence)
    }

    #[cfg(test)]
    // Birth tokens are tag bits OR'd with decimal fake PIDs; the PIDs read
    // better in decimal.
    #[allow(clippy::decimal_bitwise_operands)]
    mod tests {
        use std::cell::Cell;

        use super::*;
        use crate::process::{PID_BIRTH_FILETIME_TAG, PID_BIRTH_SYSCTL_TAG};

        fn stat(pid: u32, comm: &[u8], ticks: u64) -> Vec<u8> {
            let mut bytes = format!("{pid} (").into_bytes();
            bytes.extend_from_slice(comm);
            bytes.extend_from_slice(b") R");
            for field in 4..22 {
                bytes.extend_from_slice(format!(" {field}").as_bytes());
            }
            bytes.extend_from_slice(format!(" {ticks} 23 24\n").as_bytes());
            bytes
        }

        #[test]
        fn proc_stat_parsing_ignores_arbitrary_comm_bytes() {
            for comm in [
                b"writer".as_slice(),
                b"a (b) c)",
                b"a\nb",
                b"invalid\xff\xfe",
            ] {
                for ticks in [0, 42, u64::MAX] {
                    assert_eq!(start_ticks(&stat(123, comm, ticks), 123), Some(ticks));
                }
            }
        }

        #[test]
        fn malformed_stat_never_supplies_a_birth_token() {
            let valid = stat(123, b"writer", 9876);
            assert_eq!(start_ticks(&valid, 124), None);
            for bytes in [
                b"".as_slice(),
                b"123 writer R",
                b"bad (writer) R",
                b"123 ) (",
                b"123 (writer) R 1 2 3",
                b"123 (writer) \xff",
            ] {
                assert_eq!(start_ticks(bytes, 123), None);
            }
            let mut bad_ticks = b"123 (writer) R".to_vec();
            for _ in 4..22 {
                bad_ticks.extend_from_slice(b" 0");
            }
            bad_ticks.extend_from_slice(b" 18446744073709551616");
            assert_eq!(start_ticks(&bad_ticks, 123), None);
        }

        #[test]
        fn only_an_exact_linux_token_can_prove_pid_reuse() {
            for (birth, expected) in [
                (PID_BIRTH_PROCFS_TAG | 1234, ProcessLiveness::Alive),
                (PID_BIRTH_PROCFS_TAG | 1235, ProcessLiveness::Dead),
                (0, ProcessLiveness::Alive),
                (1235, ProcessLiveness::Alive),
                (PID_BIRTH_SYSCTL_TAG | 1235, ProcessLiveness::Alive),
                (PID_BIRTH_FILETIME_TAG | 1235, ProcessLiveness::Alive),
                (
                    PID_BIRTH_PROCFS_TAG | PID_BIRTH_SYSCTL_TAG | 1235,
                    ProcessLiveness::Alive,
                ),
                (
                    PID_BIRTH_PROCFS_TAG | PID_BIRTH_FILETIME_TAG | 1235,
                    ProcessLiveness::Alive,
                ),
            ] {
                assert_eq!(
                    alive_with(
                        123,
                        birth,
                        |pid| Ok(stat(pid, b"writer", 1234)),
                        |_| panic!("valid stat does not require a fallback probe"),
                    ),
                    expected
                );
            }
        }

        #[test]
        fn proc_read_failures_require_independent_proof_of_absence() {
            for kind in [
                io::ErrorKind::NotFound,
                io::ErrorKind::PermissionDenied,
                io::ErrorKind::Interrupted,
                io::ErrorKind::InvalidData,
                io::ErrorKind::Other,
            ] {
                for presence in [
                    ProcessLiveness::Alive,
                    ProcessLiveness::Unknown,
                    ProcessLiveness::Dead,
                ] {
                    let probes = Cell::new(0);
                    let verdict = alive_with(
                        123,
                        PID_BIRTH_PROCFS_TAG | 1234,
                        |_| Err(kind.into()),
                        |pid| {
                            assert_eq!(pid, 123);
                            probes.set(probes.get() + 1);
                            presence
                        },
                    );
                    assert_eq!(probes.get(), 1);
                    assert_eq!(
                        verdict,
                        if presence == ProcessLiveness::Dead {
                            ProcessLiveness::Dead
                        } else {
                            ProcessLiveness::Unknown
                        }
                    );
                }
            }
        }

        #[test]
        fn invalid_stat_keeps_a_possibly_live_owner() {
            for bytes in [b"truncated".to_vec(), stat(124, b"wrong pid", 1234)] {
                assert_eq!(
                    alive_with(
                        123,
                        PID_BIRTH_PROCFS_TAG | 1234,
                        |_| Ok(bytes),
                        |_| ProcessLiveness::Alive,
                    ),
                    ProcessLiveness::Unknown
                );
            }
        }

        #[test]
        fn signal_zero_never_uses_a_process_group_pid() {
            assert_eq!(presence(0), ProcessLiveness::Unknown);
            assert_eq!(presence(u32::MAX), ProcessLiveness::Unknown);
        }

        #[test]
        fn signal_probe_requires_esrch_for_absence() {
            assert_eq!(
                classify_probe_failure(Some(i64::from(libc::ESRCH)), i64::from(libc::ESRCH)),
                ProbeFailure::Absent
            );
            for code in [
                None,
                Some(0),
                Some(libc::EPERM),
                Some(libc::EACCES),
                Some(libc::EINVAL),
            ] {
                assert_eq!(
                    classify_probe_failure(code.map(i64::from), i64::from(libc::ESRCH)),
                    ProbeFailure::Ambiguous
                );
            }
        }
    }
}

#[cfg(target_os = "macos")]
mod macos {
    use super::{
        PAYLOAD_MASK, PID_BIRTH_SYSCTL_TAG, ProbeFailure, ProcessLiveness, classify_probe_failure,
        has_exact_platform_tag,
    };

    /// Outcome of reading a process's start time via `sysctl`.
    enum StartTime {
        /// Process exists; start time in microseconds since the epoch.
        Present(u64),
        /// `sysctl` reported no such process (zero-length result).
        Absent,
        /// The probe failed ambiguously.
        Error,
    }

    /// Read a process's start time (microseconds) via
    /// `proc_pidinfo(pid, PROC_PIDTBSDINFO)` -> `proc_bsdinfo.pbi_start_tv*`.
    /// (`libc` does not expose `kinfo_proc` on Darwin, so this uses libproc.)
    fn read_start_time_usec(pid: u32) -> StartTime {
        let mut info = std::mem::MaybeUninit::<libc::proc_bsdinfo>::uninit();
        let size = std::mem::size_of::<libc::proc_bsdinfo>() as libc::c_int;
        // `errno` is sticky. Clear this thread's Darwin errno slot so a zero
        // return cannot inherit an unrelated earlier `ESRCH` and falsely
        // authorize lease takeover.
        // SAFETY: `__error` returns the current thread's writable errno slot.
        unsafe { *libc::__error() = 0 };
        // SAFETY: `proc_pidinfo` writes at most `size` bytes into the uninitialized,
        // correctly sized output buffer; it returns the byte count, or <= 0 on
        // error / no such process.
        let written = unsafe {
            libc::proc_pidinfo(
                pid as libc::c_int,
                libc::PROC_PIDTBSDINFO,
                0,
                info.as_mut_ptr().cast(),
                size,
            )
        };
        // Apple libproc normalizes the underlying __proc_info -1 failure to a
        // zero return, so errno must be captured immediately even when
        // `written == 0`. A zero result alone does not prove that the PID is
        // absent.
        let error_code = (written <= 0)
            .then(|| std::io::Error::last_os_error().raw_os_error())
            .flatten()
            .map(i64::from);
        if written <= 0 {
            return match classify_probe_failure(error_code, i64::from(libc::ESRCH)) {
                ProbeFailure::Absent => StartTime::Absent,
                ProbeFailure::Ambiguous => StartTime::Error,
            };
        }
        if written < size {
            return StartTime::Error;
        }
        // SAFETY: a successful full-size `proc_pidinfo` result initialized the
        // complete `proc_bsdinfo` output object.
        let info = unsafe { info.assume_init() };
        let usec = info
            .pbi_start_tvsec
            .wrapping_mul(1_000_000)
            .wrapping_add(info.pbi_start_tvusec);
        StartTime::Present(usec)
    }

    pub(super) fn birth_token(pid: u32) -> Option<u64> {
        match read_start_time_usec(pid) {
            StartTime::Present(usec) => Some(PID_BIRTH_SYSCTL_TAG | (usec & PAYLOAD_MASK)),
            StartTime::Absent | StartTime::Error => None,
        }
    }

    pub(super) fn alive(pid: u32, pid_birth: u64) -> ProcessLiveness {
        match read_start_time_usec(pid) {
            StartTime::Absent => ProcessLiveness::Dead,
            StartTime::Error => ProcessLiveness::Unknown,
            StartTime::Present(usec) => {
                if !has_exact_platform_tag(pid_birth, PID_BIRTH_SYSCTL_TAG) {
                    // Untagged/legacy, foreign-platform, or malformed multi-tag
                    // token: cannot compare start time, so stay conservative.
                    return ProcessLiveness::Alive;
                }
                if (usec & PAYLOAD_MASK) == (pid_birth & PAYLOAD_MASK) {
                    ProcessLiveness::Alive
                } else {
                    ProcessLiveness::Dead
                }
            }
        }
    }
}

#[cfg(windows)]
mod windows_impl {
    use super::{
        PAYLOAD_MASK, PID_BIRTH_FILETIME_TAG, ProbeFailure, ProcessLiveness,
        classify_probe_failure, has_exact_platform_tag,
    };
    use windows_sys::Win32::Foundation::{
        CloseHandle, ERROR_INVALID_PARAMETER, FILETIME, GetLastError,
    };
    use windows_sys::Win32::System::Threading::{
        GetProcessTimes, OpenProcess, PROCESS_QUERY_LIMITED_INFORMATION,
    };

    /// Outcome of reading a process's creation `FILETIME`.
    enum Creation {
        /// Process exists; creation time in 100 ns ticks since 1601.
        Present(u64),
        /// No such process (`OpenProcess` reported an invalid PID).
        Absent,
        /// The probe failed ambiguously (`OpenProcess` or `GetProcessTimes`).
        Error,
    }

    fn read_creation_100ns(pid: u32) -> Creation {
        // SAFETY: `OpenProcess` takes a query-limited access mask, a non-inherit
        // flag, and a PID; it returns a handle or null. No memory is aliased.
        let handle = unsafe { OpenProcess(PROCESS_QUERY_LIMITED_INFORMATION, 0, pid) };
        if handle.is_null() {
            // SAFETY: reads the calling thread's last-error, no arguments.
            let last = unsafe { GetLastError() };
            return match classify_probe_failure(
                Some(i64::from(last)),
                i64::from(ERROR_INVALID_PARAMETER),
            ) {
                ProbeFailure::Absent => Creation::Absent,
                ProbeFailure::Ambiguous => Creation::Error,
            };
        }
        let mut creation = FILETIME {
            dwLowDateTime: 0,
            dwHighDateTime: 0,
        };
        let mut exit = creation;
        let mut kernel = creation;
        let mut user = creation;
        // SAFETY: `handle` is a live process handle; the four `FILETIME` out
        // pointers are valid, initialized locals.
        let ok = unsafe {
            GetProcessTimes(
                handle,
                &raw mut creation,
                &raw mut exit,
                &raw mut kernel,
                &raw mut user,
            )
        };
        // SAFETY: `handle` came from `OpenProcess` above and is not used again.
        unsafe {
            CloseHandle(handle);
        }
        if ok == 0 {
            return Creation::Error;
        }
        let ticks = (u64::from(creation.dwHighDateTime) << 32) | u64::from(creation.dwLowDateTime);
        Creation::Present(ticks)
    }

    pub(super) fn birth_token(pid: u32) -> Option<u64> {
        match read_creation_100ns(pid) {
            Creation::Present(ticks) => Some(PID_BIRTH_FILETIME_TAG | (ticks & PAYLOAD_MASK)),
            Creation::Absent | Creation::Error => None,
        }
    }

    pub(super) fn alive(pid: u32, pid_birth: u64) -> ProcessLiveness {
        match read_creation_100ns(pid) {
            Creation::Absent => ProcessLiveness::Dead,
            Creation::Error => ProcessLiveness::Unknown,
            Creation::Present(ticks) => {
                if !has_exact_platform_tag(pid_birth, PID_BIRTH_FILETIME_TAG) {
                    return ProcessLiveness::Alive;
                }
                if (ticks & PAYLOAD_MASK) == (pid_birth & PAYLOAD_MASK) {
                    ProcessLiveness::Alive
                } else {
                    ProcessLiveness::Dead
                }
            }
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn platform_birth_tags_are_distinct_and_high() {
        // No two tags overlap, and the payload mask clears all three.
        assert_ne!(PID_BIRTH_PROCFS_TAG, PID_BIRTH_SYSCTL_TAG);
        assert_ne!(PID_BIRTH_SYSCTL_TAG, PID_BIRTH_FILETIME_TAG);
        assert_eq!(PID_BIRTH_PROCFS_TAG & PAYLOAD_MASK, 0);
        assert_eq!(PID_BIRTH_SYSCTL_TAG & PAYLOAD_MASK, 0);
        assert_eq!(PID_BIRTH_FILETIME_TAG & PAYLOAD_MASK, 0);
    }

    #[test]
    fn platform_birth_tag_must_be_exact() {
        assert!(has_exact_platform_tag(
            PID_BIRTH_SYSCTL_TAG | 0x1234,
            PID_BIRTH_SYSCTL_TAG
        ));
        assert!(!has_exact_platform_tag(0x1234, PID_BIRTH_SYSCTL_TAG));
        assert!(!has_exact_platform_tag(
            PID_BIRTH_FILETIME_TAG | 0x1234,
            PID_BIRTH_SYSCTL_TAG
        ));
        assert!(!has_exact_platform_tag(
            PID_BIRTH_SYSCTL_TAG | PID_BIRTH_FILETIME_TAG | 0x1234,
            PID_BIRTH_SYSCTL_TAG
        ));
    }

    #[test]
    fn pid_zero_is_dead() {
        assert_eq!(process_alive(0, 0), ProcessLiveness::Dead);
    }

    #[test]
    fn macos_zero_probe_result_requires_esrch_to_prove_absence() {
        assert_eq!(
            classify_probe_failure(Some(i64::from(libc::ESRCH)), i64::from(libc::ESRCH)),
            ProbeFailure::Absent
        );
        assert_eq!(
            classify_probe_failure(Some(i64::from(libc::EACCES)), i64::from(libc::ESRCH)),
            ProbeFailure::Ambiguous
        );
        assert_eq!(
            classify_probe_failure(None, i64::from(libc::ESRCH)),
            ProbeFailure::Ambiguous
        );
    }

    #[test]
    fn windows_open_process_resource_failure_is_ambiguous() {
        const ERROR_INVALID_PARAMETER_CODE: i64 = 87;
        const ERROR_NOT_ENOUGH_MEMORY_CODE: i64 = 8;

        assert_eq!(
            classify_probe_failure(
                Some(ERROR_INVALID_PARAMETER_CODE),
                ERROR_INVALID_PARAMETER_CODE
            ),
            ProbeFailure::Absent
        );
        assert_eq!(
            classify_probe_failure(
                Some(ERROR_NOT_ENOUGH_MEMORY_CODE),
                ERROR_INVALID_PARAMETER_CODE
            ),
            ProbeFailure::Ambiguous
        );
    }

    // Unsupported platforms must keep the conservative fallback.
    #[cfg(not(any(target_os = "linux", target_os = "macos", windows)))]
    #[test]
    fn unsupported_platform_returns_unknown() {
        assert_eq!(
            process_alive(std::process::id(), 0),
            ProcessLiveness::Unknown
        );
        assert!(current_process_birth_token().is_none());
    }

    #[cfg(any(target_os = "linux", target_os = "macos", windows))]
    #[test]
    fn current_process_is_alive_with_its_own_birth_token() {
        let birth = current_process_birth_token().expect("own birth token available");
        assert_eq!(
            process_alive(std::process::id(), birth),
            ProcessLiveness::Alive,
            "the current process must read as alive with its own birth token"
        );
    }

    #[cfg(any(target_os = "linux", target_os = "macos", windows))]
    #[test]
    fn recycled_pid_birth_mismatch_reads_dead() {
        // Same PID, deliberately wrong birth payload -> reused-PID -> Dead.
        let birth = current_process_birth_token().expect("own birth token");
        let tag = birth & !PAYLOAD_MASK;
        let mismatched = tag | ((birth & PAYLOAD_MASK) ^ 0x5A5A);
        assert_eq!(
            process_alive(std::process::id(), mismatched),
            ProcessLiveness::Dead,
            "a start-time mismatch on a live PID must read as Dead (reuse-safe)"
        );
    }

    #[cfg(any(target_os = "macos", windows))]
    #[test]
    fn almost_certainly_dead_pid_reads_dead() {
        // A very high PID that is almost certainly not running. If it happens to
        // exist, the birth mismatch still yields Dead; either way, not Alive.
        #[cfg(target_os = "macos")]
        let platform_tag = PID_BIRTH_SYSCTL_TAG;
        #[cfg(windows)]
        let platform_tag = PID_BIRTH_FILETIME_TAG;
        let verdict = process_alive(0x7FFF_FFF0, platform_tag | 0x1234);
        assert_ne!(verdict, ProcessLiveness::Alive);
    }
}