reddb_server/physical/

shm.rs

//! `<data>.shm` shared-memory file substrate (gh-475).
//!
//! When provisioning is enabled (tier-wired for `Standard` and above in a
//! later slice), opening a database creates a sibling `<data>.shm` file
//! that carries a deterministic binary header recording the current owner
//! pid, generation counter, and reader registry. The header is the lock
//! protocol substrate that lets multiple embedded readers coexist on the
//! same data file and lets the next opener detect a crashed prior owner.
//!
//! ## Binary layout (v1, little-endian, 64-byte fixed header)
//!
//! ```text
//! offset size field             notes
//!      0    8 magic             ASCII "RDBSHM01"
//!      8    4 version           u32 = 1
//!     12    4 owner_pid         u32, host pid of the writer that holds the lease
//!     16    8 generation        u64, bumped on every owner takeover or heal
//!     24    8 reader_count      u64, count of attached embedded readers
//!     32    8 last_heartbeat_ms u64, owner heartbeat in unix-ms
//!     40   16 reserved          zeroed, room for v2 fields
//!     56    8 checksum          xxh3-style fold of bytes [0..56)
//! ```
//!
//! ## Lock protocol
//!
//! 1. On open, the writer attempts to claim ownership. If the magic is
//!    absent or invalid, it initialises the header with its pid and a
//!    fresh generation. If the magic is present, it inspects
//!    `owner_pid`: if the pid is no longer alive, this is a crash — the
//!    new owner bumps `generation`, rewrites the header, and the load
//!    path treats `reader_count` as authoritative for cleanup decisions
//!    in a later slice.
//! 2. Embedded readers attach by incrementing `reader_count` and
//!    detach by decrementing it. The count survives the writer crash
//!    so the next opener sees how many stale handles must be cleaned.
//! 3. mmap-ing the file is a follow-up slice; the on-disk substrate is
//!    valid without it. The file size is fixed at one OS page so mmap
//!    integration is mechanical when wired.

use std::fs::{File, OpenOptions};
use std::io::{self, Read, Seek, SeekFrom, Write};
use std::path::{Path, PathBuf};
use std::sync::atomic::{AtomicU8, Ordering};
use std::time::{SystemTime, UNIX_EPOCH};

pub const SHM_MAGIC: &[u8; 8] = b"RDBSHM01";
pub const SHM_VERSION: u32 = 1;
pub const SHM_HEADER_SIZE: usize = 64;
pub const SHM_FILE_SIZE: u64 = 4096;

static SHM_POLICY: AtomicU8 = AtomicU8::new(0);

/// Process-wide opt-in for `<data>.shm` provisioning. Default off so
/// existing single-writer flows (`minimal`) keep their current shape.
/// Tier wiring should call this with `true` when `tier >= Standard`.
/// Escape hatch: `REDDB_SHM_PROVISION=1`.
pub fn set_shm_provisioning_enabled(enabled: bool) {
    SHM_POLICY.store(if enabled { 1 } else { 2 }, Ordering::Relaxed);
}

/// Whether the open path should provision a `<data>.shm` file.
pub fn shm_provisioning_enabled() -> bool {
    match SHM_POLICY.load(Ordering::Relaxed) {
        1 => true,
        2 => false,
        _ => std::env::var("REDDB_SHM_PROVISION")
            .ok()
            .map(|v| matches!(v.as_str(), "1" | "true" | "TRUE" | "yes" | "on"))
            .unwrap_or(false),
    }
}

/// Sibling path of the `-shm` substrate file for a given data file.
pub fn shm_path_for(data_path: &Path) -> PathBuf {
    let file_name = data_path
        .file_name()
        .map(|n| n.to_string_lossy().into_owned())
        .unwrap_or_else(|| "data.rdb".to_string());
    let shm_file = format!("{file_name}-shm");
    match data_path.parent() {
        Some(parent) if !parent.as_os_str().is_empty() => parent.join(shm_file),
        _ => PathBuf::from(shm_file),
    }
}

/// Outcome of a provisioning attempt — distinguishes a clean takeover
/// from a crash recovery for diagnostics and tests.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum ShmProvisionState {
    /// File did not exist; created fresh.
    Created,
    /// Existing owner pid is still alive; attached as an additional handle.
    AttachedToLiveOwner,
    /// Existing owner pid is dead; took ownership and bumped generation.
    RecoveredFromCrash,
    /// File existed but the header was unreadable; reinitialised.
    HealedCorruptHeader,
}

#[derive(Debug, Clone)]
pub struct ShmHeader {
    pub version: u32,
    pub owner_pid: u32,
    pub generation: u64,
    pub reader_count: u64,
    pub last_heartbeat_ms: u64,
}

impl ShmHeader {
    fn encode(&self) -> [u8; SHM_HEADER_SIZE] {
        let mut buf = [0u8; SHM_HEADER_SIZE];
        buf[0..8].copy_from_slice(SHM_MAGIC);
        buf[8..12].copy_from_slice(&self.version.to_le_bytes());
        buf[12..16].copy_from_slice(&self.owner_pid.to_le_bytes());
        buf[16..24].copy_from_slice(&self.generation.to_le_bytes());
        buf[24..32].copy_from_slice(&self.reader_count.to_le_bytes());
        buf[32..40].copy_from_slice(&self.last_heartbeat_ms.to_le_bytes());
        let checksum = fold_checksum(&buf[..56]);
        buf[56..64].copy_from_slice(&checksum.to_le_bytes());
        buf
    }

    fn decode(buf: &[u8; SHM_HEADER_SIZE]) -> io::Result<Self> {
        if &buf[0..8] != SHM_MAGIC {
            return Err(io::Error::new(io::ErrorKind::InvalidData, "shm magic mismatch"));
        }
        let stored_checksum = u64::from_le_bytes(buf[56..64].try_into().unwrap());
        let computed = fold_checksum(&buf[..56]);
        if stored_checksum != computed {
            return Err(io::Error::new(io::ErrorKind::InvalidData, "shm checksum mismatch"));
        }
        Ok(Self {
            version: u32::from_le_bytes(buf[8..12].try_into().unwrap()),
            owner_pid: u32::from_le_bytes(buf[12..16].try_into().unwrap()),
            generation: u64::from_le_bytes(buf[16..24].try_into().unwrap()),
            reader_count: u64::from_le_bytes(buf[24..32].try_into().unwrap()),
            last_heartbeat_ms: u64::from_le_bytes(buf[32..40].try_into().unwrap()),
        })
    }
}

/// Handle to the provisioned `-shm` file. Drop semantics intentionally
/// minimal in this slice — callers must invoke [`Self::detach_reader`]
/// explicitly to mirror the eventual mmap-backed lifecycle.
pub struct ShmHandle {
    pub path: PathBuf,
    pub header: ShmHeader,
    pub state: ShmProvisionState,
    file: File,
}

impl ShmHandle {
    /// Current generation counter. Bumps on every crash recovery so
    /// observers can detect that ownership changed between snapshots.
    pub fn generation(&self) -> u64 {
        self.header.generation
    }

    /// Increment the on-disk reader counter. Returns the new count.
    pub fn attach_reader(&mut self) -> io::Result<u64> {
        self.header.reader_count = self.header.reader_count.saturating_add(1);
        self.rewrite_header()?;
        Ok(self.header.reader_count)
    }

    /// Decrement the on-disk reader counter (saturating). Returns new count.
    pub fn detach_reader(&mut self) -> io::Result<u64> {
        self.header.reader_count = self.header.reader_count.saturating_sub(1);
        self.rewrite_header()?;
        Ok(self.header.reader_count)
    }

    /// Refresh `last_heartbeat_ms` to the current unix-ms.
    pub fn heartbeat(&mut self) -> io::Result<()> {
        self.header.last_heartbeat_ms = unix_ms_now();
        self.rewrite_header()
    }

    fn rewrite_header(&mut self) -> io::Result<()> {
        let buf = self.header.encode();
        self.file.seek(SeekFrom::Start(0))?;
        self.file.write_all(&buf)?;
        self.file.sync_data()?;
        Ok(())
    }
}

/// Provision the `-shm` substrate for a data file. Idempotent; safe to
/// call from every open. See module docs for the lock protocol.
pub fn provision_shm(data_path: &Path) -> io::Result<ShmHandle> {
    let path = shm_path_for(data_path);
    if let Some(parent) = path.parent() {
        if !parent.as_os_str().is_empty() {
            std::fs::create_dir_all(parent)?;
        }
    }

    let mut file = OpenOptions::new()
        .read(true)
        .write(true)
        .create(true)
        .truncate(false)
        .open(&path)?;

    let metadata = file.metadata()?;
    let fresh = metadata.len() == 0;

    if fresh {
        file.set_len(SHM_FILE_SIZE)?;
        let header = ShmHeader {
            version: SHM_VERSION,
            owner_pid: current_pid(),
            generation: 1,
            reader_count: 0,
            last_heartbeat_ms: unix_ms_now(),
        };
        file.seek(SeekFrom::Start(0))?;
        file.write_all(&header.encode())?;
        file.sync_data()?;
        return Ok(ShmHandle {
            path,
            header,
            state: ShmProvisionState::Created,
            file,
        });
    }

    let mut buf = [0u8; SHM_HEADER_SIZE];
    file.seek(SeekFrom::Start(0))?;
    let existing = match file.read_exact(&mut buf) {
        Ok(()) => ShmHeader::decode(&buf).ok(),
        Err(_) => None,
    };

    let (header, state) = match existing {
        Some(prev) if pid_alive(prev.owner_pid) && prev.owner_pid != current_pid() => {
            // Attach to live owner — increment reader_count, keep generation.
            let next = ShmHeader {
                version: SHM_VERSION,
                owner_pid: prev.owner_pid,
                generation: prev.generation,
                reader_count: prev.reader_count.saturating_add(1),
                last_heartbeat_ms: prev.last_heartbeat_ms,
            };
            (next, ShmProvisionState::AttachedToLiveOwner)
        }
        Some(prev) if prev.owner_pid == current_pid() => {
            // Same-process reopen; refresh heartbeat, keep counters.
            let next = ShmHeader {
                version: SHM_VERSION,
                owner_pid: prev.owner_pid,
                generation: prev.generation,
                reader_count: prev.reader_count,
                last_heartbeat_ms: unix_ms_now(),
            };
            (next, ShmProvisionState::AttachedToLiveOwner)
        }
        Some(prev) => {
            // Owner is dead — take over, bump generation, clear reader count.
            let next = ShmHeader {
                version: SHM_VERSION,
                owner_pid: current_pid(),
                generation: prev.generation.saturating_add(1),
                reader_count: 0,
                last_heartbeat_ms: unix_ms_now(),
            };
            (next, ShmProvisionState::RecoveredFromCrash)
        }
        None => {
            // File exists but header is unreadable — heal in place.
            let next = ShmHeader {
                version: SHM_VERSION,
                owner_pid: current_pid(),
                generation: 1,
                reader_count: 0,
                last_heartbeat_ms: unix_ms_now(),
            };
            file.set_len(SHM_FILE_SIZE)?;
            (next, ShmProvisionState::HealedCorruptHeader)
        }
    };

    file.seek(SeekFrom::Start(0))?;
    file.write_all(&header.encode())?;
    file.sync_data()?;

    Ok(ShmHandle {
        path,
        header,
        state,
        file,
    })
}

/// Read the current header without taking ownership. Returns `Ok(None)`
/// when the file does not exist; surfaces a real I/O error if the file
/// is present but unreadable.
pub fn read_shm_header(data_path: &Path) -> io::Result<Option<ShmHeader>> {
    let path = shm_path_for(data_path);
    if !path.exists() {
        return Ok(None);
    }
    let mut file = OpenOptions::new().read(true).open(&path)?;
    let mut buf = [0u8; SHM_HEADER_SIZE];
    file.read_exact(&mut buf)?;
    ShmHeader::decode(&buf).map(Some)
}

fn fold_checksum(bytes: &[u8]) -> u64 {
    let mut acc: u64 = 0xcbf29ce484222325;
    for &byte in bytes {
        acc ^= byte as u64;
        acc = acc.wrapping_mul(0x100000001b3);
    }
    acc
}

fn unix_ms_now() -> u64 {
    SystemTime::now()
        .duration_since(UNIX_EPOCH)
        .map(|d| d.as_millis() as u64)
        .unwrap_or(0)
}

fn current_pid() -> u32 {
    std::process::id()
}

#[cfg(unix)]
fn pid_alive(pid: u32) -> bool {
    if pid == 0 {
        return false;
    }
    // `kill(pid, 0)` returns 0 if the process exists, -1 otherwise.
    // EPERM still implies the process exists (we just can't signal it).
    let rc = unsafe { libc::kill(pid as libc::pid_t, 0) };
    if rc == 0 {
        return true;
    }
    io::Error::last_os_error()
        .raw_os_error()
        .map(|e| e == libc::EPERM)
        .unwrap_or(false)
}

#[cfg(not(unix))]
fn pid_alive(_pid: u32) -> bool {
    // Conservative: assume alive on non-unix until a platform-specific
    // probe is wired. Crash recovery on those platforms is a follow-up.
    true
}