//! system/services/volume-manager — the storage layer's policy home
//! (docs/file-system-development/storage-architecture.md). Beside the device
//! manager: that owns the DEVICE tree, this owns the VOLUME layer. It probes a
//! storage provider's partition table, confines each filesystem to its
//! partition, spawns one filesystem per volume, and answers that filesystem's
//! startup hello with the range-confined block channel — so the filesystem
//! never finds its storage by name and never sees the whole device. It
//! supervises the filesystems it spawns, exactly as the device manager
//! supervises drivers.
//!
//! This increment (V3b) is the flip: the FAT service stops acquiring its own
//! volume and is spawned here instead, confined to its partition, and handed
//! its channel over the volume-manager protocol. Single volume for now; the
//! mount map (volumes.csv) and multi-volume land next.

const std = @import("std");
const channel = @import("channel");
const device_manager_protocol = @import("device-manager-protocol");
const volume_manager_protocol = @import("volume-manager-protocol");
const driver = @import("driver");
const ipc = @import("ipc");
const block = @import("block");
const memory = @import("memory");
const logging = @import("logging");
const process = @import("process");
const service = @import("service");
const time = @import("time");
const envelope = @import("envelope");
const partition = @import("partition.zig");

const Serve = volume_manager_protocol.Protocol.Provider(void);
const Invocation = envelope.Invocation;
const Answer = envelope.Answer;

/// The single volume this increment handles: its provider channel, its block
/// sub-range, its identity, the id it is addressed by, and the filesystem
/// process serving it (0 until spawned; reset on death for respawn).
const Volume = struct {
    storage: block.Device,
    storage_device_id: u64, // the device-manager id this volume's provider serves
    base_lba: u64,
    block_count: u64,
    identity: partition.Identity,
    id: u64,
    filesystem_pid: u32 = 0,
};

/// The filesystem binary a probed volume is served by. The signature->binary
/// map (filesystems.csv) lands with the identity ladder; for now every FAT-shaped
/// volume gets the FAT service.
const filesystem_binary = "/system/services/fat";
const volume_id: u64 = 1;

var service_endpoint: ipc.Handle = 0;
var manager_handle: ?ipc.Handle = null;
var bounce: memory.DmaRegion = undefined;
var bounce_ready = false;
/// The currently-mounted volume, or null while no storage is present. The whole
/// removal lifecycle is this field going null and back: the poll sees the
/// storage provider leave the device tree (a pulled stick), kills the filesystem
/// and clears this; when it returns, the poll re-acquires and re-mounts.
var volume: ?Volume = null;
var logged_no_volume = false;
/// How often the poll checks whether the storage provider is present. Fast
/// enough that an unplug unmounts promptly; the poll is a bare device-manager
/// enumerate, no channel work, so it is cheap to run continuously.
const poll_interval_ms = 500;

// Filesystem supervision, mirroring the device manager's (device-manager.zig):
// a clean exit is not restarted, a fault restarts with backoff, and a fast
// crash loop gives up rather than spinning. Without this a faulting filesystem
// respawns in a zero-delay loop.
const fast_death_ns: u64 = 2_000_000_000;
const crash_loop_cap: u32 = 3;
const backoff_base_ms: u64 = 300;
var fs_restarts: u32 = 0;
var fs_spawn_ns: u64 = 0;
var fs_failed = false;
/// A fat restart is due at `restart_due_ns`; the poll loop performs it once the
/// backoff has elapsed (one timer, folded into the poll — no second timer).
var restart_pending = false;
var restart_due_ns: u64 = 0;

fn deviceManager() ?ipc.Handle {
    if (manager_handle) |h| return h;
    const handle = channel.openEndpoint("device-manager") orelse return null;
    manager_handle = handle;
    return handle;
}

const OpenedStorage = struct { device_id: u64, device: block.Device };

/// The first mass-storage provider whose block channel actually opens, with its
/// device id. A device-manager tree can carry more than one entry of the
/// mass-storage identity — a phantom that no driver is bound to answers a
/// consumer hello with NO channel — so this tries each and takes the first that
/// yields a channel, exactly as a filesystem's own acquisition loop does.
/// Called only when there is no volume (an insertion), so the hellos it makes
/// are not per-poll churn.
fn openAnyStorage() ?OpenedStorage {
    const manager = deviceManager() orelse return null;
    const Entry = device_manager_protocol.ChildEntry;
    var start: u64 = 0;
    while (true) {
        const enumerate = envelope.Header{ .operation = envelope.operation_enumerate, .target = start };
        var reply: [device_manager_protocol.message_maximum]u8 = undefined;
        const length = ipc.call(manager, std.mem.asBytes(&enumerate), &reply) catch return null;
        const status = envelope.statusOf(reply[0..length]) orelse return null;
        if (status.status != 0) return null;
        const carried = @min(@as(usize, status.len), length -| envelope.prefix_size);
        const tail = reply[envelope.prefix_size..][0..carried];
        const count = tail.len / @sizeOf(Entry);
        if (count == 0) return null;
        var index: usize = 0;
        while (index < count) : (index += 1) {
            const entry = std.mem.bytesToValue(Entry, tail[index * @sizeOf(Entry) ..][0..@sizeOf(Entry)]);
            if (entry.device_id == device_manager_protocol.no_device) continue;
            if ((entry.identity >> 16) & 0xff != 0x08 or (entry.identity >> 8) & 0xff != 0x06) continue;
            const exchanged = driver.helloOn(manager, .consumer, entry.device_id, null, true) orelse continue;
            const provider = exchanged.channel orelse continue; // a phantom / not-yet-bound entry
            return .{ .device_id = entry.device_id, .device = .{ .endpoint = provider } };
        }
        start += count;
    }
}

/// Whether `device_id` is still in the device-manager tree — a bare enumerate,
/// no consumer-hello, so it is cheap to call every poll. This is how removal is
/// detected: the specific device the mounted volume sits on disappears.
fn isDevicePresent(device_id: u64) bool {
    const manager = deviceManager() orelse return false;
    const Entry = device_manager_protocol.ChildEntry;
    var start: u64 = 0;
    while (true) {
        const enumerate = envelope.Header{ .operation = envelope.operation_enumerate, .target = start };
        var reply: [device_manager_protocol.message_maximum]u8 = undefined;
        const length = ipc.call(manager, std.mem.asBytes(&enumerate), &reply) catch return false;
        const status = envelope.statusOf(reply[0..length]) orelse return false;
        if (status.status != 0) return false;
        const carried = @min(@as(usize, status.len), length -| envelope.prefix_size);
        const tail = reply[envelope.prefix_size..][0..carried];
        const count = tail.len / @sizeOf(Entry);
        if (count == 0) return false;
        var index: usize = 0;
        while (index < count) : (index += 1) {
            const entry = std.mem.bytesToValue(Entry, tail[index * @sizeOf(Entry) ..][0..@sizeOf(Entry)]);
            if (entry.device_id == device_id) return true;
        }
        start += count;
    }
}

/// Spawn the filesystem for `v`, confine it to the volume's range, and record
/// its pid. The confinement is defined for the fresh pid BEFORE the filesystem
/// runs, so its first read is already bounded; the volume manager is the
/// confinement controller (it defines the first range on the device).
fn spawnFilesystem(v: *Volume) void {
    if (fs_failed) return;
    const pid = process.spawnSupervised(filesystem_binary, &.{"1"}, service_endpoint) orelse {
        _ = logging.write("volume-manager: could not spawn the filesystem; retrying\n");
        armRestart();
        return;
    };
    if (!v.storage.defineRange(pid, v.base_lba, v.block_count)) {
        _ = logging.write("volume-manager: could not confine the filesystem to its volume; retrying\n");
        _ = process.kill(pid);
        armRestart();
        return;
    }
    v.filesystem_pid = pid;
    fs_spawn_ns = time.clock();
    std.log.info("volume 0x{x} -> {s} (pid {d}), lba {d}, {d} blocks", .{ v.identity.key, filesystem_binary, pid, v.base_lba, v.block_count });
}

/// Schedule a fat restart after backoff; the poll loop performs it once due.
fn armRestart() void {
    const delay = if (fs_restarts == 0) backoff_base_ms else backoff_base_ms << @intCast(@min(fs_restarts - 1, 5));
    restart_due_ns = time.clock() + delay * 1_000_000;
    restart_pending = true;
}

/// A storage provider just appeared: open its channel, read block 0, parse the
/// volume, and spawn its filesystem. On any failure the channel is closed (so a
/// present-but-unreadable device does not leak a handle every poll) and `volume`
/// stays null — the next poll retries. A fresh medium gets a fresh supervision
/// budget.
fn bringUpVolume() void {
    if (!bounce_ready) {
        bounce = memory.dmaAlloc(512, memory.dma_coherent | memory.dma_shareable) orelse return;
        bounce_ready = true;
    }
    const opened = openAnyStorage() orelse return;
    const device = opened.device;
    // Attach the read buffer to THIS device (a no-op without an enforcing IOMMU).
    // The handle is kept, not closed, so it can be re-attached to the next
    // device after a replug.
    if (bounce.handle) |handle| {
        if (!device.attach(handle)) {
            _ = ipc.close(device.endpoint);
            return;
        }
    }
    const geometry = device.geometry() orelse {
        _ = ipc.close(device.endpoint);
        return;
    };
    const ProbeReader = struct {
        device: block.Device,
        fn readSector(context: *anyopaque, lba: u64, buffer: *[partition.sector_bytes]u8) bool {
            const self: *@This() = @ptrCast(@alignCast(context));
            if (!self.device.read(lba, 1, bounce.physical)) return false;
            const src: [*]const u8 = @ptrFromInt(bounce.virtual);
            @memcpy(buffer, src[0..partition.sector_bytes]);
            return true;
        }
    };
    var probe = ProbeReader{ .device = device };
    const reader = partition.SectorReader{ .context = &probe, .readFn = ProbeReader.readSector };
    const found = partition.firstVolume(reader, geometry.block_count) orelse {
        if (!logged_no_volume) {
            _ = logging.write("volume-manager: storage present but no recognizable volume\n");
            logged_no_volume = true;
        }
        _ = ipc.close(device.endpoint);
        return;
    };
    logged_no_volume = false;
    fs_restarts = 0;
    fs_failed = false;
    restart_pending = false;
    volume = .{ .storage = device, .storage_device_id = opened.device_id, .base_lba = found.base_lba, .block_count = found.block_count, .identity = found.identity, .id = volume_id };
    spawnFilesystem(&volume.?);
}

/// The storage provider left the device tree (a pulled stick): kill the
/// filesystem so its mounts are retired. Retirement is lazy, not an eager
/// death-time sweep — killing the process marks the filesystem's backend
/// endpoint dead, and the VFS router drops each mount that endpoint backed on
/// the next path resolution under it (that resolve frees the slot and returns
/// not_found). Then drop the now-dead channel and clear the volume; the next
/// poll that sees storage return re-mounts.
fn removeVolume() void {
    const v = volume orelse return;
    std.log.info("storage for volume {d} removed; unmounting", .{v.id});
    if (v.filesystem_pid != 0) _ = process.kill(v.filesystem_pid);
    _ = ipc.close(v.storage.endpoint);
    volume = null;
    restart_pending = false;
    fs_restarts = 0;
    fs_failed = false;
}

/// One poll tick. Removal is checked FIRST and supersedes a pending restart: if
/// the device is gone there is nothing to restart fat onto, and respawning it
/// against the dead channel would just churn until the crash cap. Only once the
/// device is confirmed present does a due restart fire.
fn pollTick() void {
    if (volume) |v| {
        // Serving: watch for the specific device leaving (a pulled stick).
        if (!isDevicePresent(v.storage_device_id)) {
            removeVolume();
            return;
        }
        if (restart_pending and time.clock() >= restart_due_ns) {
            restart_pending = false;
            spawnFilesystem(&volume.?);
        }
    } else {
        // Idle: try to bring a present storage device up.
        bringUpVolume();
    }
}

/// A filesystem announces itself for the volume it was spawned to serve. Reply
/// with that volume's block channel (already range-confined to this filesystem's
/// badge) as the call's returned capability. No channel means the volume is not
/// ready — the filesystem retries.
fn onHello(_: void, invocation: Invocation(volume_manager_protocol.Hello), _: Answer(void)) isize {
    const v = volume orelse return 0; // not probed yet — retryable, no cap
    if (invocation.target != v.id) return 0; // unknown volume — retryable
    if (invocation.sender != v.filesystem_pid) {
        // Not the filesystem we spawned for this volume. Refuse: only the
        // confined filesystem gets the channel.
        std.log.info("refused hello for volume {d} from process {d}", .{ invocation.target, invocation.sender });
        return -envelope.EPERM;
    }
    service.replyWithCapability(v.storage.endpoint);
    std.log.info("handed volume {d} to pid {d}", .{ v.id, invocation.sender });
    return 0;
}

const handlers = Serve.Handlers{ .hello = onHello };

fn onMessage(message: []const u8, out: []u8, sender: u32, arrived: *ipc.Arrival) usize {
    // No verb takes a capability up, so the turn closes whatever arrives.
    return Serve.dispatch({}, handlers, message, sender, arrived.peek(), out);
}

fn initialise(endpoint: ipc.Handle) bool {
    service_endpoint = endpoint;
    _ = logging.write("volume-manager: starting, waiting for a storage device\n");
    _ = process.subscribeExits(endpoint);
    pollTick();
    _ = time.timerOnce(endpoint, poll_interval_ms); // the poll runs for the life of the boot
    return true;
}

fn onNotification(badge: u64) void {
    const got = ipc.Received{ .len = 0, .badge = badge, .cap = null };
    if (got.isTimer()) {
        pollTick();
        _ = time.timerOnce(service_endpoint, poll_interval_ms); // always re-arm: presence is watched continuously
        return;
    }
    // A filesystem died. The exit reason drives the decision, exactly as the
    // device manager supervises drivers: a clean exit meant to stop; a fault
    // restarts with backoff until a fast crash loop gives up. The old range is
    // reclaimed by the driver on the same death; the respawn confines afresh.
    if (got.isChildExit()) {
        const dead = got.childProcessId();
        const v = &(volume orelse return);
        if (v.filesystem_pid != dead) return;
        v.filesystem_pid = 0;
        const reason = process.exitReason(dead) orelse .fault;
        if (reason == .exited) {
            std.log.info("filesystem for volume {d} exited cleanly; not restarting", .{v.id});
            return;
        }
        const alive = time.clock() -| fs_spawn_ns;
        fs_restarts = if (alive < fast_death_ns) fs_restarts + 1 else 1;
        if (fs_restarts >= crash_loop_cap) {
            fs_failed = true;
            std.log.info("filesystem for volume {d} is failing repeatedly; giving up", .{v.id});
            return;
        }
        std.log.info("filesystem for volume {d} died ({s}); restarting", .{ v.id, @tagName(reason) });
        armRestart();
    }
}

pub fn main(init: process.Init) void {
    _ = init;
    service.run(volume_manager_protocol.message_maximum, .{
        .service = "volume-manager",
        .init = initialise,
        .on_message = onMessage,
        .on_notification = onNotification,
    });
}