//! 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.
//!
//! The manager holds a table of adopted storage DEVICES and a table of the
//! VOLUMES on them: it adopts every storage device the device-manager tree
//! carries, probes each one's whole partition table, and spawns one filesystem
//! process per volume — each confined to its partition's badge-scoped block
//! range, each supervised with its own budget. A device leaving the tree takes
//! its volumes with it.

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 fs = @import("file-system");
const partition = @import("partition.zig");
const filesystem_map = @import("filesystem-map.zig");
const volume_map = @import("volume-map.zig");

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

/// One adopted storage device: the block channel to its provider (opened once and
/// shared — refcounted per confined filesystem via the hello reply) and the
/// device-manager id it serves. A device leaving the tree takes its volumes.
const StorageDevice = struct {
    used: bool = false,
    device_id: u64 = 0,
    channel: block.Device = undefined,
};

/// One volume: which device serves it, its block sub-range, its content
/// identity, the id it is addressed by, the service binary + mount path it was
/// spawned with, the filesystem process serving it, and its own supervision
/// budget (so one volume's crash loop never touches another's).
const Volume = struct {
    used: bool = false,
    device_id: u64 = 0,
    base_lba: u64 = 0,
    block_count: u64 = 0,
    identity: partition.Identity = .{ .rung = .anonymous },
    id: u64 = 0,
    binary: []const u8 = "",
    mount_prefix: []const u8 = "",
    filesystem_pid: u32 = 0,
    // Per-volume supervision, mirroring the device manager's: a clean exit is not
    // restarted, a fault restarts with backoff, a fast crash loop gives up.
    restarts: u32 = 0,
    spawn_ns: u64 = 0,
    failed: bool = false,
    restart_pending: bool = false,
    restart_due_ns: u64 = 0,
};

// The mount map, read from configuration at boot (the policy home, storage-
// architecture.md): filesystems.csv (content signature -> service binary) and
// volumes.csv (an optional id -> mount-prefix override). The sources are held
// for the process life so the parsed rules' slices into them stay valid.
/// bound: bytes of filesystems.csv / volumes.csv the manager reads
/// decided-by: ours
/// protects: the config source buffers below
/// at-limit: truncate - a longer file is cut; a row split by the cut is malformed
/// observed-by: the per-file "malformed/truncated" log line
const config_source_bytes = 2048;
var filesystems_source: [config_source_bytes]u8 = undefined;
var volumes_source: [config_source_bytes]u8 = undefined;
/// bound: filesystem-map rules held (one per content signature)
/// decided-by: ours
/// protects: the filesystem_rules table
/// at-limit: truncate - extra rows are dropped and the "truncated" note logged
/// observed-by: the "truncated" log line
const maximum_filesystem_rules = 8;
/// bound: volumes.csv override rows held (one per pinned volume id)
/// decided-by: ours
/// protects: the volume_rules table
/// at-limit: truncate - extra rows are dropped and the "truncated" note logged
/// observed-by: the "truncated" log line
const maximum_volume_rules = 64;
var filesystem_rules: [maximum_filesystem_rules]filesystem_map.Rule = undefined;
var filesystem_rule_count: usize = 0;
var volume_rules: [maximum_volume_rules]volume_map.Override = undefined;
var volume_rule_count: usize = 0;
/// bound: bytes of a composed /volumes/<id> mount path
/// decided-by: ours
/// protects: the per-volume mount_prefix buffers below
/// at-limit: truncate - bufPrint fails; the volume mounts at a fallback path (logged)
/// observed-by: the fallback path in the log
const mount_path_maximum = 64;

/// bound: volumes the manager serves at once
/// decided-by: ours
/// protects: the volumes table and its per-volume mount-path buffers
/// at-limit: truncate - a further partition is left unserved and logged (real
///   machines carry a handful of volumes, far under this)
/// observed-by: the "volume table full" log line
const maximum_volumes = 16;
/// bound: storage devices the manager adopts at once
/// decided-by: ours
/// protects: the devices table
/// at-limit: truncate - a further device is left unadopted and logged
/// observed-by: the "device table full" log line
const maximum_devices = 8;
var devices = [_]StorageDevice{.{}} ** maximum_devices;
var volumes = [_]Volume{.{}} ** maximum_volumes;
/// Each volume's composed default mount path lives in its slot's buffer; a
/// volumes.csv override is used in place (a slice into volumes_source, no buffer).
var mount_prefix_bufs: [maximum_volumes][mount_path_maximum]u8 = undefined;
var next_volume_id: u64 = 1; // monotonic — never reused, so a stale id can't address the wrong child

var service_endpoint: ipc.Handle = 0;
var manager_handle: ?ipc.Handle = null;
var bounce: memory.DmaRegion = undefined;
var bounce_ready = false;
/// How often the poll checks device presence and fires due restarts. 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).
const fast_death_ns: u64 = 2_000_000_000;
const crash_loop_cap: u32 = 3;
const backoff_base_ms: u64 = 300;
/// bytes to format a u64 volume id as decimal (20 digits fit)
const id_decimal_bytes = 24;

// --- table lookups -----------------------------------------------------------

fn deviceById(id: u64) ?*StorageDevice {
    for (&devices) |*d| if (d.used and d.device_id == id) return d;
    return null;
}
fn claimDevice() ?*StorageDevice {
    for (&devices) |*d| if (!d.used) return d;
    return null;
}
fn volumeById(id: u64) ?*Volume {
    for (&volumes) |*v| if (v.used and v.id == id) return v;
    return null;
}
fn volumeByPid(pid: u32) ?*Volume {
    for (&volumes) |*v| if (v.used and v.filesystem_pid == pid) return v;
    return null;
}
fn firstUsedVolume() ?*Volume {
    for (&volumes) |*v| if (v.used) return v;
    return null;
}
fn claimVolumeIndex() ?usize {
    for (&volumes, 0..) |*v, i| if (!v.used) return i;
    return null;
}

// --- device-manager plumbing -------------------------------------------------

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 opens and is NOT already
/// adopted, 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. Skips already-adopted devices so a re-poll does
/// not re-open a device it already serves.
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;
            if (deviceById(entry.device_id) != null) continue; // already adopted
            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 a 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;
    }
}

// --- lifecycle ---------------------------------------------------------------

/// Spawn the filesystem for `v`, confine it to the volume's range on its device's
/// channel, 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 (v.failed) return;
    const dev = deviceById(v.device_id) orelse return; // its device left — poll will clean up
    var id_str_buf: [id_decimal_bytes]u8 = undefined;
    const id_str = std.fmt.bufPrint(&id_str_buf, "{d}", .{v.id}) catch "1";
    const pid = process.spawnSupervised(v.binary, &.{ id_str, v.mount_prefix }, service_endpoint) orelse {
        _ = logging.write("volume-manager: could not spawn the filesystem; retrying\n");
        armRestart(v);
        return;
    };
    if (!dev.channel.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(v);
        return;
    }
    v.filesystem_pid = pid;
    v.spawn_ns = time.clock();
    std.log.info("volume 0x{x} -> {s} (pid {d}), lba {d}, {d} blocks", .{ v.identity.key, v.binary, pid, v.base_lba, v.block_count });
}

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

/// Compose a volume's mount path (its id-path `/volumes/<id>`, or a volumes.csv
/// override) into its slot's buffer, and return the slice.
fn composeMountPrefix(slot: usize, identity: partition.Identity) []const u8 {
    var id_buf: [volume_map.id_maximum]u8 = undefined;
    const id = volume_map.idString(identity, &id_buf);
    return volume_map.overrideFor(volume_rules[0..volume_rule_count], id) orelse
        (std.fmt.bufPrint(&mount_prefix_bufs[slot], "/volumes/{s}", .{id}) catch "/volumes/unknown");
}

/// Adopt the next present, not-yet-adopted storage device: take its channel,
/// probe its whole partition table, and spawn a filesystem per volume it carries.
/// Returns true when it consumed a device (so the caller can loop to adopt every
/// present device in one tick), false when none remain or the device table is full.
///
/// A device is adopted exactly once and kept until it leaves the tree — even when
/// it carries no volume we can serve, or its geometry cannot be read. Keeping the
/// empty/unreadable device adopted (rather than dropping and re-probing) is what
/// lets openAnyStorage advance PAST it to the devices behind it; dropping it would
/// make openAnyStorage hand back the same unservable device every tick and starve
/// the rest. A genuine removal frees the slot (removeDevice); a re-insert gets a
/// fresh device id and is probed anew.
fn bringUpVolume() bool {
    if (!bounce_ready) {
        bounce = memory.dmaAlloc(512, memory.dma_coherent | memory.dma_shareable) orelse return false;
        bounce_ready = true;
    }
    const opened = openAnyStorage() orelse return false;
    const dev = claimDevice() orelse {
        _ = logging.write("volume-manager: device table full; a storage device is left unadopted\n");
        _ = ipc.close(opened.device.endpoint);
        return false;
    };
    dev.* = .{ .used = true, .device_id = opened.device_id, .channel = opened.device };
    // Consume this device's medium_changed events (the second of the removal
    // lifecycle's two triggers: the device stays in the tree while its medium
    // leaves — a card reader, an eject). Best effort: a provider that never
    // publishes the event simply never wakes us, and device-pull is still caught
    // by the presence poll.
    _ = opened.device.subscribeMedium(service_endpoint);
    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 after a replug. A
    // failed attach or geometry read leaves the device adopted but empty — we just
    // cannot read it, and the slot still watches it for removal.
    if (bounce.handle) |handle| {
        if (!device.attach(handle)) {
            _ = logging.write("volume-manager: could not attach the read buffer to a storage device; no volume served\n");
            return true;
        }
    }
    const geometry = device.geometry() orelse {
        _ = logging.write("volume-manager: could not read a storage device's geometry; no volume served\n");
        return true;
    };
    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 };
    var found: [maximum_volumes]partition.Volume = undefined;
    const n = partition.allVolumes(reader, geometry.block_count, found[0..]);
    if (n == 0) {
        std.log.info("device {d} present but carries no recognizable volume", .{dev.device_id});
        return true;
    }
    for (found[0..n]) |fv| {
        // Pick the service binary from the volume's content signature. A signature
        // no filesystems.csv row serves goes unserved (logged), like an unbound
        // device — the manager does not guess.
        const binary = filesystem_map.match(filesystem_rules[0..filesystem_rule_count], fv.signature) orelse {
            _ = logging.write("volume-manager: no filesystem serves this volume's content; unserved\n");
            continue;
        };
        const slot = claimVolumeIndex() orelse {
            _ = logging.write("volume-manager: volume table full; a volume is left unserved\n");
            break;
        };
        volumes[slot] = .{
            .used = true,
            .device_id = dev.device_id,
            .base_lba = fv.base_lba,
            .block_count = fv.block_count,
            .identity = fv.identity,
            .id = next_volume_id,
            .binary = binary,
            .mount_prefix = composeMountPrefix(slot, fv.identity),
        };
        next_volume_id += 1;
        spawnFilesystem(&volumes[slot]);
    }
    return true;
}

/// Close a device's channel and free its slot. No volumes are touched (the caller
/// ensures none remain, or there never were any).
fn dropDevice(dev: *StorageDevice) void {
    // Free the driver's subscriber slot before the channel closes. On a still-live
    // channel (a medium eject) this frees the slot; on a dead one (a device pull)
    // the call fails fast and the exit sweep frees it anyway.
    _ = dev.channel.unsubscribeMedium();
    _ = ipc.close(dev.channel.endpoint);
    dev.* = .{};
}

/// Retire one volume: kill its 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 free the volume slot.
fn removeVolumeState(v: *Volume) void {
    std.log.info("storage for volume {d} removed; unmounting", .{v.id});
    if (v.filesystem_pid != 0) _ = process.kill(v.filesystem_pid);
    v.* = .{};
}

/// A storage device left the tree (a pulled stick): retire every volume it served
/// and drop its channel. One removal path, whether the device is pulled cleanly
/// or vanishes.
fn removeDevice(dev: *StorageDevice) void {
    for (&volumes) |*v| {
        if (v.used and v.device_id == dev.device_id) removeVolumeState(v);
    }
    dropDevice(dev);
}

/// One poll tick. Device removal is reconciled FIRST and supersedes a pending
/// restart: a volume whose device left is retired before its restart could fire,
/// so nothing respawns against a dead channel. Then due restarts fire for present
/// volumes; then, if no device is adopted, a present device is brought up.
fn pollTick() void {
    for (&devices) |*dev| {
        if (dev.used and !isDevicePresent(dev.device_id)) removeDevice(dev);
    }
    for (&volumes) |*v| {
        if (v.used and v.restart_pending and time.clock() >= v.restart_due_ns) {
            v.restart_pending = false;
            spawnFilesystem(v);
        }
    }
    // Adopt every present, not-yet-adopted storage device. Each call consumes at
    // most one device (openAnyStorage skips the adopted), so the loop terminates
    // once none remain; the maximum_devices guard is insurance against a logic
    // slip, never the normal exit.
    var adopted: usize = 0;
    while (adopted < maximum_devices and bringUpVolume()) : (adopted += 1) {}
}

/// 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 = volumeById(invocation.target) orelse return 0; // not probed yet — retryable, no cap
    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;
    }
    const dev = deviceById(v.device_id) orelse return 0; // its device left — retryable
    service.replyWithCapability(dev.channel.endpoint);
    std.log.info("handed volume {d} to pid {d}", .{ v.id, invocation.sender });
    return 0;
}

/// Answer a `volumes` query with a mounted volume's descriptor — its id (its
/// mount path is /volumes/<id> unless overridden), its actual mount path, and its
/// display label. Software keys on the id; a UI shows the label. Returns the first
/// mounted volume for now; a full enumerate is a later refinement. Empty reply
/// means no volume is mounted.
fn onVolumes(_: void, _: Invocation(volume_manager_protocol.Volumes), answer: Answer(void)) isize {
    const v = firstUsedVolume() orelse return 0;
    var id_buf: [volume_map.id_maximum]u8 = undefined;
    const info = volume_manager_protocol.VolumeInfo{
        .id = volume_map.idString(v.identity, &id_buf),
        .mount_path = v.mount_prefix,
        .label = v.identity.labelSlice(),
    };
    const encoded = info.encode(answer.tail()) orelse return 0;
    return @intCast(encoded.len);
}

const handlers = Serve.Handlers{ .hello = onHello, .volumes = onVolumes };

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);
}

/// Read a config file into `buf`, returning the byte count (0 if missing).
fn readConfig(path: []const u8, buf: []u8) usize {
    var file = fs.open(path, .{}) orelse {
        std.log.info("volume-manager: {s} missing", .{path});
        return 0;
    };
    defer file.close();
    var used: usize = 0;
    while (used < buf.len) {
        const nn = file.read(buf[used..]) orelse break;
        if (nn == 0) break;
        used += nn;
    }
    return used;
}

/// Load the mount map from configuration once at boot (mirrors the device
/// manager's registry load). A missing or empty filesystems.csv means no volume
/// is served; volumes.csv is optional — no rows means every volume takes its
/// default /volumes/<id> path.
fn loadTables() void {
    const fs_used = readConfig("/system/configuration/filesystems.csv", &filesystems_source);
    const fr = filesystem_map.parse(filesystems_source[0..fs_used], &filesystem_rules);
    filesystem_rule_count = fr.count;
    if (fr.malformed != 0 or fr.truncated) std.log.info("filesystems.csv: {d} malformed, truncated={}", .{ fr.malformed, fr.truncated });

    const vol_used = readConfig("/system/configuration/volumes.csv", &volumes_source);
    const vr = volume_map.parse(volumes_source[0..vol_used], &volume_rules);
    volume_rule_count = vr.count;
    if (vr.malformed != 0 or vr.truncated) std.log.info("volumes.csv: {d} malformed, truncated={}", .{ vr.malformed, vr.truncated });
}

fn initialise(endpoint: ipc.Handle) bool {
    service_endpoint = endpoint;
    _ = logging.write("volume-manager: starting, waiting for a storage device\n");
    loadTables();
    _ = 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 = volumeByPid(dead) orelse 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() -| v.spawn_ns;
        v.restarts = if (alive < fast_death_ns) v.restarts + 1 else 1;
        if (v.restarts >= crash_loop_cap) {
            v.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(v);
    }
}

var last_medium_change: u32 = 0;

fn anyVolumeOn(device_id: u64) bool {
    for (&volumes) |*v| if (v.used and v.device_id == device_id) return true;
    return false;
}

/// A storage device published `medium_changed` — the second removal trigger: the
/// device stays in the tree while its medium leaves or returns (a card reader, an
/// eject). This arrives as a buffered async message, NOT a protocol request, so it
/// never reaches `Serve.dispatch` (its event op number collides with the manager's
/// own `hello`); it is decoded here by hand. Single-volume scope: the event names
/// no device, so `absent` retires every adopted device (its volumes unmount and
/// the poll re-adopts the still-present device with its now-empty medium), and
/// `present` frees any empty adopted device so the poll re-probes and remounts it.
fn onMediumEvent(payload: []const u8) void {
    const event = block.decodeMediumChanged(payload) orelse return;
    if (event.change_count == last_medium_change) return; // a coalesced or re-delivered edge
    last_medium_change = event.change_count;
    if (event.present == 0) {
        std.log.info("medium left a storage device; unmounting its volume(s)", .{});
        for (&devices) |*dev| {
            if (dev.used) removeDevice(dev);
        }
    } else {
        for (&devices) |*dev| {
            if (dev.used and !anyVolumeOn(dev.device_id)) removeDevice(dev);
        }
    }
}

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,
        .on_buffered_message = onMediumEvent,
    });
}