rstango 0.4.0

//! A device: its interface, its model, and every request a client makes of it.

use std::collections::{HashMap, HashSet, VecDeque};
use std::sync::atomic::{AtomicBool, Ordering};
use std::time::{Duration, Instant, SystemTime};
use std::sync::{Arc, Mutex, Weak};
use std::fmt::{self, Debug};
use std::future::Future;
use std::borrow::Cow;
use std::any::Any;

use futures::future::{join, ready, BoxFuture};
use tracing::{debug, error, info, warn};
use futures::stream::BoxStream;

use crate::dev::{All, AttrConfig, AttrInfo, AttrMemorized, AttrQuality, AttrRead, AttrReady, AttrValueEvent, CommandInfo, DevInfo, Named, NamedResult, RecordedResult, ToNames, ToNamesNotAll, 
    DevSource, DevState};
use crate::value::{IntoValue, Scalar, Spectrum, Value};
use crate::error::{reason, Error, ErrorKind, Result};
use crate::database::{GetPropSpec, SetPropSpec};
use crate::local::{self, LocalDevHandle};
use crate::name::{self, Name, ArcName};
use crate::backend::BackendDev;
use crate::pool;

use super::def::{AttrDef, CommandDef, DeleteFn, InitFn, StateFn, StatusFn, WriteFn};
use super::{reserved_name, Model, DEFAULT_HISTORY_DEPTH, DEFAULT_STATE, DEFAULT_STATUS, INIT_NAME, 
    MISSING_STATUS, RESERVED_ATTRS, RESERVED_COMMANDS, STATE_NAME, STATUS_NAME};
use super::polling::{self, PollItem, PolledHistory, PolledKind, Polled, DEFAULT_VALIDITY_PERIODS};
use super::class::{AnyClass, Class};
use super::attr::{Attr, AttrAlarm, AttrRole};
use super::command::Command;
use super::server::Server;


/// The model-erased view of a [`Dev`], which is how a server refers to the devices it
/// serves, whatever the model of the class they belong to.
pub(super) trait AnyDev: Debug + Send + Sync {

    /// The name of that device, shared so that it is cheaply copied out.
    fn name(&self) -> &Arc<str>;

    /// The class of that device.
    fn class(&self) -> &dyn AnyClass;

    /// Initialize that device: its model, if it has one, is deleted, and a new one is built
    /// by the init function of its class, on the pool of the server when that may block.
    /// This only fails when the initialization never ran to its end, its function having
    /// panicked: one that failed is kept by the device instead.
    fn init(&self) -> BoxFuture<'_, Result<()>>;

    /// Stop serving that device, for good, and delete its model, if it has one, giving it
    /// back to the delete function of its class so that it may release what it owns: what
    /// a device goes through once it is removed from its server, or when the server shuts
    /// down. Not to be confused with the deletion of the model alone, which is also what an
    /// initialization starts with, see [`Dev::init`].
    /// 
    /// The backend is told first, so that no new request reaches a device that is being
    /// deleted, see [`crate::backend::BackendServer::remove`]. A request it was answering
    /// may still be running, and the model is only deleted once it is through with it,
    /// which is why a request of that device must never wait for this.
    fn withdraw(&self) -> BoxFuture<'_, Result<()>>;

    /// Mark that device as withdrawn, see [`Dev::is_withdrawn`], and take it out of the
    /// local registry, so that no client of this process finds it anymore: the first step
    /// of [`Self::withdraw`], and all that becomes of the devices of a server that ended
    /// without being stopped.
    fn withdraw_now(&self);

    /// Start serving that device through the backend of its server, giving back the ITR it
    /// is reached at. This is quick and never blocks, see
    /// [`crate::backend::BackendServer::add`].
    fn serve(&self) -> Result<Arc<str>>;

    /// That device as [`Any`], for it to be downcast to the [`Dev`] it is.
    fn into_any(self: Arc<Self>) -> Arc<dyn Any + Send + Sync>;

    /// Poll those objects of that device once, which are due together, see
    /// [`super::polling`]: read the attributes, or execute the commands, as a client asking
    /// the device does, on the pool of the server when that may block, and record what
    /// each gave in its history, valid for as long as its item says. A polled attribute is
    /// also sent as a periodic event once its period has passed since the last one was
    /// sent, see [`Dev::poll_attrs`], and when the last one was sent is given back for
    /// each item, never for a command.
    ///
    /// An object the device does not have anymore is left out. This only fails when the
    /// pool dropped the poll before it ran to its end, see [`pool::run`]: a read or an
    /// execution that failed is recorded as such.
    fn poll(&self, kind: PolledKind, items: &[PollItem]) -> BoxFuture<'static, Result<Vec<Option<Instant>>>>;

    /// What the history of that object of that device holds, for its polling status, see
    /// [`PolledHistory`], absent if the device has no such object.
    fn polled_history(&self, kind: PolledKind, name: &str) -> Option<PolledHistory>;

    /// The name of that object of that device, as its definition gives it, failing if the
    /// device has no such object or if it cannot be polled, see [`Attr::is_pollable`] and
    /// [`Command::is_pollable`].
    fn check_pollable(&self, kind: PolledKind, name: &str) -> Result<Arc<str>>;

}

/// The handle to a device from within its own execution functions, giving access to the
/// device outside of its model.
/// 
/// A device can be served while having no model: until it is initialized, or when its
/// initialization failed. It is then in UNKNOWN or FAULT state, with a status describing
/// why it has no model. The idea behind this is that the Init command, which deletes then
/// initializes a device, might fail, however we don't want to stop serving the device
/// for that. This also allows a server to start if any of its devices fails to init, the
/// device will be available but it will not be usable in any way, it will just be possible
/// to call Init and try to fix it, individually from the other devices that might be
/// running.
pub struct Dev<M: Model> {
    /// Self reference, for what a device hands the backend of itself.
    this: Weak<Self>,
    /// The name of the device.
    name: Arc<str>,
    /// The class this device is an instance of, through which its definition is reached.
    class: Arc<Class<M>>,
    /// The interface of this device, see [`DevMutable`]. It is locked very shortly so 
    /// there is no need for 
    mutable: Mutex<DevMutable<M>>,
    /// The internal model of the device, absent until the device is initialized.
    /// 
    /// An asynchronous lock, held across the awaits of the asynchronous handlers.
    model: async_lock::RwLock<Option<Result<M>>>,
    /// The device recorded as served by this process while it is served, which is how a
    /// client of this process reaches it directly, see [`crate::local`].
    local_handle: Mutex<Option<LocalDevHandle>>,
    /// Set once this device is withdrawn from its server, see [`Self::is_withdrawn`].
    withdrawn: AtomicBool,
}

impl<M: Model> Dev<M> {

    pub(super) fn new(class: Arc<Class<M>>, name: Arc<str>) -> Arc<Self> {
        let state_attr = Arc::new(Attr::new_with_role(Arc::clone(&class.def().state_attr), AttrRole::State));
        let status_attr = Arc::new(Attr::new_with_role(Arc::clone(&class.def().status_attr), AttrRole::Status));
        Arc::new_cyclic(|this| Self {
            this: this.clone(),
            name,
            class,
            mutable: Mutex::new(DevMutable {
                black_box: VecDeque::new(),
                commands: HashMap::new(),
                attrs: HashMap::new(),
                state_attr,
                status_attr,
                auto_alarm_states: Arc::from([DevState::On]),
            }),
            model: async_lock::RwLock::new(None),
            local_handle: Mutex::new(None),
            withdrawn: AtomicBool::new(false),
        })
    }

    /// The name of this device.
    #[inline]
    pub fn name(&self) -> &str {
        &self.name
    }

    /// The class this device is an instance of.
    #[inline]
    pub fn class(&self) -> &Class<M> {
        &self.class
    }

    /// The server serving this device.
    #[inline]
    pub fn server(&self) -> &Server {
        self.class.server()
    }

    /// Whether this device is gone for good: removed from its server and deleted, or its
    /// server ended. Nothing of it is served from then on, and pushing events or setting
    /// anything of its attributes fails, which is how a thread of its own learns to stop. A
    /// device added again under the same name, which is what a restart is, is another
    /// device, which this one never becomes.
    #[inline]
    pub fn is_withdrawn(&self) -> bool {
        self.withdrawn.load(Ordering::Acquire)
    }

    /// Fail once this device is withdrawn, see [`Self::is_withdrawn`].
    fn check_withdrawn(&self) -> Result<()> {
        if self.is_withdrawn() {
            return Err(Error::new(ErrorKind::DevNotConnected)
                .with_description(format!("Device {:?} is no longer served", self.name)));
        }
        Ok(())
    }

    // The properties of a device are the database's: everything below asks the database 
    // of the server for what it holds of this device, see the very same methods on
    // [`crate::database::DatabaseProxy`]. The blocking forms are for synchronous handlers, 
    // which run on the pool, and must never run in an asynchronous one.

    /// The names of the properties of this device that match the given filter.
    pub async fn list_prop_async(&self, filter: &str) -> Result<Spectrum<str>> {
        self.server().database().list_dev_prop_async(&self.name, filter).await
    }

    /// The names of the properties of this device, see [`Self::list_prop_async`].
    pub fn list_prop(&self, filter: &str) -> Result<Spectrum<str>> {
        self.server().database().list_dev_prop(&self.name, filter)
    }

    /// The properties of this device, parsed into the shape the given names ask for, see
    /// [`crate::database::DatabaseProxy::get_dev_prop`].
    pub fn get_prop_async<Spec, Names: ToNames>(&self, names: Names) -> impl Future<Output = Result<Spec::Output>> + Send
    where
        Spec: GetPropSpec<Names>,
    {
        self.server().database().get_dev_prop_async::<Spec, Names>(&self.name, names)
    }

    /// The properties of this device, see [`Self::get_prop_async`].
    pub fn get_prop<Spec, Names: ToNames>(&self, names: Names) -> Result<Spec::Output>
    where
        Spec: GetPropSpec<Names>,
    {
        self.server().database().get_dev_prop::<Spec, Names>(&self.name, names)
    }

    /// Write the given properties of this device.
    pub fn set_prop_async<'a, Spec>(&'a self, spec: Spec) -> impl Future<Output = Result<()>> + Send
    where
        Spec: SetPropSpec<'a>,
    {
        self.server().database().set_dev_prop_async(&self.name, spec)
    }

    /// Write the given properties of this device, see [`Self::set_prop_async`].
    pub fn set_prop<'a, Spec>(&'a self, spec: Spec) -> Result<()>
    where
        Spec: SetPropSpec<'a>,
    {
        self.server().database().set_dev_prop(&self.name, spec)
    }

    /// Delete the given properties of this device.
    pub fn delete_prop_async<Names: ToNamesNotAll>(&self, names: Names) -> impl Future<Output = Result<()>> + Send {
        self.server().database().delete_dev_prop_async(&self.name, names)
    }

    /// Delete the given properties of this device, see [`Self::delete_prop_async`].
    pub fn delete_prop<Names: ToNamesNotAll>(&self, names: Names) -> Result<()> {
        self.server().database().delete_dev_prop(&self.name, names)
    }

    /// This device, shared, which is how a handler, only given a reference to it, hands it
    /// to a thread of its own. Holding it does not keep it served, see
    /// [`Self::is_withdrawn`].
    #[inline]
    pub fn arc(&self) -> Arc<Self> {
        // A device is only ever reached through a reference to it, so it is alive.
        self.this.upgrade().unwrap()
    }

    /// Add a command to that device only, from its definition, which is what a device
    /// whose interface depends on its properties does at initialization; a command every
    /// device of the class has is defined on the class instead, see [`ClassDef::command`].
    /// 
    /// A name the class already defines is refused rather than shadowed: the device was
    /// given every command of its class before its initialization ran.
    /// 
    /// [`ClassDef::command`]: super::def::ClassDef::command
    pub fn add_command(&self, def: CommandDef<M>) -> Result<()> {

        // Refused rather than panicked on, unlike the same name given to a class, see
        // [`ClassDef::command`]: what a device adds to itself it usually reads from its
        // properties, so a reserved name here is a configuration to report, not a
        // definition to fix.
        if !name::check_tango_command_name(&def.name) {
            return Err(Error::new(ErrorKind::CommandNameInvalid)
                .with_description(format!("Command {:?} of device {:?} has an invalid name, it should \
                    be made of letters, digits, '_' and '-', at most 255 long", def.name, self.name)));
        }

        if let Some(reserved) = reserved_name(&def.name, RESERVED_COMMANDS) {
            return Err(Error::new(ErrorKind::CommandNameInvalid)
                .with_description(format!("Command {:?} cannot be defined on device {:?}, \
                    {reserved:?} is answered by the server itself", def.name, self.name)));
        }

        let mut mutable = self.mutable.lock().unwrap();
        if mutable.commands.contains_key(Name::new(&def.name)) {
            return Err(Error::new(ErrorKind::CommandAlreadyDefined)
                .with_description(format!("Command {:?} is already defined on device {:?}", def.name, self.name)));
        }

        let name = ArcName::from(&*def.name);
        mutable.commands.insert(name, Arc::new(Command::new(Arc::new(def))));
        Ok(())

    }

    /// Remove a command from that device, telling whether it was defined.
    pub fn remove_command(&self, name: &str) -> bool {
        self.mutable.lock().unwrap().commands.remove(Name::new(name)).is_some()
    }

    /// Add an attribute to that device only, from its definition, see
    /// [`Self::add_command`] and [`ClassDef::attr`].
    /// 
    /// An attribute added by the initialization of the device is configured from the
    /// database once the initialization returns. One added at any other time keeps the
    /// configuration of its definition.
    /// 
    /// [`ClassDef::attr`]: super::def::ClassDef::attr
    pub fn add_attr(&self, def: AttrDef<M>) -> Result<()> {

        if !name::check_tango_attr_name(&def.name) {
            return Err(Error::new(ErrorKind::AttrNameInvalid)
                .with_description(format!("Attribute {:?} of device {:?} has an invalid name, it should \
                    be made of letters, digits, '_' and '-', at most 255 long", 
                    def.name, self.name)));
        }

        if let Some(reserved) = reserved_name(&def.name, RESERVED_ATTRS) {
            return Err(Error::new(ErrorKind::AttrNameInvalid)
                .with_description(format!("Attribute {:?} cannot be defined on device {:?}, \
                    {reserved:?} is answered by the server itself", 
                    def.name, self.name)));
        }

        if let Err(err) = def.check() {
            return Err(Error::new(ErrorKind::Reason(reason::API_AttrWrongDefined))
                .with_description(format!("Attribute {:?} cannot be defined on device {:?}, {err}",
                    def.name, self.name)));
        }

        let mut mutable = self.mutable.lock().unwrap();
        if mutable.attrs.contains_key(Name::new(&def.name)) {
            return Err(Error::new(ErrorKind::AttrAlreadyDefined)
                .with_description(format!("Attribute {:?} is already defined on device {:?}", 
                    def.name, self.name)));
        }

        let name = ArcName::from(&*def.name);
        mutable.attrs.insert(name, Arc::new(Attr::new(Arc::new(def))));
        Ok(())

    }

    /// Give the attribute of that name a new set point without writing it, which every read
    /// gives back as its write value from then on, and which the database remembers if the
    /// attribute is memorized, see [`AttrMemorized`].
    /// 
    /// This is for a device whose set point changes by itself, a motor stopped short of its
    /// target for instance, so that a client and the next initialization find the value
    /// the device actually works towards. An attribute that cannot be written, the State
    /// and the Status included, is refused.
    pub async fn set_attr_w_value_async(&self, name: &str, value: impl IntoValue) -> Result<()> {
        self.check_withdrawn()?;
        let attr = self.find_attr(name)?;
        attr.set_w_value(self, value.into_value()).await
    }

    /// The blocking form of [`Self::set_attr_w_value_async`], which must never run in an
    /// asynchronous handler; it gives up after the timeout of the database of the server.
    pub fn set_attr_w_value(&self, name: &str, value: impl IntoValue) -> Result<()> {
        self.server().database().block_on(self.set_attr_w_value_async(name, value))?
    }

    /// Change the configuration of the attribute of that name, given as the whole of it,
    /// which is what a client changing it does: refused if it is incoherent, stored in the
    /// database, and pushed to the subscribers of the configuration of that attribute. Its
    /// current configuration is the one [`Attr::config`] gives.
    pub async fn set_attr_config_async(&self, name: &str, config: &AttrConfig) -> Result<()> {
        self.check_withdrawn()?;
        self.change_attr_config(name, config).await
    }

    /// The blocking form of [`Self::set_attr_config_async`], which must never run in an
    /// asynchronous handler; it gives up after the timeout of the database of the server.
    pub fn set_attr_config(&self, name: &str, config: &AttrConfig) -> Result<()> {
        self.server().database().block_on(self.set_attr_config_async(name, config))?
    }

    /// Set the states in which this device is put in ALARM when one of its attributes is in
    /// alarm, which is only [`DevState::On`] unless set otherwise, an empty list leaving
    /// the state of the device to its state function alone.
    /// 
    /// When the state function gives one of those states, the attributes that have any
    /// alarm configured are read, and a single one of them in alarm or in warning makes
    /// the device ALARM, its status telling which and why, the way cppTango does. Getting
    /// the state of the device then borrows its model the way reading those attributes
    /// does.
    pub fn set_auto_alarm_states(&self, states: &[DevState]) {
        self.mutable.lock().unwrap().auto_alarm_states = Arc::from(states);
    }

    /// Remove an attribute from that device, telling whether it was defined.
    pub fn remove_attr(&self, name: &str) -> bool {
        self.mutable.lock().unwrap().attrs.remove(Name::new(name)).is_some()
    }

    /// The command of that name on this device, absent when it has none of that name: the
    /// State, the Status and the Init every device has, whatever its class defines, or else
    /// one of its interface.
    /// 
    /// Every request on a command goes through this, and it is shared rather than borrowed
    /// from the interface store: the interface store is unlocked as soon as the command is
    /// out of it, because a handler is free to change the interface of its own device, and
    /// that store is the lock taken *under* the model, never around it.
    pub fn get_command(&self, name: &str) -> Option<Arc<Command<M>>> {
        if name.eq_ignore_ascii_case(STATE_NAME) {
            Some(Arc::new(Command::new_with_attr(self.state_attr())))
        } else if name.eq_ignore_ascii_case(STATUS_NAME) {
            Some(Arc::new(Command::new_with_attr(self.status_attr())))
        } else if name.eq_ignore_ascii_case(INIT_NAME) {
            Some(Arc::new(Command::new_with_init()))
        } else {
            self.mutable.lock().unwrap().commands.get(Name::new(name)).map(Arc::clone)
        }
    }

    /// The attribute of that name on this device, the State and the Status included,
    /// reached the way [`Self::get_command`] reaches a command.
    pub fn get_attr(&self, name: &str) -> Option<Arc<Attr<M>>> {
        let mutable = self.mutable.lock().unwrap();
        if name.eq_ignore_ascii_case(STATE_NAME) {
            Some(Arc::clone(&mutable.state_attr))
        } else if name.eq_ignore_ascii_case(STATUS_NAME) {
            Some(Arc::clone(&mutable.status_attr))
        } else {
            mutable.attrs.get(Name::new(name)).map(Arc::clone)
        }
    }

    /// Every command of this device: the three every device has first, then its whole
    /// interface.
    pub fn commands(&self) -> Vec<Arc<Command<M>>> {
        let mutable = self.mutable.lock().unwrap();
        [
            Command::new_with_attr(Arc::clone(&mutable.state_attr)), 
            Command::new_with_attr(Arc::clone(&mutable.status_attr)), 
            Command::new_with_init(),
        ]
            .into_iter()
            .map(Arc::new)
            .chain(mutable.commands.values().map(Arc::clone))
            .collect()
    }

    /// Every attribute of this device: the two every device has first, then its whole
    /// interface.
    /// 
    /// TODO: The order of the interface is the one of the interface store, which is a hash
    /// map and therefore arbitrary; a client reading a whole interface expects a stable one.
    pub fn attrs(&self) -> Vec<Arc<Attr<M>>> {
        let mutable = self.mutable.lock().unwrap();
        [&mutable.state_attr, &mutable.status_attr]
            .into_iter()
            .chain(mutable.attrs.values())
            .map(Arc::clone)
            .collect()
    }

    // The events of the attributes of this device are pushed to the subscriptions its
    // server keeps of them, see [`Server::events`]. A push never blocks and needs no
    // executor, so that it is made from anywhere, a thread of the device's own included.
    // Nothing is sent of an event no client subscribed to.

    /// Push an event of that kind for the attribute of that name, the state and the status
    /// included: a reading, or the error reading it, failing if the device has no such
    /// attribute, or if that attribute does not declare it pushes such events, see
    /// [`AttrDef::push_change_event`]. See the methods below for what a device pushes most.
    ///
    /// A reading must have a read value, so an attribute that cannot be read pushes none,
    /// and be of a valid quality for the state and the status. A reading with no write value
    /// is given the set point of its attribute, and one with a write value makes it the set
    /// point, the way a read does, so that an event and a read of a writable attribute
    /// carry the same, as with cppTango.
    pub fn push_attr_event(&self, name: &str, event: AttrValueEvent, mut read: Result<AttrRead<Value>>) -> Result<()> {
        self.check_withdrawn()?;
        let attr = self.find_attr(name)?;
        attr.check_push_event(self, event)?;
        if let Ok(read) = &mut read {
            attr.check_pushed(read)?;
            attr.merge_w_value(read);
        }
        self.server().push_attr_value(&self.name, attr.name(), event, read);
        Ok(())
    }

    /// Push a change event for the attribute of that name, a value of a valid quality read
    /// now, see [`Self::push_attr_event`].
    pub fn push_change_event(&self, name: &str, value: impl IntoValue) -> Result<()> {
        self.push_attr_event(name, AttrValueEvent::Change, Ok(read_now(value)))
    }

    /// Push an archive event for the attribute of that name, see [`Self::push_change_event`].
    pub fn push_archive_event(&self, name: &str, value: impl IntoValue) -> Result<()> {
        self.push_attr_event(name, AttrValueEvent::Archive, Ok(read_now(value)))
    }

    /// Push a user event for the attribute of that name, see [`Self::push_change_event`].
    pub fn push_user_event(&self, name: &str, value: impl IntoValue) -> Result<()> {
        self.push_attr_event(name, AttrValueEvent::User, Ok(read_now(value)))
    }

    /// Push an alarm event for the attribute of that name, see [`Self::push_change_event`].
    pub fn push_alarm_event(&self, name: &str, value: impl IntoValue) -> Result<()> {
        self.push_attr_event(name, AttrValueEvent::Alarm, Ok(read_now(value)))
    }

    /// Push that the attribute of that name is ready to be read, with a counter of the
    /// device's own telling which of its readings it is, see [`AttrReady`]. This fails if
    /// the device has no such attribute, or does not declare it announced, see
    /// [`AttrDef::push_data_ready_event`].
    pub fn push_data_ready_event(&self, name: &str, counter: i32) -> Result<()> {
        self.check_withdrawn()?;
        let attr = self.find_attr(name)?;
        attr.check_data_ready(self)?;
        let ready = AttrReady { data_kind: attr.data_kind()?, counter };
        self.server().push_attr_ready(&self.name, attr.name(), Ok(ready));
        Ok(())
    }

    /// Initialize this device: delete its model, if it has one, and build a new one with
    /// the init function of its class. A failure is kept in place of the model, reported as
    /// FAULT and in the status.
    /// 
    /// This runs when a synchronization picks the device up, and on every `Init` command.
    /// 
    /// The model is taken exclusively for the whole initialization, so that a request
    /// arriving in the middle of it waits for it rather than seeing a half built device.
    /// This is also why the init function of a class must not reach the model of the very
    /// device it is building: it is that lock it would be asking for.
    /// 
    /// The attributes are configured from the database around the init function: those of
    /// the class before it, so that it finds them configured, and those it added after it,
    /// see [`Self::load_all_attr_config`].
    pub(super) async fn init(&self) {

        let def = self.class.def();
        let mut model = self.model.write().await;

        // A device withdrawn meanwhile had its model deleted for good, which is never
        // built again: the Init command that asked for it is told so, see
        // [`Self::check_model`].
        if self.is_withdrawn() {
            return;
        }

        // An initialization is a delete-then-init, which is what the Init command of a
        // device is: a device that was running is deleted first, and a device whose
        // previous initialization failed has only that error to drop.
        self.delete_model(&mut model).await;

        // The interface goes with the model it was built for: whatever the previous
        // initialization added to this device alone is gone, and the next one is free to
        // add it again, possibly differently.
        //
        // What the class defines is put back, so that the device starts again from the
        // interface every device of its class has and its initialization only adds to it,
        // see [`Self::add_attr`].
        self.reset_interface();
        self.load_all_attr_config().await;

        let mut res = match &def.init {
            InitFn::Sync(init) => init(self),
            InitFn::Async(init) => init(self).await,
        };

        // We re-load attribute config afterward in case new attributes have been added,
        // in which case only their config will be loaded. The memorized values are then
        // written back, which needs the model.
        if let Ok(new_model) = &mut res {
            self.load_all_attr_config().await;
            for attr in self.attrs() {
                attr.restore(self, new_model).await;
            }
        }

        // Initialize, we just log the result but keep the error for state/status.
        *model = Some(match res {
            Ok(new_model) => {
                info!(dev = %self.name, class = %def.name, "Initialized the device");
                Ok(new_model)
            }
            Err(err) => {
                error!(dev = %self.name, class = %def.name, %err, "Failed to initialize the device");
                Err(err)
            }
        });

        // Whatever the initialization gave, what the device has is polled, a device that
        // failed recording the failure of every poll, as cppTango does.
        self.setup_polling().await;

    }

    /// Give the polling of the server what this device is polled for, and the histories of
    /// its attributes and commands their depth, from what the database holds for it and
    /// what its definitions ask for, see [`super::polling`] for which has the last word.
    ///
    /// A property that cannot be read is taken as absent, and an entry of it that names
    /// an object the device does not have, or one that cannot be polled, is left out, both
    /// with a warning: the device is polled for the rest all the same.
    async fn setup_polling(&self) {

        const PROPS: [&str; 6] = [
            "polled_attr", 
            "polled_cmd", 
            "poll_ring_depth", 
            "attr_poll_ring_depth", 
            "cmd_poll_ring_depth", 
            "poll_old_factor",
        ];

        let Some(poller) = self.server().poller() else {
            return;
        };

        let props = match self.server().database().get_dev_prop_async::<Spectrum<str>, _>(&self.name, PROPS).await {
            Ok(props) => props,
            Err(err) => {
                warn!(dev = %self.name, %err, "Failed to read the polling properties of the device, polling it as defined");
                PROPS.map(|_| Ok(Spectrum::<str>::from(&[] as &[&str])))
            }
        };

        let [
            polled_attr,
            polled_cmd,
            ring_depth,
            attr_depths,
            cmd_depths,
            validity_periods,
        ] = props.into_iter()
            .zip(PROPS)
            .map(|(prop, name)| prop.unwrap_or_else(|err| {
                warn!(dev = %self.name, prop = %name, %err, "Failed to read a polling property of the device, ignoring it");
                Spectrum::<str>::from(&[] as &[&str])
            }))
            .collect::<Vec<_>>()
            .try_into()
            .expect("one value per property");

        // The depths of the histories, of every attribute and command whether polled or
        // not, a device being free to record in them itself.
        let ring_depth = first_number(&self.name, PROPS[2], &ring_depth).unwrap_or(DEFAULT_HISTORY_DEPTH);
        let validity_periods = first_number(&self.name, PROPS[5], &validity_periods)
            .filter(|&factor| factor > 0)
            .unwrap_or(DEFAULT_VALIDITY_PERIODS);

        let attr_depths = polling::parse_depths(&self.name, PROPS[3], attr_depths.iter()).into_iter()
            .map(|(name, depth)| (ArcName::from(name), depth))
            .collect::<HashMap<_, _>>();
        let cmd_depths = polling::parse_depths(&self.name, PROPS[4], cmd_depths.iter()).into_iter()
            .map(|(name, depth)| (ArcName::from(name), depth))
            .collect::<HashMap<_, _>>();

        let (attrs, commands) = (self.attrs(), self.commands());

        for attr in &attrs {
            attr.set_history_depth(attr_depths.get(Name::new(attr.name())).copied().unwrap_or(ring_depth));
        }

        // The State and the Status have the history of their attribute, which their
        // command only gives a depth of its own when the property names it.
        for command in &commands {
            match cmd_depths.get(Name::new(command.name())) {
                Some(&depth) => command.set_history_depth(depth),
                None if !command.involves_state() => command.set_history_depth(ring_depth),
                None => {}
            }
        }

        // The periods, the database first, then the definitions for what it does not list.
        let mut polled = Vec::new();
        for (kind, prop) in [(PolledKind::Attr, &polled_attr), (PolledKind::Command, &polled_cmd)] {

            let prop_name = match kind {
                PolledKind::Attr => PROPS[0],
                PolledKind::Command => PROPS[1],
            };

            let mut listed = HashSet::new();
            for (name, period) in polling::parse_periods(&self.name, prop_name, prop.iter()) {
                match self.check_pollable(kind, name) {
                    Ok(name) => {
                        listed.insert(ArcName::from(Arc::clone(&name)));
                        polled.push(Polled { kind, name, period });
                    }
                    Err(err) => warn!(dev = %self.name, prop = %prop_name, %name, %err, "Ignored a polled object"),
                }
            }

            let defined = match kind {
                PolledKind::Attr => attrs.iter()
                    .filter_map(|attr| attr.poll_period().map(|period| (attr.name(), period)))
                    .collect::<Vec<_>>(),
                PolledKind::Command => commands.iter()
                    .filter_map(|command| command.poll_period().map(|period| (command.name(), period)))
                    .collect::<Vec<_>>(),
            };

            for (name, period) in defined {
                if !listed.contains(Name::new(name)) {
                    polled.push(Polled { kind, name: Arc::from(name), period: period.max(polling::MIN_PERIOD) });
                }
            }

        }

        debug!(dev = %self.name, polled = polled.len(), "Polling the device");
        poller.set_dev(&(self.arc() as Arc<dyn AnyDev>), validity_periods, polled);

    }

    /// Poll those attributes, see [`AnyDev::poll`]: read at once, as a client reading
    /// them together does, see [`BackendDev::read_attr`], each reading recorded in the
    /// history of its attribute and sent as a periodic event, see [`Self::record_poll`].
    fn poll_attrs(self: Arc<Self>, items: &[PollItem]) -> BoxFuture<'static, Result<Vec<Option<Instant>>>> {

        let mut last_periodics = items.iter()
            .map(|item| item.last_periodic)
            .collect::<Vec<_>>();

        // A device withdrawn meanwhile is not polled anymore, see [`Self::is_withdrawn`].
        if self.is_withdrawn() {
            return Box::pin(ready(Ok(last_periodics)));
        }

        // Each attribute along with the index of its item.
        let mut polled = Vec::new();
        for (index, item) in items.iter().enumerate() {
            match self.find_attr(&item.name) {
                Ok(attr) => polled.push((index, attr, item.valid_for)),
                Err(err) => debug!(dev = %self.name, attr = %item.name, %err, "Failed to poll the attribute"),
            }
        }

        if polled.is_empty() {
            return Box::pin(ready(Ok(last_periodics)));
        }

        let targets = polled.iter()
            .map(|(_, attr, _)| Ok(Arc::clone(attr)))
            .collect::<Vec<_>>();

        let ctx = ReadCtx::new(&self, polled.iter().any(|(_, attr, _)| attr.role() != AttrRole::Interface));
        let blocking = polled.iter().any(|(_, attr, _)| attr.read_may_block(&self, &ctx));

        pool::run(blocking, async move {

            // A request that failed as a whole, the device having no model for instance, is
            // the failure of every reading.
            let mut reads = self.exec_read_attr(targets, ctx, DevSource::Dev).await
                .map(Vec::into_iter);

            for (index, attr, valid_for) in polled {
                let read = match &mut reads {
                    Ok(reads) => reads.next().expect("every attribute is read").value,
                    Err(err) => Err(err.clone()),
                };
                last_periodics[index] = self.record_poll(&attr, read, valid_for, last_periodics[index]);
            }

            Ok(last_periodics)

        })

    }

    /// Record that reading of that attribute polled in its history, valid for that long,
    /// and send it as a periodic event once the period of that event has passed since the
    /// last one was, see [`crate::dev::AttrPeriodicEventConfig`], giving back when the
    /// last one was sent. A read that failed is both recorded and sent as the error it
    /// failed with.
    fn record_poll(&self, attr: &Attr<M>, read: Result<AttrRead<Value>>, valid_for: Duration, last_periodic: Option<Instant>) -> Option<Instant> {

        let read = read.and_then(|read| attr.check_pushed(&read).map(|()| read));

        if let Err(err) = attr.push_history(read.clone(), Some(valid_for)) {
            debug!(dev = %self.name, attr = %attr.name(), %err, "Failed to record the attribute polled");
        }

        let now = Instant::now();
        let period = attr.config().periodic_event.period.unwrap_or(polling::DEFAULT_PERIODIC_EVENT_PERIOD);
        if last_periodic.is_some_and(|last| now.duration_since(last) < period.mul_f64(polling::PERIODIC_EVENT_TOLERANCE)) {
            return last_periodic;
        }

        let read = read.map(|mut read| {
            attr.merge_w_value(&mut read);
            read
        });

        self.server().push_attr_value(&self.name, attr.name(), AttrValueEvent::Periodic, read);
        Some(now)

    }

    /// Poll those commands, see [`AnyDev::poll`]: executed together, with no argument,
    /// see [`Self::exec_many_command`], each result recorded in the history of its command.
    /// A command sends no periodic event, so none was ever sent.
    fn poll_commands(self: Arc<Self>, items: &[PollItem]) -> BoxFuture<'static, Result<Vec<Option<Instant>>>> {

        let none = vec![None; items.len()];

        // A device withdrawn meanwhile is not polled anymore, see [`Self::is_withdrawn`].
        if self.is_withdrawn() {
            return Box::pin(ready(Ok(none)));
        }

        // Each command along with how long what it gives is valid for.
        let mut polled = Vec::new();
        for item in items {
            match self.find_command(&item.name) {
                Ok(command) => polled.push((command, item.valid_for)),
                Err(err) => debug!(dev = %self.name, command = %item.name, %err, "Failed to poll the command"),
            }
        }

        if polled.is_empty() {
            return Box::pin(ready(Ok(none)));
        }

        let targets = polled.iter()
            .map(|(command, _)| (Ok(Arc::clone(command)), Value::Void))
            .collect::<Vec<_>>();

        let ctx = ReadCtx::new(&self, polled.iter().any(|(command, _)| command.involves_state()));
        let blocking = polled.iter().any(|(command, _)| command.may_block(&self, &ctx));

        pool::run(blocking, async move {

            // A request that failed as a whole is the failure of every command.
            let mut results = self.exec_many_command(targets, ctx, DevSource::Dev).await
                .map(Vec::into_iter);

            for (command, valid_for) in polled {
                let res = match &mut results {
                    Ok(results) => results.next().expect("every command is executed").value,
                    Err(err) => Err(err.clone()),
                };
                if let Err(err) = command.push_history(res, Some(valid_for)) {
                    debug!(dev = %self.name, command = %command.name(), %err, "Failed to record the command polled");
                }
            }

            Ok(none)

        })

    }

    /// Delete the model of this device, if it has one, giving it back to the delete
    /// function of its class so that it may release what it owns, taking the model
    /// already locked so that [`Self::init`] can delete and initialize under a single
    /// lock.
    /// 
    /// The device itself keeps existing, with no model, until it is initialized again:
    /// every request needing the model fails until then, but its state and its status
    /// still answer, saying why it has none, see [`Self::no_model_answer`].
    async fn delete_model(&self, model: &mut Option<Result<M>>) {

        // Only a model that was actually built is deleted: the error of a failed
        // initialization is dropped, there being nothing to give back.
        let Some(Ok(prev)) = model.take() else {
            return;
        };

        let def = self.class.def();
        debug!(dev = %self.name, class = %def.name, "Deleting the model of the device");

        match &def.delete {
            Some(DeleteFn::Sync(delete)) => delete(prev, self),
            Some(DeleteFn::Async(delete)) => delete(prev, self).await,
            None => {}
        }

    }

    /// Give this device the interface of its class and nothing else, dropping whatever it
    /// had added to its own: what a device starts every initialization from, and therefore
    /// what it has no part of until it is initialized once.
    fn reset_interface(&self) {

        let def = self.class.def();
        let mut mutable = self.mutable.lock().unwrap();

        mutable.commands.clear();
        mutable.attrs.clear();
        mutable.state_attr = Arc::new(Attr::new_with_role(Arc::clone(&def.state_attr), AttrRole::State));
        mutable.status_attr = Arc::new(Attr::new_with_role(Arc::clone(&def.status_attr), AttrRole::Status));

        for def in &def.commands {
            let name = ArcName::from(&*def.name);
            mutable.commands.insert(name, Arc::new(Command::new(Arc::clone(def))));
        }

        for def in &def.attrs {
            let name = ArcName::from(&*def.name);
            mutable.attrs.insert(name, Arc::new(Attr::new(Arc::clone(def))));
        }

    }

    /// Configure every attribute of this device that is not yet from what the database holds
    /// for its class and for this device, both asked at once, see [`Attr::load_config`].
    /// 
    /// A request that fails does not fail the device: it is kept as the error of every
    /// attribute it was for, which a client finds out reading any of them.
    async fn load_all_attr_config(&self) {

        // Find all attributes that are not yet loaded from the database, the State and the
        // Status included.
        let attrs = {
            let mutable = self.mutable.lock().unwrap();
            [&mutable.state_attr, &mutable.status_attr]
                .into_iter()
                .chain(mutable.attrs.values())
                .filter(|attr| !attr.is_loaded())
                .map(Arc::clone)
                .collect::<Vec<_>>()
        };

        self.load_given_attr_config(&attrs).await;

    }

    /// Configure those attributes from what the database holds for them, see
    /// [`Self::load_all_attr_config`].
    async fn load_given_attr_config(&self, attrs: &[Arc<Attr<M>>]) {

        if attrs.is_empty() {
            return;
        }

        // Now we collect the names of the attributes.
        let names = attrs
            .iter()
            .map(|attr| attr.name())
            .collect::<Vec<_>>();

        // Waiting on both class attr properties and dev attr properties.
        let (
            class_props, 
            dev_props,
        ) = join(
            self.class.attr_props(&names), 
            self.server()
                .database()
                .get_dev_attr_prop_async::<Spectrum<str>, _, _>(&self.name, names.as_slice(), All),
        ).await;

        let dev_props = dev_props.and_then(|props| props
                .into_iter()
                .collect::<Result<Vec<_>>>());

        for (index, attr) in attrs.iter().enumerate() {
            attr.load_config(self,
                class_props.as_ref().map(|props| &*props[index]),
                dev_props.as_ref().map(|props| &*props[index]));
        }

    }

    /// What a device that has no model answers for an attribute or a command of that role,
    /// given the error its initialization failed with, if it failed at all: the State and
    /// the Status say what the device is, the FAULT of a failed initialization along with
    /// the error it failed with, which is what keeping that error is for, or the UNKNOWN of
    /// a device that has not been initialized yet along with saying so. Everything else
    /// fails with why the device has no model, see [`init_err`].
    fn no_model_answer(&self, role: AttrRole, failure: Option<&Error>) -> Result<Value> {
        match (role, failure) {
            (AttrRole::State, Some(_)) => Ok(DevState::Fault.into()),
            (AttrRole::State, None) => Ok(DevState::Unknown.into()),
            (AttrRole::Status, Some(err)) => Ok(make_failed_status(err).into()),
            (AttrRole::Status, None) => Ok(Scalar::from(MISSING_STATUS).into()),
            (AttrRole::Interface, _) => Err(init_err(failure, &self.name)),
        }
    }

    /// The state of the device derived from its model, which is already borrowed with the
    /// access [`Self::state_access`] asked for, which is how its State attribute is read,
    /// see [`Attr::read`], so that a request reading the state among other attributes
    /// derives it under the same borrow as the rest of them.
    /// 
    /// The state the state function gives is ALARM when an attribute is in alarm, see
    /// [`Self::set_auto_alarm_states`].
    pub(super) async fn model_state(&self, model: &mut ModelRef<'_, M>, ctx: &mut ReadCtx<M>) -> Result<DevState> {
        let state = self.model_state_inner(model, ctx).await?;
        if self.attrs_in_alarm(ctx, state, model, false).await.is_empty() {
            Ok(state)
        } else {
            Ok(DevState::Alarm)
        }
    }

    /// The counterpart of [`Self::model_state`] for the status of the device, which tells
    /// every attribute in alarm after what the status function gives.
    pub(super) async fn model_status(&self, model: &mut ModelRef<'_, M>, ctx: &mut ReadCtx<M>) -> Result<Scalar<str>> {

        let status = match &self.class.def().status {
            Some(StatusFn::SharedSync(func)) => func(model.shared(), self),
            Some(StatusFn::ExclusiveSync(func)) => func(model.exclusive()?, self),
            Some(StatusFn::SharedAsync(func)) => func(model.shared(), self).await,
            Some(StatusFn::ExclusiveAsync(func)) => func(model.exclusive()?, self).await,
            None => Ok(Scalar::from(DEFAULT_STATUS)),
        }?;

        if !ctx.has_alarm() {
            return Ok(status);
        }

        let state = self.model_state_inner(model, ctx).await?;
        let alarms = self.attrs_in_alarm(ctx, state, model, true).await;
        if alarms.is_empty() {
            return Ok(status);
        }

        let mut status = status.value().to_owned();
        for (attr, alarm) in alarms {
            status.push('\n');
            status.push_str(&alarm.describe(attr.name()));
        }

        Ok(Scalar::from(status.as_str()))

    }

    /// The state the state function of the class gives, see [`Self::model_state`], called
    /// once for the whole request, see [`ReadCtx::state`].
    async fn model_state_inner(&self, model: &mut ModelRef<'_, M>, ctx: &mut ReadCtx<M>) -> Result<DevState> {

        if let Some(state) = ctx.state {
            return Ok(state);
        }

        let state = match &self.class.def().state {
            Some(StateFn::SharedSync(func)) => func(model.shared(), self),
            Some(StateFn::ExclusiveSync(func)) => func(model.exclusive()?, self),
            Some(StateFn::SharedAsync(func)) => func(model.shared(), self).await,
            Some(StateFn::ExclusiveAsync(func)) => func(model.exclusive()?, self).await,
            None => Ok(DEFAULT_STATE),
        }?;

        ctx.state = Some(state);
        Ok(state)

    }

    /// The attributes in alarm, with the alarm each is in, when the state function gave
    /// a state that alarms put the device in ALARM in, see [`Self::set_auto_alarm_states`].
    /// Only the first is looked for unless every one is asked for.
    /// 
    /// An attribute the request already read is not read again, see [`AlarmAttr::read`].
    async fn attrs_in_alarm(&self, ctx: &mut ReadCtx<M>, state: DevState, model: &mut ModelRef<'_, M>, every: bool) -> Vec<(Arc<Attr<M>>, AttrAlarm)> {

        let mut found = Vec::new();
        if !ctx.may_alarm(state) {
            return found;
        }

        // Taken out of the context while its attributes are read with it: none of them is
        // the State nor the Status, so none of their reads looks at it.
        let mut alarm_ctx = ctx.alarm.take();
        for alarm_attr in alarm_ctx.iter_mut().flat_map(|alarm| &mut alarm.attrs) {

            let alarm = match alarm_attr.read {
                Some(alarm) => alarm,
                None => {
                    
                    // The attribute hasn't been read in the request, so we read it here
                    // just to know its alarm state, we ignore the value! Like cppTango
                    // does for a polled attribute, its latest reading recorded is used
                    // when there is one, whatever the source of the request.
                    let alarm = alarm_attr.attr.read_with_alarm(self, model, ctx, DevSource::CacheDev).await
                        .ok()
                        .and_then(|(_, alarm)| alarm);

                    alarm_attr.read = Some(alarm);
                    alarm
                    
                }
            };

            if let Some(alarm) = alarm {
                found.push((Arc::clone(&alarm_attr.attr), alarm));
                if !every {
                    break;
                }
            }

        }

        ctx.alarm = alarm_ctx;
        found

    }

    /// The access deriving the state of the device needs to its model, which reads the
    /// attributes that may put it in ALARM as well.
    pub(super) fn state_access(&self, ctx: &ReadCtx<M>) -> Access {
        let access = match &self.class.def().state {
            Some(StateFn::ExclusiveSync(_) | StateFn::ExclusiveAsync(_)) => Access::Exclusive,
            _ => Access::Shared,
        };
        access.merge(ctx.access(self))
    }

    /// The access deriving the status of the device needs to its model, which derives its
    /// state as well when the attributes may put it in ALARM.
    pub(super) fn status_access(&self, ctx: &ReadCtx<M>) -> Access {
        let access = match &self.class.def().status {
            Some(StatusFn::ExclusiveSync(_) | StatusFn::ExclusiveAsync(_)) => Access::Exclusive,
            _ => Access::Shared,
        };
        if ctx.has_alarm() { access.merge(self.state_access(ctx)) } else { access }
    }

    /// Whether initializing this device may block, its init function or its delete
    /// function being synchronous, see [`pool::run`].
    /// 
    /// The memorized values the initialization writes back count too, though only those of
    /// the attributes of the class are known beforehand.
    pub(super) fn init_may_block(&self) -> bool {
        let def = self.class.def();
        matches!(def.init, InitFn::Sync(_)) || self.delete_may_block() || def.attrs.iter().any(|attr| {
            attr.memorized == Some(AttrMemorized::RestoreAndWrite)
                && matches!(attr.write, Some(WriteFn::SharedSync(_) | WriteFn::ExclusiveSync(_)))
        })
    }

    /// Whether deleting the model of this device may block, its delete function being
    /// synchronous, see [`pool::run`].
    fn delete_may_block(&self) -> bool {
        matches!(self.class.def().delete, Some(DeleteFn::Sync(_)))
    }

    /// Whether deriving the state of this device may block, see [`pool::run`], reading
    /// the attributes that may put it in ALARM included.
    pub(super) fn state_may_block(&self, ctx: &ReadCtx<M>) -> bool {
        matches!(&self.class.def().state, Some(StateFn::SharedSync(_) | StateFn::ExclusiveSync(_)))
            || ctx.may_block(self)
    }

    /// Whether deriving the status of this device may block, see [`pool::run`], deriving
    /// its state included when the attributes may put it in ALARM.
    pub(super) fn status_may_block(&self, ctx: &ReadCtx<M>) -> bool {
        matches!(&self.class.def().status, Some(StatusFn::SharedSync(_) | StatusFn::ExclusiveSync(_)))
            || (ctx.has_alarm() && self.state_may_block(ctx))
    }

    /// Lock the model of this device with the given access, whether it has one or not.
    async fn lock(&self, access: Access) -> ModelGuard<'_, M> {
        match access {
            Access::Shared => ModelGuard::Shared(self.model.read().await),
            Access::Exclusive => ModelGuard::Exclusive(self.model.write().await),
        }
    }

    /// Lock the model of this device with the given access, which is how every request
    /// that needs the model reaches it.
    /// 
    /// A device that has no model fails instead: with the error of its failed
    /// initialization, so that the caller is told the actual cause, or with
    /// [`ErrorKind::DevNotConnected`] when it has not been initialized yet.
    pub(super) async fn lock_model(&self, access: Access) -> Result<ModelGuard<'_, M>> {
        let guard = self.lock(access).await;
        match guard.get() {
            Some(Ok(_)) => Ok(guard),
            Some(Err(err)) => Err(init_err(Some(err), &self.name)),
            None => Err(init_err(None, &self.name)),
        }
    }

    /// Fail if this device has no model to serve a request with, with the error saying
    /// why: what a request that does not itself need the model answers with.
    pub(super) async fn check_model(&self) -> Result<()> {
        match &*self.model.read().await {
            Some(Ok(_)) => Ok(()),
            Some(Err(err)) => Err(init_err(Some(err), &self.name)),
            None => Err(init_err(None, &self.name)),
        }
    }

    /// The command of that name, see [`Self::get_command`], and the error a client is
    /// answered with when the device has no command of that name.
    fn find_command(&self, name: &str) -> Result<Arc<Command<M>>> {
        self.get_command(name)
            .ok_or_else(|| Error::new(ErrorKind::CommandNotFound)
                .with_description(format!("Command {name:?} is not defined on device {:?}", self.name)))
    }

    /// The counterpart of [`Self::find_command`] for the attributes.
    fn find_attr(&self, name: &str) -> Result<Arc<Attr<M>>> {
        self.get_attr(name)
            .ok_or_else(|| Error::new(ErrorKind::AttrNotFound)
                .with_description(format!("Attribute {name:?} is not defined on device {:?}", self.name)))
    }

    /// The State attribute of this device, kept out of its interface, which reads the state
    /// through the state function of its class, see [`Self::model_state`].
    fn state_attr(&self) -> Arc<Attr<M>> {
        Arc::clone(&self.mutable.lock().unwrap().state_attr)
    }

    /// The Status attribute of this device, see [`Self::state_attr`].
    fn status_attr(&self) -> Arc<Attr<M>> {
        Arc::clone(&self.mutable.lock().unwrap().status_attr)
    }

    /// The attributes of the given names, or every one when none is given, resolved before
    /// the model is borrowed, since resolving one locks the interface store. A name that
    /// resolves to nothing only fails that one attribute: the others are still answered.
    fn attr_targets(&self, names: Option<Spectrum<str>>) -> Vec<Resolved<Arc<Attr<M>>>> {
        match names {
            Some(names) => scalars(&names)
                .map(|name| {
                    let target = self.find_attr(name.value());
                    resolved(name, target)
                })
                .collect(),
            None => self.attrs().into_iter().map(Ok).collect(),
        }
    }

    /// The counterpart of [`Self::attr_targets`] for the commands.
    fn command_targets(&self, names: Option<Spectrum<str>>) -> Vec<Resolved<Arc<Command<M>>>> {
        match names {
            Some(names) => scalars(&names)
                .map(|name| {
                    let target = self.find_command(name.value());
                    resolved(name, target)
                })
                .collect(),
            None => self.commands().into_iter().map(Ok).collect(),
        }
    }

    /// Execute a command, resolved beforehand, see [`BackendDev::command`]: under its own
    /// borrow of the model, as [`Self::exec_many_command`] executes several, a device that
    /// has no model answering the same, see [`Self::no_model_answer`]. The Init is executed
    /// apart, since it rebuilds the model rather than borrowing it, see [`Self::init`].
    async fn exec_command(&self, target: Result<Arc<Command<M>>>, input: Value, mut ctx: ReadCtx<M>, source: DevSource) -> Result<Value> {

        // A device whose initialization failed only has what its class defines, so a name
        // it does not have may well be one it would have had: its own failure is what a
        // client needs to hear then, not that the name is unknown.
        let command = match target {
            Ok(command) => command,
            Err(err) => return Err(self.check_model().await.err().unwrap_or(err)),
        };

        // The Init is never answered from the cache, and an initialization that failed is a
        // failed Init: the device keeps that error to report it in its status, and the
        // client that asked for the initialization is given it too.
        if command.is_init() {

            if source == DevSource::Cache {
                return Err(command.not_polled_err(self));
            }

            self.init().await;
            self.check_model().await?;
            return Ok(Value::Void);

        }

        let mut guard = self.lock(command.exec_access(self, &ctx)).await;
        if let Some(failure) = guard.failure() {
            return self.no_model_answer(command.attr_role(), failure.as_ref());
        }

        command.exec(self, &mut guard.model(), input, &mut ctx, source).await

    }

    /// Execute commands, resolved beforehand, each with its argument, under a single borrow
    /// of the model, exclusive as soon as one of the execution functions asks for it: the
    /// counterpart for commands of [`Self::exec_read_attr`], which is how the commands
    /// polled together are executed, see [`Self::poll_commands`]. A single command is
    /// executed the same way, without the batch, see [`Self::exec_command`].
    ///
    /// The Init cannot be among them, see [`Self::exec_command`].
    async fn exec_many_command(&self, targets: Vec<(Resolved<Arc<Command<M>>>, Value)>, mut ctx: ReadCtx<M>, source: DevSource) -> Result<Vec<NamedResult<Value>>> {

        let access = targets.iter()
            .filter_map(|(target, _)| target.as_ref().ok())
            .fold(Access::Shared, |access, command| access.merge(command.exec_access(self, &ctx)));

        let mut guard = self.lock(access).await;

        // A device that has no model answers each command on its own, see
        // [`Self::no_model_answer`], as a multiple read does.
        if let Some(failure) = guard.failure() {
            return Ok(targets.into_iter()
                .map(|(target, _)| match target {
                    Ok(command) => Named::new(Scalar::from(command.name()), self.no_model_answer(command.attr_role(), failure.as_ref())),
                    Err(unresolved) => unresolved.map(Err),
                })
                .collect());
        }

        let mut model = guard.model();
        let mut results = Vec::with_capacity(targets.len());

        for (target, input) in targets {
            results.push(match target {
                Ok(command) => {
                    let res = command.exec(self, &mut model, input, &mut ctx, source).await;
                    Named::new(Scalar::from(command.name()), res)
                }
                Err(unresolved) => unresolved.map(Err),
            });
        }

        Ok(results)

    }

    /// Read attributes, resolved beforehand, see [`BackendDev::read_attr`].
    async fn exec_read_attr(&self, targets: Vec<Resolved<Arc<Attr<M>>>>, mut ctx: ReadCtx<M>, source: DevSource) -> Result<Vec<NamedResult<AttrRead<Value>>>> {

        // They are all read under a single borrow of the model, which is what makes the
        // values of a multiple read consistent with each other, and that borrow is
        // exclusive as soon as one of the read functions asks for it.
        let access = targets.iter()
            .filter_map(|target| target.as_ref().ok())
            .fold(Access::Shared, |access, attr| access.merge(attr.read_access(self, &ctx)));

        let mut guard = self.lock(access).await;

        // A device that has no model answers each attribute on its own, see
        // [`Self::no_model_answer`]: a client reading the State of a broken device is told
        // it is broken, and why, not that the request failed.
        if let Some(failure) = guard.failure() {
            return Ok(targets.into_iter()
                .map(|target| match target {
                    Ok(attr) => Named::new(Scalar::from(attr.name()), self.no_model_answer(attr.role(), failure.as_ref()).map(read_now)),
                    Err(unresolved) => unresolved.map(Err),
                })
                .collect());
        }

        // The attributes are read first and the state and the status derived after them, so
        // that an attribute that may put the device in ALARM is read once, its reading being
        // what the state is derived from, see [`ReadCtx::record`]. Each is answered where it
        // was asked for all the same.
        let mut model = guard.model();
        let mut reads = vec![const { None }; targets.len()];

        for interface in [true, false] {
            for (target, slot) in targets.iter().zip(&mut reads) {
                if let Ok(attr) = target
                && (attr.role() == AttrRole::Interface) == interface {
                    let read = attr.read_with_alarm(self, &mut model, &mut ctx, source).await;
                    ctx.record(attr, read.as_ref().ok().and_then(|(_, alarm)| *alarm));
                    *slot = Some(Named::new(Scalar::from(attr.name()), read.map(|(read, _)| read)));
                }
            }
        }

        for (target, slot) in targets.into_iter().zip(&mut reads) {
            if let Err(unresolved) = target {
                *slot = Some(unresolved.map(Err));
            }
        }

        Ok(reads.into_iter().map(|read| read.expect("every target is read")).collect())

    }

    /// Write attributes, resolved beforehand, see [`BackendDev::write_attr`].
    async fn exec_write_attr(&self, targets: Vec<(Result<Arc<Attr<M>>>, Value)>) -> Result<Vec<Result<()>>> {

        // Written under a single borrow of the model, so that a client writing several
        // attributes at once has them written together.
        let access = targets.iter()
            .filter_map(|(target, _)| target.as_ref().ok())
            .fold(Access::Shared, |access, attr| access.merge(attr.write_access()));

        let mut guard = self.lock_model(access).await?;
        let mut model = guard.model();

        let mut writes = Vec::with_capacity(targets.len());
        for (target, value) in targets {
            writes.push(match target {
                Ok(attr) => attr.write(self, &mut model, value).await,
                Err(err) => Err(err),
            });
        }

        Ok(writes)

    }

    /// Change the configuration of attributes, each given as the whole configuration it is
    /// to have, see [`BackendDev::set_attr_config`].
    /// 
    /// Every attribute is changed on its own, one failing leaving the others to be changed:
    /// on the device first, and then in the database, the way cppTango does.
    async fn exec_set_attr_config(&self, configs: Vec<Named<AttrConfig>>) -> Vec<Result<()>> {
        let mut results = Vec::with_capacity(configs.len());
        for Named { name, value } in configs {
            results.push(self.change_attr_config(name.value(), &value).await);
        }
        results
    }

    /// Change the configuration of one attribute, see [`Self::exec_set_attr_config`].
    /// 
    /// A change the database fails to store is the one the device has all the same, until
    /// it is configured from the database again, and is therefore pushed all the same.
    async fn change_attr_config(&self, name: &str, config: &AttrConfig) -> Result<()> {

        let attr = self.find_attr(name)?;
        let class_props = self.class.attr_props(&[attr.name()]).await?;
        let changes = attr.change_config(&class_props[0], config)
            .map_err(|err| Error::new(ErrorKind::Reason(reason::API_IncoherentValues))
                .with_description(format!("Configuration of attribute {:?} of device {:?} \
                    cannot be changed, {err}", attr.name(), self.name)))?;

        if changes.is_empty() {
            return Ok(());
        }

        self.server().push_attr_config(&self.name, attr.name(), attr.info());

        let database = self.server().database();

        if !changes.put.is_empty() {
            let put = changes.put.into_iter()
                .map(|(prop, values)| (prop.name(), values))
                .collect::<Vec<_>>();
            database.set_dev_attr_prop_async(&self.name, (attr.name(), put)).await?;
        }

        if !changes.del.is_empty() {
            let del = changes.del.iter().map(|prop| prop.name()).collect::<Vec<_>>();
            database.delete_dev_attr_prop_async(&self.name, attr.name(), del.as_slice()).await?;
        }

        Ok(())

    }

    /// Write then read back attributes, resolved beforehand, see
    /// [`BackendDev::write_read_attr`].
    async fn exec_write_read_attr(&self, targets: Vec<(Resolved<Arc<Attr<M>>>, Value)>, mut ctx: ReadCtx<M>) -> Result<Vec<NamedResult<AttrRead<Value>>>> {

        // Every value is written and read back under a single borrow of the model, which is
        // the whole point of asking for the two together: what a client is given is what the
        // device had right after its write, with nothing of anyone else in between.
        let access = targets.iter()
            .filter_map(|(target, _)| target.as_ref().ok())
            .fold(Access::Shared, |access, attr| access.merge(attr.write_access()).merge(attr.read_access(self, &ctx)));

        let mut guard = self.lock_model(access).await?;
        let mut model = guard.model();

        let mut reads = Vec::with_capacity(targets.len());
        for (target, value) in targets {
            reads.push(match target {
                // A write that failed is not read back: what a client gets of that
                // attribute is the reason its write was refused.
                Ok(attr) => {
                    let read = match attr.write(self, &mut model, value).await {
                        Ok(()) => attr.read(self, &mut model, &mut ctx, DevSource::Dev).await,
                        Err(err) => Err(err),
                    };
                    Named::new(Scalar::from(attr.name()), read)
                }
                Err(unresolved) => unresolved.map(Err),
            });
        }

        Ok(reads)

    }
}

impl<M: Model> AnyDev for Dev<M> {

    #[inline]
    fn name(&self) -> &Arc<str> {
        &self.name
    }

    #[inline]
    fn class(&self) -> &dyn AnyClass {
        &*self.class
    }

    fn init(&self) -> BoxFuture<'_, Result<()>> {
        let dev = self.arc();
        pool::run(self.init_may_block(), async move {
            dev.init().await;
            Ok(())
        })
    }

    fn withdraw(&self) -> BoxFuture<'_, Result<()>> {
        let dev = self.arc();
        Box::pin(async move {

            // Out of reach of the clients of this process first, as of the others next.
            dev.withdraw_now();
            if let Some(backend) = dev.server().backend() {
                backend.remove(&dev.name);
            }

            pool::run(dev.delete_may_block(), async move {
                dev.delete_model(&mut *dev.model.write().await).await;
                Ok(())
            }).await

        })
    }

    fn withdraw_now(&self) {
        self.withdrawn.store(true, Ordering::Release);
        self.local_handle.lock().unwrap().take();
        if let Some(poller) = self.server().poller() {
            poller.remove_dev(self);
        }
    }

    fn serve(&self) -> Result<Arc<str>> {
        let backend = self.server().backend_checked()?;
        let dev = DevImpl::from_arc(self.arc()) as Arc<dyn BackendDev>;
        let itr = backend.add(&self.name, Arc::clone(&dev))?;
        if self.server().local {
            let local_handle = local::register(&dev, Arc::clone(&itr), backend.host(), backend.port(), Arc::clone(&self.name));
            *self.local_handle.lock().unwrap() = Some(local_handle);
        }
        Ok(itr)
    }

    #[inline]
    fn into_any(self: Arc<Self>) -> Arc<dyn Any + Send + Sync> {
        self
    }

    fn poll(&self, kind: PolledKind, items: &[PollItem]) -> BoxFuture<'static, Result<Vec<Option<Instant>>>> {
        match kind {
            PolledKind::Attr => self.arc().poll_attrs(items),
            PolledKind::Command => self.arc().poll_commands(items),
        }
    }

    fn polled_history(&self, kind: PolledKind, name: &str) -> Option<PolledHistory> {

        /// How many of the latest results the polling status tells the times of.
        const RECORDS: usize = 5;

        let (depth, entries) = match kind {
            PolledKind::Attr => {
                let attr = self.get_attr(name)?;
                let entries = attr.history(RECORDS).into_iter()
                    .map(|entry| (entry.time, entry.value.err()))
                    .collect::<Vec<_>>();
                (attr.history_depth(), entries)
            }
            PolledKind::Command => {
                let command = self.get_command(name)?;
                let entries = command.history(RECORDS).into_iter()
                    .map(|entry| (entry.time, entry.value.err()))
                    .collect::<Vec<_>>();
                (command.history_depth(), entries)
            }
        };

        let failure = entries.last().and_then(|(_, err)| err.clone());
        Some(PolledHistory {
            depth,
            times: entries.into_iter().map(|(time, _)| time).collect(),
            failure,
        })

    }

    fn check_pollable(&self, kind: PolledKind, name: &str) -> Result<Arc<str>> {
        match kind {
            PolledKind::Attr => {
                let attr = self.find_attr(name)?;
                if !attr.is_pollable() {
                    return Err(Error::new(ErrorKind::Reason(reason::API_NotSupported))
                        .with_description(format!("Attribute {:?} of device {:?} cannot be polled, it is write-only", 
                            attr.name(), self.name)));
                }
                Ok(Arc::from(attr.name()))
            }
            PolledKind::Command => {
                let command = self.find_command(name)?;
                if !command.is_pollable() {
                    return Err(Error::new(ErrorKind::Reason(reason::API_NotSupported))
                        .with_description(format!("Command {:?} of device {:?} cannot be polled, only one taking no argument can", 
                            command.name(), self.name)));
                }
                Ok(Arc::from(command.name()))
            }
        }
    }

}

/// The first value of that property parsed as a number, absent if it has none, or with a
/// warning if it is not one.
fn first_number<T: std::str::FromStr>(dev: &str, prop: &str, values: &Spectrum<str>) -> Option<T> {
    let value = values.iter().next()?;
    match value.trim().parse() {
        Ok(number) => Some(number),
        Err(_) => {
            warn!(%dev, %prop, %value, "Ignored an invalid polling property");
            None
        }
    }
}

impl<M: Model> Debug for Dev<M> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Dev").field("name", &self.name).finish_non_exhaustive()
    }
}

/// The interface of a single device, which every device owns its own copy of because it
/// can be altered device by device.
struct DevMutable<M: Model> {
    /// The black box of the device where past commands, attribute reads/writes and other
    /// accesses to this device.
    black_box: VecDeque<String>,
    /// The commands the device currently registers.
    commands: HashMap<ArcName, Arc<Command<M>>>,
    attrs: HashMap<ArcName, Arc<Attr<M>>>,
    /// The State and the Status of the device, kept out of the interface since every device
    /// has them whatever its class, see [`Dev::state_attr`].
    state_attr: Arc<Attr<M>>,
    status_attr: Arc<Attr<M>>,
    /// The states in which the device is put in ALARM when one of its attributes is in
    /// alarm, see [`Dev::set_auto_alarm_states`].
    auto_alarm_states: Arc<[DevState]>,
}

/// What a request reading a device learns of it as it goes, so that it reads nothing twice:
/// what may put the device in ALARM, along with what the request read of it, and the state
/// the state function gave, see [`Dev::set_auto_alarm_states`].
pub(super) struct ReadCtx<M: Model> {
    /// What may put the device in ALARM, which only a request involving the state or the
    /// status takes, and only when anything may, see [`ReadCtx::new`].
    alarm: Option<AlarmCtx<M>>,
    /// The state the state function gave, once called, so that the state and the status
    /// asked for together call it once.
    state: Option<DevState>,
}

/// What may put a device in ALARM, taken out of the interface store once for a whole
/// request, so that what the request borrows the model for and what it reads are the same:
/// the states in which the device may be put in ALARM, and the attributes that have any
/// alarm configured, by name.
struct AlarmCtx<M: Model> {
    states: Arc<[DevState]>,
    attrs: Vec<AlarmAttr<M>>,
}

/// An attribute that may put a device in ALARM, see [`AlarmCtx`].
struct AlarmAttr<M: Model> {
    attr: Arc<Attr<M>>,
    /// What the request found reading the attribute, so that it is read only once:
    /// - `None`: not read yet;
    /// - `Some(None)`: read, and in no alarm, or could not be read;
    /// - `Some(Some(alarm))`: read, and in that alarm.
    read: Option<Option<AttrAlarm>>,
}

impl<M: Model> ReadCtx<M> {

    /// The context of a request on that device, which takes what may put the device in
    /// ALARM out of its interface store only if the request involves its state or its
    /// status, and only keeps it if the device has states to be put in ALARM in and
    /// attributes that have any alarm configured.
    fn new(dev: &Dev<M>, involves_state_status: bool) -> Self {

        let alarm = involves_state_status.then(|| {

            let mutable = dev.mutable.lock().unwrap();
            if mutable.auto_alarm_states.is_empty() {
                return None;
            }

            let mut attrs = mutable.attrs.values()
                .filter(|attr| attr.has_alarms())
                .map(|attr| AlarmAttr {
                    attr: Arc::clone(attr),
                    read: None,
                })
                .collect::<Vec<_>>();

            if attrs.is_empty() {
                return None;
            }

            attrs.sort_by(|a, b| a.attr.name().cmp(b.attr.name()));
            Some(AlarmCtx {
                states: Arc::clone(&mutable.auto_alarm_states),
                attrs,
            })

        }).flatten();

        Self { 
            alarm, 
            state: None,
        }

    }

    /// Whether anything may put the device in ALARM, in any state.
    fn has_alarm(&self) -> bool {
        self.alarm.is_some()
    }

    /// Whether anything may put the device in ALARM when the state function gave that
    /// state, see [`Dev::set_auto_alarm_states`].
    fn may_alarm(&self, state: DevState) -> bool {
        self.alarm.as_ref().is_some_and(|alarm| alarm.states.contains(&state))
    }

    /// Keep the alarm the request found that attribute in, if it may put the device in
    /// ALARM, so that it is not read again.
    fn record(&mut self, attr: &Arc<Attr<M>>, alarm: Option<AttrAlarm>) {
        
        let alarm_attr = self.alarm.iter_mut()
            .flat_map(|alarm| &mut alarm.attrs)
            .find(|alarm_attr| Arc::ptr_eq(&alarm_attr.attr, attr));

        if let Some(alarm_attr) = alarm_attr {
            alarm_attr.read = Some(alarm);
        }
        
    }

    /// The access reading the attributes that may put the device in ALARM needs to the
    /// model.
    fn access(&self, dev: &Dev<M>) -> Access {
        self.alarm.iter()
            .flat_map(|alarm| &alarm.attrs)
            .fold(Access::Shared, |access, alarm_attr| access.merge(alarm_attr.attr.read_access(dev, self)))
    }

    /// Whether reading the attributes that may put the device in ALARM may block, see
    /// [`pool::run`].
    fn may_block(&self, dev: &Dev<M>) -> bool {
        self.alarm.iter()
            .flat_map(|alarm| &alarm.attrs)
            .any(|alarm_attr| alarm_attr.attr.read_may_block(dev, self))
    }

}

/// A reading of a value of a valid quality, made now, which is what a device pushes most.
fn read_now(value: impl IntoValue) -> AttrRead<Value> {
    AttrRead {
        value: Some(value.into_value()),
        w_value: None,
        quality: AttrQuality::Valid,
        time: SystemTime::now(),
    }
}

/// The access a request needs to the model of a device: shared when every handler it runs
/// takes the model by reference, so that such requests run in parallel, and exclusive as
/// soon as one of them takes it mutably.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Access {
    Shared,
    Exclusive,
}

impl Access {

    /// The access covering both, which is exclusive as soon as either is: a request
    /// touching several attributes at once borrows the model once, for what the most
    /// demanding of its handlers needs.
    pub fn merge(self, other: Self) -> Self {
        match (self, other) {
            (Self::Shared, Self::Shared) => Self::Shared,
            _ => Self::Exclusive,
        }
    }

}

/// The lock of the model of a device, held for the duration of a request with the access
/// that request asked for.
pub enum ModelGuard<'a, M> {
    Shared(async_lock::RwLockReadGuard<'a, Option<Result<M>>>),
    Exclusive(async_lock::RwLockWriteGuard<'a, Option<Result<M>>>),
}

impl<M> ModelGuard<'_, M> {

    /// The model of the device if it has one, or the error of its failed initialization.
    #[inline]
    fn get(&self) -> &Option<Result<M>> {
        match self {
            Self::Shared(guard) => guard,
            Self::Exclusive(guard) => guard,
        }
    }

    /// Why the device has no model, if it has none: the error its initialization failed
    /// with, if it failed at all, see [`Dev::no_model_answer`].
    fn failure(&self) -> Option<Option<Error>> {
        match self.get() {
            Some(Ok(_)) => None,
            Some(Err(err)) => Some(Some(err.clone())),
            None => Some(None),
        }
    }

    /// The model, borrowed with the access the lock was taken with.
    /// 
    /// # Panics
    /// 
    /// If the device has no model, which [`Dev::lock_model`] checks beforehand.
    pub(super) fn model(&mut self) -> ModelRef<'_, M> {
        match self {
            Self::Shared(guard) => match &**guard {
                Some(Ok(model)) => ModelRef::Shared(model),
                _ => panic!("the device has no model"),
            }
            Self::Exclusive(guard) => match &mut **guard {
                Some(Ok(model)) => ModelRef::Exclusive(model),
                _ => panic!("the device has no model"),
            }
        }
    }

}

/// The model of a device, borrowed for the duration of a request with the access that
/// request asked for, which is what lets every handler of a single request run under one
/// borrow of the model.
pub enum ModelRef<'m, M> {
    Shared(&'m M),
    Exclusive(&'m mut M),
}

impl<M> ModelRef<'_, M> {

    /// The model, shared, which either access gives.
    #[inline]
    pub fn shared(&self) -> &M {
        match self {
            Self::Shared(model) => model,
            Self::Exclusive(model) => model,
        }
    }

    /// The model, exclusively, which only an exclusive access gives.
    /// 
    /// Failing here is a bug of the server and not of the device: the access is computed
    /// from the very handlers the request then runs, see [`Access::merge`], so a handler
    /// asking for the model mutably has been counted before it is called.
    #[inline]
    pub fn exclusive(&mut self) -> Result<&mut M> {
        match self {
            Self::Exclusive(model) => Ok(model),
            Self::Shared(_) => Err(Error::new(ErrorKind::Internal)
                .with_description("The model was borrowed shared while a handler of that \
                    request needs it exclusively")),
        }
    }

}

/// The status of a device whose initialization failed: the error it failed with, which is
/// what a client reads to know why the device is not running.
fn make_failed_status(err: &Error) -> Scalar<str> {
    Scalar::from(format!("The initialization of the device failed:\n{err}").as_str())
}

/// The error every request on a device that has no model fails with, given the error
/// its initialization failed with, if it failed at all.
fn init_err(failure: Option<&Error>, name: &str) -> Error {
    match failure {
        Some(err) => Error::new(ErrorKind::DevInitFailed)
            .with_description(format!("Device {name:?} failed to initialize"))
            .with_parent(err.clone()),
        None => Error::new(ErrorKind::DevNotConnected)
            .with_description(format!("Device {name:?} is not initialized")),
    }
}

/// The documentation of a definition as a client is told it, an empty one being none.
pub fn some_doc(doc: &Cow<'static, str>) -> Option<Cow<'static, str>> {
    (!doc.is_empty()).then(|| doc.clone())
}

// =================== //
//   Device backend    //
// =================== //

/// The backend of a device, which is the device itself seen through a transparent wrapper,
/// so that the backend methods are not methods of [`Dev`] itself. The backend serving a
/// device holds it as such, and it is reached directly by a proxy of this process when the
/// server is local, see [`crate::local`].
#[repr(transparent)]
struct DevImpl<M: Model>(Dev<M>);

impl<M: Model> DevImpl<M> {

    /// See a device as its backend.
    #[inline]
    fn from_arc(dev: Arc<Dev<M>>) -> Arc<Self> {
        // SAFETY: DevImpl is a repr(transparent) wrapper around Dev, so both have the same
        // layout and an Arc of one is an Arc of the other.
        unsafe { Arc::from_raw(Arc::into_raw(dev) as *const Self) }
    }

}

impl<M: Model> Debug for DevImpl<M> {
    #[inline]
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        Debug::fmt(&self.0, f)
    }
}

/// What touches the model is run on the pool of the server when one of the handlers it
/// runs may block, and polled by the executor of the backend otherwise, see [`pool::run`];
/// what only reads the definitions and the interface is answered on the call itself.
///
/// Every request is refused once the device is withdrawn, see [`Dev::is_withdrawn`], which
/// a client holding the device may still be asking it. A request withdrawn meanwhile finds
/// no model once it holds its lock, the withdrawal deleting it under that lock, and is
/// answered as a device that is not initialized.
impl<M: Model> BackendDev for DevImpl<M> {

    fn name(&self) -> BoxFuture<'_, Result<Arc<str>>> {

        if let Err(err) = self.0.check_withdrawn() {
            return Box::pin(ready(Err(err)));
        }

        Box::pin(ready(Ok(Arc::clone(&self.0.name))))

    }

    fn admin_dev_name(&self) -> BoxFuture<'_, Result<Arc<str>>> {

        if let Err(err) = self.0.check_withdrawn() {
            return Box::pin(ready(Err(err)));
        }

        Box::pin(ready(Ok(Arc::clone(&self.0.server().admin_dev_name))))

    }

    fn description(&self) -> BoxFuture<'_, Result<Arc<str>>> {

        if let Err(err) = self.0.check_withdrawn() {
            return Box::pin(ready(Err(err)));
        }

        Box::pin(ready(Ok(Arc::from(&*self.0.class.def().doc))))

    }

    /// None of this is the device's own: the class it is an instance of, and the server
    /// running it, are what a client is told here, so all of it is read off the definition
    /// of its class and off the server it belongs to.
    fn info(&self) -> BoxFuture<'_, Result<DevInfo>> {

        if let Err(err) = self.0.check_withdrawn() {
            return Box::pin(ready(Err(err)));
        }

        let def = self.0.class.def();
        let server = self.0.server();
        let backend = match server.backend_checked() {
            Ok(backend) => backend,
            Err(err) => return Box::pin(ready(Err(err))),
        };

        Box::pin(ready(Ok(DevInfo {
            class: def.name.to_string(),
            server_id: server.name().to_string(),
            server_host: backend.host().to_string(),
            server_version: backend.version(),
            doc_url: def.doc_url.clone(),
            // The description of the class, this crate having the one string where Tango
            // has two: what the device *is* and what it says of itself are the same here.
            dev_type: Some(def.doc.clone()),
            version_info: vec![
                (Cow::Borrowed("rstango"), Cow::Borrowed(env!("CARGO_PKG_VERSION"))),
            ],
        })))

    }

    fn ping(&self) -> BoxFuture<'_, Result<()>> {

        if let Err(err) = self.0.check_withdrawn() {
            return Box::pin(ready(Err(err)));
        }

        Box::pin(ready(Ok(())))

    }

    fn state(&self) -> BoxFuture<'_, Result<DevState>> {

        if let Err(err) = self.0.check_withdrawn() {
            return Box::pin(ready(Err(err)));
        }

        let dev = self.0.arc();
        let command = Arc::new(Command::new_with_attr(dev.state_attr()));
        let ctx = ReadCtx::new(&dev, true);
        pool::run(command.may_block(&dev, &ctx), async move {
            DevState::try_from(dev.exec_command(Ok(command), Value::Void, ctx, DevSource::CacheDev).await?)
        })

    }

    fn status(&self) -> BoxFuture<'_, Result<Scalar<str>>> {

        if let Err(err) = self.0.check_withdrawn() {
            return Box::pin(ready(Err(err)));
        }

        let dev = self.0.arc();
        let command = Arc::new(Command::new_with_attr(dev.status_attr()));
        let ctx = ReadCtx::new(&dev, true);
        pool::run(command.may_block(&dev, &ctx), async move {
            Scalar::<str>::try_from(dev.exec_command(Ok(command), Value::Void, ctx, DevSource::CacheDev).await?)
        })

    }

    fn black_box(&self, max: Option<usize>) -> BoxFuture<'_, Result<Spectrum<str>>> {

        if let Err(err) = self.0.check_withdrawn() {
            return Box::pin(ready(Err(err)));
        }

        let mutable = self.0.mutable.lock().unwrap();
        let black_box = Spectrum::from_strs(mutable.black_box.iter().take(max.unwrap_or(usize::MAX)))
            .expect("strings of more than 4 GiB");

        Box::pin(ready(Ok(black_box)))

    }

    fn command(&self, name: Scalar<str>, input: Value, source: DevSource) -> BoxFuture<'_, Result<Value>> {
        
        if let Err(err) = self.0.check_withdrawn() {
            return Box::pin(ready(Err(err)));
        }

        let dev = self.0.arc();
        let target = dev.find_command(name.value());
        let ctx = ReadCtx::new(&dev, target.as_ref().is_ok_and(|command| command.involves_state()));

        let blocking = target.as_ref().is_ok_and(|command| command.may_block(&dev, &ctx));
        pool::run(blocking, async move {
            let res = dev.exec_command(target, input, ctx, source).await;
            if let Err(err) = &res {
                debug!(dev = %dev.name, command = %name.value(), %err, "Failed to execute the command");
            }
            res
        })

    }

    fn read_attr(&self, names: Option<Spectrum<str>>, source: DevSource) -> BoxFuture<'_, Result<Vec<NamedResult<AttrRead<Value>>>>> {
        
        if let Err(err) = self.0.check_withdrawn() {
            return Box::pin(ready(Err(err)));
        }

        let dev = self.0.arc();
        let targets = dev.attr_targets(names);
        let ctx = ReadCtx::new(&dev, targets.iter().flatten().any(|attr| attr.role() != AttrRole::Interface));

        let blocking = targets.iter().flatten().any(|attr| attr.read_may_block(&dev, &ctx));
        pool::run(blocking, async move { dev.exec_read_attr(targets, ctx, source).await })

    }

    fn write_attr(&self, values: Vec<Named<Value>>) -> BoxFuture<'_, Result<Vec<Result<()>>>> {
        
        if let Err(err) = self.0.check_withdrawn() {
            return Box::pin(ready(Err(err)));
        }

        let dev = self.0.arc();
        let targets = values.into_iter()
            .map(|write| (dev.find_attr(write.name.value()), write.value))
            .collect::<Vec<_>>();
        
        let blocking = targets.iter()
            .filter_map(|(target, _)| target.as_ref().ok())
            .any(|attr| attr.write_may_block());
        pool::run(blocking, async move { dev.exec_write_attr(targets).await })
        
    }

    fn write_read_attr(&self, values: Vec<Named<Value>>) -> BoxFuture<'_, Result<Vec<NamedResult<AttrRead<Value>>>>> {
        
        if let Err(err) = self.0.check_withdrawn() {
            return Box::pin(ready(Err(err)));
        }

        let dev = self.0.arc();
        let targets = values.into_iter()
            .map(|write| {
                let target = dev.find_attr(write.name.value());
                (resolved(write.name, target), write.value)
            })
            .collect::<Vec<_>>();
        
        let ctx = ReadCtx::new(&dev, false);
        let blocking = targets.iter()
            .filter_map(|(target, _)| target.as_ref().ok())
            .any(|attr| attr.write_may_block() || attr.read_may_block(&dev, &ctx));
        pool::run(blocking, async move { dev.exec_write_read_attr(targets, ctx).await })

    }

    /// The definitions and nothing else, so this borrows no model: a device that failed to
    /// initialize still describes the interface of its class, which is how a client knows
    /// to ask it for the state and the status that say it failed.
    fn command_info(&self, names: Option<Spectrum<str>>) -> BoxFuture<'_, Result<Vec<NamedResult<CommandInfo>>>> {
        
        if let Err(err) = self.0.check_withdrawn() {
            return Box::pin(ready(Err(err)));
        }

        let infos = self.0.command_targets(names).into_iter()
            .map(|target| match target {
                Ok(target) => Named::new(Scalar::from(target.name()), Ok(target.info())),
                Err(unresolved) => unresolved.map(Err),
            })
            .collect();
        
        Box::pin(ready(Ok(infos)))

    }

    /// The counterpart of [`Self::command_info`], and the same absence of any model.
    fn attr_info(&self, names: Option<Spectrum<str>>) -> BoxFuture<'_, Result<Vec<NamedResult<AttrInfo>>>> {
        
        if let Err(err) = self.0.check_withdrawn() {
            return Box::pin(ready(Err(err)));
        }

        let infos = self.0.attr_targets(names).into_iter()
            .map(|target| match target {
                Ok(target) => Named::new(Scalar::from(target.name()), target.info()),
                Err(unresolved) => unresolved.map(Err),
            })
            .collect();

        Box::pin(ready(Ok(infos)))

    }

    /// Change the configuration of each attribute on its own, see [`Dev::exec_set_attr_config`].
    fn set_attr_config(&self, configs: Vec<Named<AttrConfig>>) -> BoxFuture<'_, Result<Vec<Result<()>>>> {

        if let Err(err) = self.0.check_withdrawn() {
            return Box::pin(ready(Err(err)));
        }

        Box::pin(async move { Ok(self.0.exec_set_attr_config(configs).await) })

    }

    /// What the device recorded itself, or its polling recorded, in the history of that
    /// command, see [`Command::history`]: a command nothing was recorded of fails rather
    /// than giving an empty history, whether it is polled or not.
    fn command_history(&self, name: Scalar<str>, max: usize) -> BoxFuture<'_, Result<Vec<RecordedResult<Value>>>> {

        if let Err(err) = self.0.check_withdrawn() {
            return Box::pin(ready(Err(err)));
        }

        Box::pin(ready(self.0.find_command(name.value()).and_then(|command| {
            let history = command.history(max);
            if history.is_empty() {
                return Err(Error::new(ErrorKind::CommandNotPolled)
                    .with_description(format!("Command {:?} of device {:?} has no history recorded", 
                        command.name(), self.0.name)));
            }
            Ok(history)
        })))

    }

    /// The counterpart of [`Self::command_history`] for the attributes, see [`Attr::history`].
    fn attr_history(&self, name: Scalar<str>, max: usize) -> BoxFuture<'_, Result<Vec<RecordedResult<AttrRead<Value>>>>> {

        if let Err(err) = self.0.check_withdrawn() {
            return Box::pin(ready(Err(err)));
        }

        Box::pin(ready(self.0.find_attr(name.value()).and_then(|attr| {
            let history = attr.history(max);
            if history.is_empty() {
                return Err(Error::new(ErrorKind::AttrNotPolled)
                    .with_description(format!("Attribute {:?} of device {:?} has no history recorded", 
                        attr.name(), self.0.name)));
            }
            Ok(history)
        })))

    }

    // The subscriptions are the server's, which keeps them for as long as the device is
    // served under that name, see [`Server::events`]. Only an attribute the device has can
    // be subscribed to, the state and the status included, as cppTango allows, and only to
    // an event something sends, see [`Attr::check_push_event`].

    fn subscribe_attr_value(&self, _admin: Arc<dyn BackendDev>, name: Scalar<str>, event: AttrValueEvent) -> BoxFuture<'_, Result<BoxStream<'static, Result<AttrRead<Value>>>>> {

        if let Err(err) = self.0.check_withdrawn() {
            return Box::pin(ready(Err(err)));
        }

        Box::pin(ready(self.0.find_attr(name.value()).and_then(|target| {
            target.check_push_event(&self.0, event)?;
            Ok(self.0.server().subscribe_attr_value(&self.0.name, target.name(), event))
        })))

    }

    fn subscribe_attr_ready(&self, _admin: Arc<dyn BackendDev>, name: Scalar<str>) -> BoxFuture<'_, Result<BoxStream<'static, Result<AttrReady>>>> {

        if let Err(err) = self.0.check_withdrawn() {
            return Box::pin(ready(Err(err)));
        }

        Box::pin(ready(self.0.find_attr(name.value()).and_then(|target| {
            target.check_data_ready(&self.0)?;
            Ok(self.0.server().subscribe_attr_ready(&self.0.name, target.name()))
        })))

    }

    fn subscribe_attr_config(&self, _admin: Arc<dyn BackendDev>, name: Scalar<str>) -> BoxFuture<'_, Result<BoxStream<'static, Result<AttrInfo>>>> {

        if let Err(err) = self.0.check_withdrawn() {
            return Box::pin(ready(Err(err)));
        }

        Box::pin(ready(self.0.find_attr(name.value()).map(|target| {
            self.0.server().subscribe_attr_config(&self.0.name, target.name())
        })))

    }

}

/// What a name a request gave resolved to, which names itself, or the name itself when it
/// resolved to nothing, along with the reason: the very name the request gave then.
type Resolved<T> = std::result::Result<T, Named<Error>>;

fn resolved<T>(name: Scalar<str>, res: Result<T>) -> Resolved<T> {
    res.map_err(|err| Named::new(name, err))
}

/// Every name a request gave, each a scalar of its own sharing them.
fn scalars(names: &Spectrum<str>) -> impl Iterator<Item = Scalar<str>> + '_ {
    (0..names.len()).map(|index| names.get_scalar([index]).expect("a name at every index"))
}


#[cfg(test)]
mod tests {

    use std::sync::Mutex;
    use std::thread;
    use std::time::Duration;

    use futures::channel::oneshot;
    use futures::executor::block_on;
    use futures::{FutureExt, StreamExt};

    use crate::dev::{ToNames, WriteAttrSpec};
    use crate::value::FromValue;

    use super::*;
    use super::super::def::{AttrDef, ClassDef, CommandDef};
    use super::super::server::ServerBuilder;
    use crate::testing::TestContext;

    /// A server with a single device of that class, which is added but not synchronized
    /// yet, on a database listing no other device, the server being served by the context.
    fn serve<M: Model>(def: ClassDef<M>) -> (TestContext, Arc<DevImpl<M>>) {
        let ctx = TestContext::new().unwrap();
        ctx.database().add_server("Test/test", &[]).unwrap();
        ctx.database().add_dev("test/dev/1", &def.name, "Test/test").unwrap();
        let server = ctx.serve(ServerBuilder::new("Test", "test")).unwrap();
        let class = server.add_class(def).unwrap();
        let dev = class.add_dev("test/dev/1").unwrap();
        let dev = DevImpl::from_arc(dev);
        drop(class);
        (ctx, dev)
    }

    #[test]
    fn push_event() {

        let (ctx, dev) = serve(model_def());
        let server = ctx.server("Test/test").unwrap();
        server.sync().unwrap();
        let mut state = server.subscribe_attr_value("test/dev/1", "State", AttrValueEvent::Change);
        let mut count = server.subscribe_attr_value("test/dev/1", "Count", AttrValueEvent::Change);

        thread::scope(|scope| {
            scope.spawn(|| dev.0.push_change_event("state", DevState::Moving).unwrap());
        });

        let read = block_on(state.next()).unwrap().unwrap();
        assert_eq!(read.value, Some(Value::from(DevState::Moving)));
        assert_eq!(read.w_value, None);
        assert_eq!(read.quality, AttrQuality::Valid);

        // A writable attribute is pushed with its set point, as it is read.
        dev.0.set_attr_w_value("Count", 5).unwrap();
        dev.0.push_change_event("Count", 3).unwrap();
        let read = block_on(count.next()).unwrap().unwrap();
        assert_eq!(read.value, Some(Value::from(3)));
        assert_eq!(read.w_value, Some(Value::from(5)));

        assert!(dev.0.push_change_event("Missing", 1.0).is_err());

        // A reading with no read value, or a State of another quality than valid, is refused.
        let mut empty = read_now(3);
        empty.value = None;
        assert_eq!(dev.0.push_attr_event("Count", AttrValueEvent::Change, Ok(empty)).unwrap_err().kind(), ErrorKind::InvalidData);
        let alarm = read_now(DevState::Moving).with_quality(AttrQuality::Alarm);
        assert_eq!(dev.0.push_attr_event("State", AttrValueEvent::Change, Ok(alarm)).unwrap_err().kind(), ErrorKind::InvalidData);

        // An event the attribute does not declare it pushes is refused.
        let err = dev.0.push_archive_event("Count", 3).unwrap_err();
        assert_eq!(err.kind(), ErrorKind::Reason(reason::API_AttributePollingNotStarted));
        dev.0.push_user_event("Count", 3).unwrap();

    }

    #[test]
    fn set_attr_config() {

        let (ctx, dev) = serve(model_def());
        let server = ctx.server("Test/test").unwrap();
        server.sync().unwrap();
        let mut conf = server.subscribe_attr_config("test/dev/1", "Count");

        let mut config = dev.0.get_attr("Count").unwrap().config();
        config.label = Some("Counter".into());
        dev.0.set_attr_config("count", &config).unwrap();

        assert_eq!(dev.0.get_attr("Count").unwrap().config().label.as_deref(), Some("Counter"));
        let info = block_on(conf.next()).unwrap().unwrap();
        assert_eq!(info.config.label.as_deref(), Some("Counter"));

        // Nothing changed, nothing pushed.
        dev.0.set_attr_config("Count", &config).unwrap();
        assert!(conf.next().now_or_never().is_none());

    }

    /// A read from the cache is answered with the latest reading recorded while it is
    /// valid, the State command shares the history of its attribute, and a history is
    /// given oldest first.
    #[test]
    fn attr_history() {

        let (_ctx, dev) = serve(model_def());
        dev.0.server().sync().unwrap();

        let read = |name: &str, source| block_on(dev.read_attr(name.to_names(), source)).unwrap().pop().unwrap().value;

        let err = read("Count", DevSource::Cache).unwrap_err();
        assert_eq!(err.kind(), ErrorKind::AttrNotPolled);
        assert!(read("Count", DevSource::CacheDev).is_ok());
        assert_eq!(block_on(dev.attr_history(Scalar::from("Count"), 10)).unwrap_err().kind(), ErrorKind::AttrNotPolled);

        let count = dev.0.get_attr("Count").unwrap();
        count.push_history(Ok(read_now(5)), None).unwrap();
        count.push_history(Ok(read_now(6)), None).unwrap();
        assert_eq!(read("Count", DevSource::Cache).unwrap().value, Some(Value::from(6)));
        assert_eq!(read("Count", DevSource::Dev).unwrap().value, Some(Value::from(0)));

        let history = block_on(dev.attr_history(Scalar::from("Count"), 10)).unwrap();
        let values = history.into_iter().map(|entry| entry.value.unwrap().value.unwrap()).collect::<Vec<_>>();
        assert_eq!(values, [Value::from(5), Value::from(6)]);

        count.set_history_depth(1);
        assert_eq!(count.history(10).len(), 1);

        // An outdated reading is read again from the device, or refused from the cache alone.
        count.push_history(Ok(read_now(7)), Some(Duration::ZERO)).unwrap();
        assert_eq!(read("Count", DevSource::Cache).unwrap_err().kind(), ErrorKind::ValueOutdated);
        assert_eq!(read("Count", DevSource::CacheDev).unwrap().value, Some(Value::from(0)));

        count.clear_history();
        assert!(count.history(10).is_empty());

        // The State is only recorded valid, and its command answers from that history.
        let state = dev.0.get_attr("State").unwrap();
        let mut alarm = read_now(DevState::Fault);
        alarm.quality = AttrQuality::Alarm;
        assert!(state.push_history(Ok(alarm), None).is_err());
        let mut empty = read_now(DevState::Fault);
        empty.value = None;
        assert!(state.push_history(Ok(empty), None).is_err());
        state.push_history(Ok(read_now(DevState::Fault)), None).unwrap();
        let value = block_on(dev.command(Scalar::from("State"), Value::Void, DevSource::Cache)).unwrap();
        assert_eq!(DevState::try_from(value).unwrap(), DevState::Fault);
        let history = block_on(dev.command_history(Scalar::from("state"), 10)).unwrap();
        assert_eq!(history.len(), 1);

    }

    /// A command is answered from the cache the way an attribute is read from it, and only
    /// one taking no argument is recorded.
    #[test]
    fn command_history() {

        let (_ctx, dev) = serve(model_def());
        dev.0.server().sync().unwrap();

        let exec = |name: &str, source| block_on(dev.command(Scalar::from(name), Value::Void, source));

        assert_eq!(exec("GetCount", DevSource::Cache).unwrap_err().kind(), ErrorKind::CommandNotPolled);
        assert_eq!(exec("GetCount", DevSource::CacheDev).unwrap(), Value::from(0));
        assert_eq!(block_on(dev.command_history(Scalar::from("GetCount"), 10)).unwrap_err().kind(), ErrorKind::CommandNotPolled);

        let get_count = dev.0.get_command("GetCount").unwrap();
        get_count.push_history(Ok(Value::from(5)), None).unwrap();
        get_count.push_history(Err(Error::new(ErrorKind::TimedOut)), None).unwrap();
        assert_eq!(exec("GetCount", DevSource::Cache).unwrap_err().kind(), ErrorKind::TimedOut);
        assert_eq!(exec("GetCount", DevSource::Dev).unwrap(), Value::from(0));

        let history = block_on(dev.command_history(Scalar::from("getcount"), 10)).unwrap();
        assert_eq!(history.len(), 2);
        assert_eq!(history[0].value.as_ref().unwrap(), &Value::from(5));
        assert!(history[1].value.is_err());

        // An outdated result is executed again, or refused from the cache alone.
        get_count.push_history(Ok(Value::from(6)), Some(Duration::ZERO)).unwrap();
        assert_eq!(exec("GetCount", DevSource::Cache).unwrap_err().kind(), ErrorKind::ValueOutdated);
        assert_eq!(exec("GetCount", DevSource::CacheDev).unwrap(), Value::from(0));

        let incr = dev.0.get_command("Incr").unwrap();
        assert!(incr.push_history(Ok(Value::from(1)), None).is_err());
        assert!(dev.0.get_command("Init").unwrap().push_history(Ok(Value::Void), None).is_err());
        assert_eq!(exec("Init", DevSource::Cache).unwrap_err().kind(), ErrorKind::CommandNotPolled);

        // The State command records in its attribute.
        dev.0.get_command("State").unwrap().push_history(Ok(DevState::Alarm.into()), None).unwrap();
        let read = dev.0.get_attr("State").unwrap().history(1).pop().unwrap().value.unwrap();
        assert_eq!(read.value, Some(Value::from(DevState::Alarm)));

    }

    /// A motor whose initialization always fails, which is served with no model.
    fn failing_def() -> ClassDef<Motor> {
        ClassDef::<Motor>::new_async_with("Motor", |_dev| Box::pin(async {
            Err(Error::new(ErrorKind::Internal).with_description("No hardware"))
        }))
            .command(CommandDef::new("GetCount").exec_async(Motor::get_count))
            .attr(AttrDef::new("Count").read_async(Motor::count))
    }

    /// A device that has no model answers each attribute of a read on its own: the State
    /// and the Status saying what it is, the others failing with why it has no model.
    #[test]
    fn no_model_reads() {

        let (_ctx, dev) = serve(failing_def());
        let read = |names: &[&str]| block_on(dev.read_attr(names.to_names(), DevSource::Dev)).unwrap();
        let value = |read: &NamedResult<AttrRead<Value>>| read.value.as_ref().unwrap().value.clone().unwrap();

        // Not initialized yet.
        let reads = read(&["State", "Status"]);
        assert_eq!(DevState::try_from(value(&reads[0])).unwrap(), DevState::Unknown);
        assert_eq!(Scalar::<str>::try_from(value(&reads[1])).unwrap().value(), MISSING_STATUS);

        dev.0.server().sync().unwrap();

        let reads = read(&["State", "Status", "Count", "Missing"]);
        assert_eq!(DevState::try_from(value(&reads[0])).unwrap(), DevState::Fault);
        assert!(Scalar::<str>::try_from(value(&reads[1])).unwrap().value().contains("No hardware"));
        assert_eq!(reads[2].value.as_ref().unwrap_err().kind(), ErrorKind::DevInitFailed);
        assert_eq!(reads[3].value.as_ref().unwrap_err().kind(), ErrorKind::AttrNotFound);

    }

    /// Commands executed together under a single borrow, each answered in its order, as a
    /// multiple read is, a device that has no model included.
    #[test]
    fn exec_many_command() {

        let exec = |dev: &DevImpl<Motor>, names: &[&str]| {
            let targets = names.iter()
                .map(|&name| (resolved(Scalar::from(name), dev.0.find_command(name)), Value::Void))
                .collect();
            block_on(dev.0.exec_many_command(targets, ReadCtx::new(&dev.0, true), DevSource::Dev)).unwrap()
        };

        let (_ctx, dev) = serve(model_def());
        dev.0.server().sync().unwrap();

        let results = exec(&dev, &["GetCount", "State", "Missing"]);
        assert_eq!(results[0].value.as_ref().unwrap(), &Value::from(0));
        assert_eq!(DevState::try_from(results[1].value.clone().unwrap()).unwrap(), DevState::On);
        assert_eq!(results[2].value.as_ref().unwrap_err().kind(), ErrorKind::CommandNotFound);

        let (_ctx, dev) = serve(failing_def());
        dev.0.server().sync().unwrap();

        let results = exec(&dev, &["GetCount", "State", "Status"]);
        assert_eq!(results[0].value.as_ref().unwrap_err().kind(), ErrorKind::DevInitFailed);
        assert_eq!(DevState::try_from(results[1].value.clone().unwrap()).unwrap(), DevState::Fault);
        assert!(Scalar::<str>::try_from(results[2].value.clone().unwrap()).unwrap().value().contains("No hardware"));

    }

    fn thread_name() -> String {
        thread::current().name().unwrap_or_default().to_string()
    }

    struct Motor {
        count: i32,
    }

    impl Motor {

        async fn init(_dev: &Dev<Self>) -> Result<Self> {
            Ok(Self { count: 0 })
        }

        async fn incr(&mut self, _dev: &Dev<Self>, arg: i32) -> Result<i32> {
            self.count += arg;
            Ok(self.count)
        }

        async fn count(&self, _dev: &Dev<Self>) -> Result<i32> {
            Ok(self.count)
        }

        async fn get_count(&self, _dev: &Dev<Self>, _: ()) -> Result<i32> {
            Ok(self.count)
        }

        async fn set_count(&mut self, _dev: &Dev<Self>, arg: i32) -> Result<()> {
            self.count = arg;
            Ok(())
        }

        async fn state(&self, _dev: &Dev<Self>) -> Result<DevState> {
            Ok(if self.count == 0 { DevState::On } else { DevState::Moving })
        }

        fn thread(&self, _dev: &Dev<Self>) -> Result<String> {
            Ok(thread_name())
        }

        async fn thread_async(&self, _dev: &Dev<Self>) -> Result<String> {
            Ok(thread_name())
        }

    }

    fn model_def() -> ClassDef<Motor> {
        ClassDef::new_async("Motor", Motor::init)
            .state_async(Motor::state)
            .push_state_events(Some(false), None)
            .command(CommandDef::new("Incr").exec_exclusive_async(Motor::incr))
            .command(CommandDef::new("GetCount").exec_async(Motor::get_count))
            .attr(AttrDef::new("Count").read_async(Motor::count).write_exclusive_async(Motor::set_count)
                .push_change_event(false))
            .attr(AttrDef::new("Thread").read(Motor::thread))
            .attr(AttrDef::new("ThreadAsync").read_async(Motor::thread_async))
    }

    fn read_one<T: FromValue>(dev: &DevImpl<Motor>, name: &str) -> T {
        let mut reads = block_on(dev.read_attr(name.to_names(), DevSource::Dev)).unwrap();
        let read = reads.pop().unwrap().value;
        T::from_value(read.unwrap().value.unwrap()).unwrap()
    }

    /// Every asynchronous handler, from the initialization to the state, run as a client
    /// asks for it.
    #[test]
    fn async_handlers() {

        let (_ctx, dev) = serve(model_def());
        dev.0.server().sync().unwrap();

        assert_eq!(block_on(dev.state()).unwrap(), DevState::On);
        let count = block_on(dev.command(Scalar::from("Incr"), Value::from(3), DevSource::Dev)).unwrap();
        assert_eq!(i32::from_value(count).unwrap(), 3);
        assert_eq!(block_on(dev.state()).unwrap(), DevState::Moving);

        let writes = block_on(dev.write_attr(("Count", 7).into_values())).unwrap();
        assert!(writes.into_iter().all(|write| write.is_ok()));
        assert_eq!(read_one::<i32>(&dev, "Count"), 7);

        // The Init command builds the model again.
        block_on(dev.command(Scalar::from("Init"), Value::Void, DevSource::Dev)).unwrap();
        assert_eq!(read_one::<i32>(&dev, "Count"), 0);

    }

    /// A name that resolves to nothing is answered with the very name the request gave,
    /// sharing its octets, and a known one with the name of its definition.
    #[test]
    fn unknown_attr_name() {

        let (_ctx, dev) = serve(model_def());
        dev.0.server().sync().unwrap();

        let names = Spectrum::from(&["count", "Nothing"][..]);
        let reads = block_on(dev.read_attr(Some(names.clone()), DevSource::Dev)).unwrap();

        assert_eq!(reads[0].name.value(), "Count");
        assert!(reads[0].value.is_ok());

        assert_eq!(reads[1].name.value().as_ptr(), names.get([1]).unwrap().as_ptr());
        assert_eq!(reads[1].value.as_ref().unwrap_err().kind(), ErrorKind::AttrNotFound);

    }

    /// A request runs on the pool as soon as one of its handlers is synchronous, the
    /// asynchronous ones being blocked on there, and is polled where it is awaited
    /// otherwise.
    #[test]
    fn request_threads() {

        let (_ctx, dev) = serve(model_def());
        dev.0.server().sync().unwrap();

        let on_pool = |name: &str| name.starts_with("pool-");
        assert!(on_pool(&read_one::<String>(&dev, "Thread")));
        assert!(!on_pool(&read_one::<String>(&dev, "ThreadAsync")));

        let reads = block_on(dev.read_attr(["ThreadAsync", "Thread"].to_names(), DevSource::Dev)).unwrap();
        for read in reads {
            assert!(on_pool(&String::from_value(read.value.unwrap().value.unwrap()).unwrap()));
        }

    }

    /// The model stays locked across the awaits of an asynchronous handler: a request
    /// arriving meanwhile waits for it.
    #[test]
    fn lock_across_await() {

        let (tx, rx) = oneshot::channel::<()>();
        let gate = Mutex::new(Some(rx));
        let def = model_def()
            .command(CommandDef::new("Wait").exec_exclusive_async_with(move |_model: &mut Motor, _dev, _input| {
                let rx = gate.lock().unwrap().take();
                Box::pin(async move {
                    if let Some(rx) = rx {
                        let _ = rx.await;
                    }
                    Ok(Value::Void)
                })
            }));

        let (_ctx, dev) = serve(def);
        dev.0.server().sync().unwrap();

        let mut wait = dev.command(Scalar::from("Wait"), Value::Void, DevSource::Dev);
        assert!((&mut wait).now_or_never().is_none());
        let mut read = dev.read_attr("Count".to_names(), DevSource::Dev);
        assert!((&mut read).now_or_never().is_none());

        tx.send(()).unwrap();
        block_on(wait).unwrap();
        assert!(block_on(read).unwrap().pop().unwrap().value.is_ok());

    }

    /// A device whose asynchronous initialization failed keeps the error, and gives it
    /// instead of running anything.
    #[test]
    fn async_init_failed() {

        let def = ClassDef::<Motor>::new_async_with("Motor", |_dev| Box::pin(async {
            Err(Error::new(ErrorKind::Internal).with_description("No hardware"))
        }));

        let (_ctx, dev) = serve(def);
        assert_eq!(block_on(dev.state()).unwrap(), DevState::Unknown);
        dev.0.server().sync().unwrap();

        assert_eq!(block_on(dev.state()).unwrap(), DevState::Fault);
        assert!(block_on(dev.status()).unwrap().value().contains("No hardware"));
        assert!(block_on(dev.command(Scalar::from("Init"), Value::Void, DevSource::Dev)).is_err());

    }

}