Fix gRPC `BoardList*` methods concurrency issues (#1804)

* Improved streaming of pluggable-discoveries events (WIP)

Now the DiscoveryManager is able to start the discoveries and add/remove
them in a thread-safe way. Also the watchers may connect and disconnect
seamlessly at any time, the incoming events from the discovery are
broadcasted correctly to each active watcher.

This refactoring dramatically simplifies the DiscoveryManager design.

* Added discovery id in discovery.Event struct

* Cache active ports and transmit them when a new watcher connects

* Correctly handle discovery cleanup

* Fixed wrong test

* Correctly handle discovery cleanup and re-add

* Added some doc comments in the source code

* Move Unlock under defer

* Factored subrotuine into a function

it will be useful in the next commits.

* Do not cache ports in the DiscoveryClient

there is already a cache in the DiscoveryManager there is no need to
duplicate it.

* Discovery: eventChan must be protected by mutex when doing START_SYNC

otherwise the discovery may send some events before the eventChan is
setup (and those events will be lost)

* Increased error level for logging watchers that lags

* Updated discvoery_client to the latest API

* Report discovery start errors

* Update arduino/discovery/discovery_client/main.go
Co-authored-by: Umberto Baldi <34278123+umbynos@users.noreply.github.com>
Co-authored-by: Umberto Baldi <34278123+umbynos@users.noreply.github.com>

arduino/cores/packagemanager/package_manager_test.go

@@ -329,16 +329,14 @@ func TestPackageManagerClear(t *testing.T) {
 	packageManager := packagemanager.NewPackageManager(customHardware, customHardware, customHardware, customHardware, "test")
 	packageManager.LoadHardwareFromDirectory(customHardware)
 
-	// Creates another PackageManager but don't load the hardware
-	emptyPackageManager := packagemanager.NewPackageManager(customHardware, customHardware, customHardware, customHardware, "test")
+	// Check that the hardware is loaded
+	require.NotEmpty(t, packageManager.Packages)
 
-	// Verifies they're not equal
-	require.NotEqual(t, packageManager, emptyPackageManager)
-
-	// Clear the first PackageManager that contains loaded hardware
+	// Clear the package manager
 	packageManager.Clear()
-	// Verifies both PackageManagers are now equal
-	require.Equal(t, packageManager, emptyPackageManager)
+
+	// Check that the hardware is cleared
+	require.Empty(t, packageManager.Packages)
 }
 
 func TestFindToolsRequiredFromPlatformRelease(t *testing.T) {

arduino/discovery/discovery.go

@@ -57,7 +57,6 @@ type PluggableDiscovery struct {
 	incomingMessagesError error
 	state                 int
 	eventChan             chan<- *Event
-	cachedPorts           map[string]*Port
 }
 
 type discoveryMessage struct {
@@ -121,8 +120,9 @@ func (p *Port) String() string {
 
 // Event is a pluggable discovery event
 type Event struct {
-	Type string
-	Port *Port
+	Type        string
+	Port        *Port
+	DiscoveryID string
 }
 
 // New create and connect to the given pluggable discovery
@@ -131,7 +131,6 @@ func New(id string, args ...string) *PluggableDiscovery {
 		id:          id,
 		processArgs: args,
 		state:       Dead,
-		cachedPorts: map[string]*Port{},
 	}
 }
 
@@ -176,9 +175,8 @@ func (disc *PluggableDiscovery) jsonDecodeLoop(in io.Reader, outChan chan<- *dis
 				return
 			}
 			disc.statusMutex.Lock()
-			disc.cachedPorts[msg.Port.Address+"|"+msg.Port.Protocol] = msg.Port
 			if disc.eventChan != nil {
-				disc.eventChan <- &Event{"add", msg.Port}
+				disc.eventChan <- &Event{"add", msg.Port, disc.GetID()}
 			}
 			disc.statusMutex.Unlock()
 		} else if msg.EventType == "remove" {
@@ -187,9 +185,8 @@ func (disc *PluggableDiscovery) jsonDecodeLoop(in io.Reader, outChan chan<- *dis
 				return
 			}
 			disc.statusMutex.Lock()
-			delete(disc.cachedPorts, msg.Port.Address+"|"+msg.Port.Protocol)
 			if disc.eventChan != nil {
-				disc.eventChan <- &Event{"remove", msg.Port}
+				disc.eventChan <- &Event{"remove", msg.Port, disc.GetID()}
 			}
 			disc.statusMutex.Unlock()
 		} else {
@@ -276,10 +273,7 @@ func (disc *PluggableDiscovery) killProcess() error {
 	}
 	disc.statusMutex.Lock()
 	defer disc.statusMutex.Unlock()
-	if disc.eventChan != nil {
-		close(disc.eventChan)
-		disc.eventChan = nil
-	}
+	disc.stopSync()
 	disc.state = Dead
 	logrus.Infof("killed discovery %s process", disc.id)
 	return nil
@@ -366,13 +360,17 @@ func (disc *PluggableDiscovery) Stop() error {
 	}
 	disc.statusMutex.Lock()
 	defer disc.statusMutex.Unlock()
-	disc.cachedPorts = map[string]*Port{}
+	disc.stopSync()
+	disc.state = Idling
+	return nil
+}
+
+func (disc *PluggableDiscovery) stopSync() {
 	if disc.eventChan != nil {
+		disc.eventChan <- &Event{"stop", nil, disc.GetID()}
 		close(disc.eventChan)
 		disc.eventChan = nil
 	}
-	disc.state = Idling
-	return nil
 }
 
 // Quit terminates the discovery. No more commands can be accepted by the discovery.
@@ -409,6 +407,9 @@ func (disc *PluggableDiscovery) List() ([]*Port, error) {
 // The event channel must be consumed as quickly as possible since it may block the
 // discovery if it becomes full. The channel size is configurable.
 func (disc *PluggableDiscovery) StartSync(size int) (<-chan *Event, error) {
+	disc.statusMutex.Lock()
+	defer disc.statusMutex.Unlock()
+
 	if err := disc.sendCommand("START_SYNC\n"); err != nil {
 		return nil, err
 	}
@@ -423,29 +424,10 @@ func (disc *PluggableDiscovery) StartSync(size int) (<-chan *Event, error) {
 		return nil, errors.Errorf(tr("communication out of sync, expected '%[1]s', received '%[2]s'"), "OK", msg.Message)
 	}
 
-	disc.statusMutex.Lock()
-	defer disc.statusMutex.Unlock()
 	disc.state = Syncing
-	disc.cachedPorts = map[string]*Port{}
-	if disc.eventChan != nil {
-		// In case there is already an existing event channel in use we close it
-		// before creating a new one.
-		close(disc.eventChan)
-	}
+	// In case there is already an existing event channel in use we close it before creating a new one.
+	disc.stopSync()
 	c := make(chan *Event, size)
 	disc.eventChan = c
 	return c, nil
 }
-
-// ListCachedPorts returns a list of the available ports. The list is a cache of all the
-// add/remove events happened from the StartSync call and it will not consume any
-// resource from the underliying discovery.
-func (disc *PluggableDiscovery) ListCachedPorts() []*Port {
-	disc.statusMutex.Lock()
-	defer disc.statusMutex.Unlock()
-	res := []*Port{}
-	for _, port := range disc.cachedPorts {
-		res = append(res, port)
-	}
-	return res
-}

arduino/discovery/discovery_client/go.mod

@@ -7,6 +7,7 @@ replace github.com/arduino/arduino-cli => ../../..
 require (
 	github.com/arduino/arduino-cli v0.0.0-00010101000000-000000000000
 	github.com/gizak/termui/v3 v3.1.0
+	github.com/sirupsen/logrus v1.4.2
 )
 
 require (
@@ -20,7 +21,6 @@ require (
 	github.com/nsf/termbox-go v0.0.0-20190121233118-02980233997d // indirect
 	github.com/pkg/errors v0.9.1 // indirect
 	github.com/rivo/uniseg v0.2.0 // indirect
-	github.com/sirupsen/logrus v1.4.2 // indirect
 	golang.org/x/net v0.0.0-20210505024714-0287a6fb4125 // indirect
 	golang.org/x/sys v0.0.0-20220520151302-bc2c85ada10a // indirect
 	golang.org/x/text v0.3.6 // indirect

arduino/discovery/discovery_client/main.go

@@ -21,36 +21,28 @@ import (
 	"log"
 	"os"
 	"sort"
-	"time"
 
 	"github.com/arduino/arduino-cli/arduino/discovery"
+	"github.com/arduino/arduino-cli/arduino/discovery/discoverymanager"
 	ui "github.com/gizak/termui/v3"
 	"github.com/gizak/termui/v3/widgets"
+	"github.com/sirupsen/logrus"
 )
 
 func main() {
-	discoveries := []*discovery.PluggableDiscovery{}
-	discEvent := make(chan *discovery.Event)
+	logrus.SetLevel(logrus.ErrorLevel)
+	dm := discoverymanager.New()
 	for _, discCmd := range os.Args[1:] {
-		disc := discovery.New("", discCmd)
-		if err := disc.Run(); err != nil {
-			log.Fatal("Error starting discovery:", err)
-		}
-		if err := disc.Start(); err != nil {
-			log.Fatal("Error starting discovery:", err)
-		}
-		eventChan, err := disc.StartSync(10)
-		if err != nil {
-			log.Fatal("Error starting discovery:", err)
-		}
-		go func() {
-			for msg := range eventChan {
-				discEvent <- msg
-			}
-		}()
-		discoveries = append(discoveries, disc)
+		disc := discovery.New(discCmd, discCmd)
+		dm.Add(disc)
 	}
+	dm.Start()
 
+	activePorts := map[string]*discovery.Port{}
+	watcher, err := dm.Watch()
+	if err != nil {
+		log.Fatalf("failed to start discoveries: %v", err)
+	}
 	if err := ui.Init(); err != nil {
 		log.Fatalf("failed to initialize termui: %v", err)
 	}
@@ -66,15 +58,20 @@ func main() {
 	updateList := func() {
 		rows := []string{}
 		rows = append(rows, "Available ports list:")
-		for _, disc := range discoveries {
-			for i, port := range disc.ListCachedPorts() {
-				rows = append(rows, fmt.Sprintf(" [%04d] Address: %s", i, port.AddressLabel))
-				rows = append(rows, fmt.Sprintf("        Protocol: %s", port.ProtocolLabel))
-				keys := port.Properties.Keys()
-				sort.Strings(keys)
-				for _, k := range keys {
-					rows = append(rows, fmt.Sprintf("                  %s=%s", k, port.Properties.Get(k)))
-				}
+
+		ids := sort.StringSlice{}
+		for id := range activePorts {
+			ids = append(ids, id)
+		}
+		ids.Sort()
+		for _, id := range ids {
+			port := activePorts[id]
+			rows = append(rows, fmt.Sprintf("> Address: %s", port.AddressLabel))
+			rows = append(rows, fmt.Sprintf("  Protocol: %s", port.ProtocolLabel))
+			keys := port.Properties.Keys()
+			sort.Strings(keys)
+			for _, k := range keys {
+				rows = append(rows, fmt.Sprintf("                  %s=%s", k, port.Properties.Get(k)))
 			}
 		}
 		l.Rows = rows
@@ -123,20 +120,16 @@ out:
 				previousKey = e.ID
 			}
 
-		case <-discEvent:
+		case ev := <-watcher.Feed():
+			if ev.Type == "add" {
+				activePorts[ev.Port.Address+"|"+ev.Port.Protocol] = ev.Port
+			}
+			if ev.Type == "remove" {
+				delete(activePorts, ev.Port.Address+"|"+ev.Port.Protocol)
+			}
 			updateList()
 		}
 
 		ui.Render(l)
 	}
-
-	for _, disc := range discoveries {
-		disc.Quit()
-		fmt.Println("Discovery QUITed")
-		for disc.State() == discovery.Alive {
-			time.Sleep(time.Millisecond)
-		}
-		fmt.Println("Discovery correctly terminated")
-	}
-
 }

arduino/discovery/discoverymanager/discoverymanager.go

@@ -18,6 +18,7 @@ package discoverymanager
 import (
 	"fmt"
 	"sync"
+	"time"
 
 	"github.com/arduino/arduino-cli/arduino/discovery"
 	"github.com/arduino/arduino-cli/i18n"
@@ -25,11 +26,20 @@ import (
 	"github.com/sirupsen/logrus"
 )
 
-// DiscoveryManager is required to handle multiple pluggable-discovery that
-// may be shared across platforms
+// DiscoveryManager manages the many-to-many communication between all pluggable
+// discoveries and all watchers. Each PluggableDiscovery, once started, will
+// produce a sequence of "events". These events will be broadcasted to all
+// listening Watcher.
+// The DiscoveryManager will not start the discoveries until the Start method
+// is called.
 type DiscoveryManager struct {
-	discoveriesMutex sync.Mutex
-	discoveries      map[string]*discovery.PluggableDiscovery
+	discoveriesMutex   sync.Mutex
+	discoveries        map[string]*discovery.PluggableDiscovery // all registered PluggableDiscovery
+	discoveriesRunning bool                                     // set to true once discoveries are started
+	feed               chan *discovery.Event                    // all events will pass through this channel
+	watchersMutex      sync.Mutex
+	watchers           map[*PortWatcher]bool                  // all registered Watcher
+	watchersCache      map[string]map[string]*discovery.Event // this is a cache of all active ports
 }
 
 var tr = i18n.Tr
@@ -37,15 +47,24 @@ var tr = i18n.Tr
 // New creates a new DiscoveryManager
 func New() *DiscoveryManager {
 	return &DiscoveryManager{
-		discoveries: map[string]*discovery.PluggableDiscovery{},
+		discoveries:   map[string]*discovery.PluggableDiscovery{},
+		watchers:      map[*PortWatcher]bool{},
+		feed:          make(chan *discovery.Event, 50),
+		watchersCache: map[string]map[string]*discovery.Event{},
 	}
 }
 
 // Clear resets the DiscoveryManager to its initial state
 func (dm *DiscoveryManager) Clear() {
-	dm.QuitAll()
 	dm.discoveriesMutex.Lock()
 	defer dm.discoveriesMutex.Unlock()
+
+	if dm.discoveriesRunning {
+		for _, d := range dm.discoveries {
+			d.Quit()
+			logrus.Infof("Closed and removed discovery %s", d.GetID())
+		}
+	}
 	dm.discoveries = map[string]*discovery.PluggableDiscovery{}
 }
 
@@ -60,234 +79,200 @@ func (dm *DiscoveryManager) IDs() []string {
 	return ids
 }
 
-// Add adds a discovery to the list of managed discoveries
-func (dm *DiscoveryManager) Add(disc *discovery.PluggableDiscovery) error {
-	id := disc.GetID()
+// Start starts all the discoveries in this DiscoveryManager.
+// If the discoveries are already running, this function does nothing.
+func (dm *DiscoveryManager) Start() []error {
 	dm.discoveriesMutex.Lock()
 	defer dm.discoveriesMutex.Unlock()
-	if _, has := dm.discoveries[id]; has {
-		return errors.Errorf(tr("pluggable discovery already added: %s"), id)
+	if dm.discoveriesRunning {
+		return nil
 	}
-	dm.discoveries[id] = disc
-	return nil
-}
 
-// remove quits and deletes the discovery with specified id
-// from the discoveries managed by this DiscoveryManager
-func (dm *DiscoveryManager) remove(id string) {
-	dm.discoveriesMutex.Lock()
-	d := dm.discoveries[id]
-	delete(dm.discoveries, id)
-	dm.discoveriesMutex.Unlock()
-	d.Quit()
-	logrus.Infof("Closed and removed discovery %s", id)
-}
+	go func() {
+		// Send all events coming from the feed channel to all active watchers
+		for ev := range dm.feed {
+			dm.feedEvent(ev)
+		}
+	}()
+
+	errs := []error{}
+	var errsLock sync.Mutex
 
-// parallelize runs function f concurrently for each discovery.
-// Returns a list of errors returned by each call of f.
-func (dm *DiscoveryManager) parallelize(f func(d *discovery.PluggableDiscovery) error) []error {
 	var wg sync.WaitGroup
-	errChan := make(chan error)
-	dm.discoveriesMutex.Lock()
-	discoveries := []*discovery.PluggableDiscovery{}
 	for _, d := range dm.discoveries {
-		discoveries = append(discoveries, d)
-	}
-	dm.discoveriesMutex.Unlock()
-	for _, d := range discoveries {
 		wg.Add(1)
 		go func(d *discovery.PluggableDiscovery) {
-			defer wg.Done()
-			if err := f(d); err != nil {
-				errChan <- err
+			if err := dm.startDiscovery(d); err != nil {
+				errsLock.Lock()
+				errs = append(errs, err)
+				errsLock.Unlock()
 			}
+			wg.Done()
 		}(d)
 	}
+	wg.Wait()
+	dm.discoveriesRunning = true
 
-	// Wait in a goroutine to collect eventual errors running a discovery.
-	// When all goroutines that are calling discoveries are done close the errors chan.
-	go func() {
-		wg.Wait()
-		close(errChan)
-	}()
-
-	errs := []error{}
-	for err := range errChan {
-		errs = append(errs, err)
-	}
 	return errs
 }
 
-// RunAll the discoveries for this DiscoveryManager,
-// returns an error for each discovery failing to run
-func (dm *DiscoveryManager) RunAll() []error {
-	return dm.parallelize(func(d *discovery.PluggableDiscovery) error {
-		if d.State() != discovery.Dead {
-			// This discovery is already alive, nothing to do
-			return nil
-		}
+// Add adds a discovery to the list of managed discoveries
+func (dm *DiscoveryManager) Add(d *discovery.PluggableDiscovery) error {
+	dm.discoveriesMutex.Lock()
+	defer dm.discoveriesMutex.Unlock()
 
-		if err := d.Run(); err != nil {
-			dm.remove(d.GetID())
-			return fmt.Errorf(tr("discovery %[1]s process not started: %[2]w"), d.GetID(), err)
-		}
-		return nil
-	})
+	id := d.GetID()
+	if _, has := dm.discoveries[id]; has {
+		return errors.Errorf(tr("pluggable discovery already added: %s"), id)
+	}
+	dm.discoveries[id] = d
+
+	if dm.discoveriesRunning {
+		dm.startDiscovery(d)
+	}
+	return nil
 }
 
-// StartAll the discoveries for this DiscoveryManager,
-// returns an error for each discovery failing to start
-func (dm *DiscoveryManager) StartAll() []error {
-	return dm.parallelize(func(d *discovery.PluggableDiscovery) error {
-		state := d.State()
-		if state != discovery.Idling {
-			// Already started
-			return nil
-		}
-		if err := d.Start(); err != nil {
-			dm.remove(d.GetID())
-			return fmt.Errorf(tr("starting discovery %[1]s: %[2]w"), d.GetID(), err)
-		}
-		return nil
-	})
+// PortWatcher is a watcher for all discovery events (port connection/disconnection)
+type PortWatcher struct {
+	closeCB func()
+	feed    chan *discovery.Event
 }
 
-// StartSyncAll the discoveries for this DiscoveryManager,
-// returns an error for each discovery failing to start syncing
-func (dm *DiscoveryManager) StartSyncAll() (<-chan *discovery.Event, []error) {
-	eventSink := make(chan *discovery.Event, 5)
-	var wg sync.WaitGroup
-	errs := dm.parallelize(func(d *discovery.PluggableDiscovery) error {
-		state := d.State()
-		if state != discovery.Idling || state == discovery.Syncing {
-			// Already syncing
-			return nil
-		}
+// Feed returns the feed of events coming from the discoveries
+func (pw *PortWatcher) Feed() <-chan *discovery.Event {
+	return pw.feed
+}
 
-		eventCh, err := d.StartSync(5)
-		if err != nil {
-			dm.remove(d.GetID())
-			return fmt.Errorf(tr("start syncing discovery %[1]s: %[2]w"), d.GetID(), err)
-		}
+// Close closes the PortWatcher
+func (pw *PortWatcher) Close() {
+	pw.closeCB()
+}
 
-		wg.Add(1)
-		go func() {
-			for ev := range eventCh {
-				eventSink <- ev
-			}
-			wg.Done()
-		}()
-		return nil
-	})
+// Watch starts a watcher for all discovery events (port connection/disconnection).
+// The watcher must be closed when it is no longer needed with the Close method.
+func (dm *DiscoveryManager) Watch() (*PortWatcher, error) {
+	dm.Start()
+
+	watcher := &PortWatcher{
+		feed: make(chan *discovery.Event, 10),
+	}
+	watcher.closeCB = func() {
+		dm.watchersMutex.Lock()
+		defer dm.watchersMutex.Unlock()
+		delete(dm.watchers, watcher)
+		close(watcher.feed)
+	}
 	go func() {
-		wg.Wait()
-		eventSink <- &discovery.Event{Type: "quit"}
-		close(eventSink)
+		dm.watchersMutex.Lock()
+		// When a watcher is started, send all the current active ports first...
+		for _, cache := range dm.watchersCache {
+			for _, ev := range cache {
+				watcher.feed <- ev
+			}
+		}
+		// ...and after that add the watcher to the list of watchers receiving events
+		dm.watchers[watcher] = true
+		dm.watchersMutex.Unlock()
 	}()
-	return eventSink, errs
+	return watcher, nil
 }
 
-// StopAll the discoveries for this DiscoveryManager,
-// returns an error for each discovery failing to stop
-func (dm *DiscoveryManager) StopAll() []error {
-	return dm.parallelize(func(d *discovery.PluggableDiscovery) error {
-		state := d.State()
-		if state != discovery.Syncing && state != discovery.Running {
-			// Not running nor syncing, nothing to stop
-			return nil
+func (dm *DiscoveryManager) startDiscovery(d *discovery.PluggableDiscovery) (discErr error) {
+	defer func() {
+		// If this function returns an error log it
+		if discErr != nil {
+			logrus.Errorf("Discovery %s failed to run: %s", d.GetID(), discErr)
 		}
+	}()
 
-		if err := d.Stop(); err != nil {
-			dm.remove(d.GetID())
-			return fmt.Errorf(tr("stopping discovery %[1]s: %[2]w"), d.GetID(), err)
-		}
-		return nil
-	})
-}
+	if err := d.Run(); err != nil {
+		return fmt.Errorf(tr("discovery %[1]s process not started: %[2]w"), d.GetID(), err)
+	}
+	eventCh, err := d.StartSync(5)
+	if err != nil {
+		return fmt.Errorf("%s: %s", tr("starting discovery %s", d.GetID()), err)
+	}
 
-// QuitAll quits all the discoveries managed by this DiscoveryManager.
-// Returns an error for each discovery that fails quitting
-func (dm *DiscoveryManager) QuitAll() []error {
-	errs := dm.parallelize(func(d *discovery.PluggableDiscovery) error {
-		if d.State() == discovery.Dead {
-			// Stop! Stop! It's already dead!
-			return nil
+	go func() {
+		// Transfer all incoming events from this discovery to the feed channel
+		for ev := range eventCh {
+			dm.feed <- ev
 		}
-
-		d.Quit()
-		return nil
-	})
-	return errs
+	}()
+	return nil
 }
 
-// List returns a list of available ports detected from all discoveries
-// and a list of errors for those discoveries that returned one.
-func (dm *DiscoveryManager) List() ([]*discovery.Port, []error) {
-	var wg sync.WaitGroup
-	// Use this struct to avoid the need of two separate
-	// channels for ports and errors.
-	type listMsg struct {
-		Err  error
-		Port *discovery.Port
-	}
-	msgChan := make(chan listMsg)
-	dm.discoveriesMutex.Lock()
-	discoveries := []*discovery.PluggableDiscovery{}
-	for _, d := range dm.discoveries {
-		discoveries = append(discoveries, d)
-	}
-	dm.discoveriesMutex.Unlock()
-	for _, d := range discoveries {
-		wg.Add(1)
-		go func(d *discovery.PluggableDiscovery) {
-			defer wg.Done()
-			if d.State() != discovery.Running {
-				// Discovery is not running, it won't return anything
-				return
-			}
-			ports, err := d.List()
-			if err != nil {
-				msgChan <- listMsg{Err: fmt.Errorf(tr("listing ports from discovery %[1]s: %[2]w"), d.GetID(), err)}
+func (dm *DiscoveryManager) feedEvent(ev *discovery.Event) {
+	dm.watchersMutex.Lock()
+	defer dm.watchersMutex.Unlock()
+
+	sendToAllWatchers := func(ev *discovery.Event) {
+		// Send the event to all watchers
+		for watcher := range dm.watchers {
+			select {
+			case watcher.feed <- ev:
+				// OK
+			case <-time.After(time.Millisecond * 500):
+				// If the watcher is not able to process event fast enough
+				// remove the watcher from the list of watchers
+				logrus.Error("Watcher is not able to process events fast enough, removing it from the list of watchers")
+				delete(dm.watchers, watcher)
 			}
-			for _, p := range ports {
-				msgChan <- listMsg{Port: p}
+		}
+	}
+
+	if ev.Type == "stop" {
+		// Send remove events for all the cached ports of the terminating discovery
+		cache := dm.watchersCache[ev.DiscoveryID]
+		for _, addEv := range cache {
+			removeEv := &discovery.Event{
+				Type: "remove",
+				Port: &discovery.Port{
+					Address:       addEv.Port.Address,
+					AddressLabel:  addEv.Port.AddressLabel,
+					Protocol:      addEv.Port.Protocol,
+					ProtocolLabel: addEv.Port.ProtocolLabel},
+				DiscoveryID: addEv.DiscoveryID,
 			}
-		}(d)
+			sendToAllWatchers(removeEv)
+		}
+
+		// Remove the cache for the terminating discovery
+		delete(dm.watchersCache, ev.DiscoveryID)
+		return
 	}
 
-	go func() {
-		// Close the channel only after all goroutines are done
-		wg.Wait()
-		close(msgChan)
-	}()
+	sendToAllWatchers(ev)
 
-	ports := []*discovery.Port{}
-	errs := []error{}
-	for msg := range msgChan {
-		if msg.Err != nil {
-			errs = append(errs, msg.Err)
-		} else {
-			ports = append(ports, msg.Port)
-		}
+	// Cache the event for the discovery
+	cache := dm.watchersCache[ev.DiscoveryID]
+	if cache == nil {
+		cache = map[string]*discovery.Event{}
+		dm.watchersCache[ev.DiscoveryID] = cache
+	}
+	eventID := ev.Port.Address + "|" + ev.Port.Protocol
+	switch ev.Type {
+	case "add":
+		cache[eventID] = ev
+	case "remove":
+		delete(cache, eventID)
+	default:
+		logrus.Errorf("Unhandled event from discovery: %s", ev.Type)
 	}
-	return ports, errs
 }
 
-// ListCachedPorts return the current list of ports detected from all discoveries
-func (dm *DiscoveryManager) ListCachedPorts() []*discovery.Port {
+// List return the current list of ports detected from all discoveries
+func (dm *DiscoveryManager) List() []*discovery.Port {
+	dm.Start()
+
 	res := []*discovery.Port{}
-	dm.discoveriesMutex.Lock()
-	discoveries := []*discovery.PluggableDiscovery{}
-	for _, d := range dm.discoveries {
-		discoveries = append(discoveries, d)
-	}
-	dm.discoveriesMutex.Unlock()
-	for _, d := range discoveries {
-		if d.State() != discovery.Syncing {
-			// Discovery is not syncing
-			continue
+	dm.watchersMutex.Lock()
+	defer dm.watchersMutex.Unlock()
+	for _, cache := range dm.watchersCache {
+		for _, ev := range cache {
+			res = append(res, ev.Port)
 		}
-		res = append(res, d.ListCachedPorts()...)
 	}
 	return res
 }

cli/arguments/completion.go

@@ -178,7 +178,7 @@ func GetInstallableLibs() []string {
 func GetConnectedBoards() []string {
 	inst := instance.CreateAndInit()
 
-	list, _ := board.List(&rpc.BoardListRequest{
+	list, _, _ := board.List(&rpc.BoardListRequest{
 		Instance: inst,
 	})
 	var res []string

cli/arguments/port.go

@@ -106,31 +106,16 @@ func (p *Port) GetPort(instance *rpc.Instance, sk *sketch.Sketch) (*discovery.Po
 		return nil, errors.New("invalid instance")
 	}
 	dm := pm.DiscoveryManager()
-	if errs := dm.RunAll(); len(errs) == len(dm.IDs()) {
-		// All discoveries failed to run, we can't do anything
-		return nil, fmt.Errorf("%v", errs)
-	} else if len(errs) > 0 {
-		// If only some discoveries failed to run just tell the user and go on
-		for _, err := range errs {
-			feedback.Error(err)
-		}
-	}
-	eventChan, errs := dm.StartSyncAll()
-	if len(errs) > 0 {
-		return nil, fmt.Errorf("%v", errs)
+	watcher, err := dm.Watch()
+	if err != nil {
+		return nil, err
 	}
-
-	defer func() {
-		// Quit all discoveries at the end.
-		if errs := dm.QuitAll(); len(errs) > 0 {
-			logrus.Errorf("quitting discoveries when getting port metadata: %v", errs)
-		}
-	}()
+	defer watcher.Close()
 
 	deadline := time.After(p.timeout.Get())
 	for {
 		select {
-		case portEvent := <-eventChan:
+		case portEvent := <-watcher.Feed():
 			if portEvent.Type != "add" {
 				continue
 			}
@@ -161,7 +146,7 @@ func (p *Port) GetSearchTimeout() time.Duration {
 // discovered Port object together with the FQBN. If the port does not match
 // exactly 1 board,
 func (p *Port) DetectFQBN(inst *rpc.Instance) (string, *rpc.Port) {
-	detectedPorts, err := board.List(&rpc.BoardListRequest{
+	detectedPorts, _, err := board.List(&rpc.BoardListRequest{
 		Instance: inst,
 		Timeout:  p.timeout.Get().Milliseconds(),
 	})

cli/board/list.go

@@ -64,22 +64,26 @@ func runListCommand(cmd *cobra.Command, args []string) {
 		os.Exit(0)
 	}
 
-	ports, err := board.List(&rpc.BoardListRequest{
+	ports, discvoeryErrors, err := board.List(&rpc.BoardListRequest{
 		Instance: inst,
 		Timeout:  timeoutArg.Get().Milliseconds(),
 	})
 	if err != nil {
 		feedback.Errorf(tr("Error detecting boards: %v"), err)
 	}
+	for _, err := range discvoeryErrors {
+		feedback.Errorf(tr("Error starting discovery: %v"), err)
+	}
 	feedback.PrintResult(result{ports})
 }
 
 func watchList(cmd *cobra.Command, inst *rpc.Instance) {
-	eventsChan, err := board.Watch(inst.Id, nil)
+	eventsChan, closeCB, err := board.Watch(inst.Id)
 	if err != nil {
 		feedback.Errorf(tr("Error detecting boards: %v"), err)
 		os.Exit(errorcodes.ErrNetwork)
 	}
+	defer closeCB()
 
 	// This is done to avoid printing the header each time a new event is received
 	if feedback.GetFormat() == feedback.Text {

commands/board/list.go

@@ -16,6 +16,7 @@
 package board
 
 import (
+	"context"
 	"encoding/json"
 	"fmt"
 	"io/ioutil"
@@ -176,32 +177,21 @@ func identify(pm *packagemanager.PackageManager, port *discovery.Port) ([]*rpc.B
 // List returns a list of boards found by the loaded discoveries.
 // In case of errors partial results from discoveries that didn't fail
 // are returned.
-func List(req *rpc.BoardListRequest) (r []*rpc.DetectedPort, e error) {
+func List(req *rpc.BoardListRequest) (r []*rpc.DetectedPort, discoveryStartErrors []error, e error) {
 	pm := commands.GetPackageManager(req.GetInstance().Id)
 	if pm == nil {
-		return nil, &arduino.InvalidInstanceError{}
+		return nil, nil, &arduino.InvalidInstanceError{}
 	}
 
 	dm := pm.DiscoveryManager()
-	if errs := dm.RunAll(); len(errs) > 0 {
-		return nil, &arduino.UnavailableError{Message: tr("Error starting board discoveries"), Cause: fmt.Errorf("%v", errs)}
-	}
-	if errs := dm.StartAll(); len(errs) > 0 {
-		return nil, &arduino.UnavailableError{Message: tr("Error starting board discoveries"), Cause: fmt.Errorf("%v", errs)}
-	}
-	defer func() {
-		if errs := dm.StopAll(); len(errs) > 0 {
-			logrus.Error(errs)
-		}
-	}()
+	discoveryStartErrors = dm.Start()
 	time.Sleep(time.Duration(req.GetTimeout()) * time.Millisecond)
 
 	retVal := []*rpc.DetectedPort{}
-	ports, errs := pm.DiscoveryManager().List()
-	for _, port := range ports {
+	for _, port := range dm.List() {
 		boards, err := identify(pm, port)
 		if err != nil {
-			return nil, err
+			return nil, discoveryStartErrors, err
 		}
 
 		// boards slice can be empty at this point if neither the cores nor the
@@ -212,92 +202,49 @@ func List(req *rpc.BoardListRequest) (r []*rpc.DetectedPort, e error) {
 		}
 		retVal = append(retVal, b)
 	}
-	if len(errs) > 0 {
-		return retVal, &arduino.UnavailableError{Message: tr("Error getting board list"), Cause: fmt.Errorf("%v", errs)}
-	}
-	return retVal, nil
+	return retVal, discoveryStartErrors, nil
 }
 
 // Watch returns a channel that receives boards connection and disconnection events.
-// The discovery process can be interrupted by sending a message to the interrupt channel.
-func Watch(instanceID int32, interrupt <-chan bool) (<-chan *rpc.BoardListWatchResponse, error) {
+// It also returns a callback function that must be used to stop and dispose the watch.
+func Watch(instanceID int32) (<-chan *rpc.BoardListWatchResponse, func(), error) {
 	pm := commands.GetPackageManager(instanceID)
 	dm := pm.DiscoveryManager()
 
-	runErrs := dm.RunAll()
-	if len(runErrs) == len(dm.IDs()) {
-		// All discoveries failed to run, we can't do anything
-		return nil, &arduino.UnavailableError{Message: tr("Error starting board discoveries"), Cause: fmt.Errorf("%v", runErrs)}
+	watcher, err := dm.Watch()
+	if err != nil {
+		return nil, nil, err
 	}
 
-	eventsChan, errs := dm.StartSyncAll()
-	if len(runErrs) > 0 {
-		errs = append(runErrs, errs...)
-	}
+	ctx, cancel := context.WithCancel(context.Background())
+	go func() {
+		<-ctx.Done()
+		watcher.Close()
+	}()
 
 	outChan := make(chan *rpc.BoardListWatchResponse)
-
 	go func() {
 		defer close(outChan)
-		for _, err := range errs {
-			outChan <- &rpc.BoardListWatchResponse{
-				EventType: "error",
-				Error:     err.Error(),
+		for event := range watcher.Feed() {
+			port := &rpc.DetectedPort{
+				Port: event.Port.ToRPC(),
 			}
-		}
-		for {
-			select {
-			case event := <-eventsChan:
-				if event.Type == "quit" {
-					// The discovery manager has closed its event channel because it's
-					// quitting all the discovery processes that are running, this
-					// means that the events channel we're listening from won't receive any
-					// more events.
-					// Handling this case is necessary when the board watcher is running and
-					// the instance being used is reinitialized since that quits all the
-					// discovery processes and reset the discovery manager. That would leave
-					// this goroutine listening forever on a "dead" channel and might even
-					// cause panics.
-					// This message avoid all this issues.
-					// It will be the client's task restarting the board watcher if necessary,
-					// this host won't attempt restarting it.
-					outChan <- &rpc.BoardListWatchResponse{
-						EventType: event.Type,
-					}
-					return
-				}
-
-				port := &rpc.DetectedPort{
-					Port: event.Port.ToRPC(),
-				}
 
-				boardsError := ""
-				if event.Type == "add" {
-					boards, err := identify(pm, event.Port)
-					if err != nil {
-						boardsError = err.Error()
-					}
-					port.MatchingBoards = boards
-				}
-				outChan <- &rpc.BoardListWatchResponse{
-					EventType: event.Type,
-					Port:      port,
-					Error:     boardsError,
-				}
-			case <-interrupt:
-				for _, err := range dm.StopAll() {
-					// Discoveries that return errors have their process
-					// closed and are removed from the list of discoveries
-					// in the manager
-					outChan <- &rpc.BoardListWatchResponse{
-						EventType: "error",
-						Error:     tr("stopping discoveries: %s", err),
-					}
+			boardsError := ""
+			if event.Type == "add" {
+				boards, err := identify(pm, event.Port)
+				if err != nil {
+					boardsError = err.Error()
 				}
-				return
+				port.MatchingBoards = boards
+			}
+			outChan <- &rpc.BoardListWatchResponse{
+				EventType: event.Type,
+				Port:      port,
+				Error:     boardsError,
 			}
 		}
 	}()
 
-	return outChan, nil
+	return outChan, cancel, nil
 }

commands/daemon/daemon.go

@@ -36,9 +36,7 @@ import (
 	"github.com/arduino/arduino-cli/i18n"
 	rpc "github.com/arduino/arduino-cli/rpc/cc/arduino/cli/commands/v1"
 	"github.com/sirupsen/logrus"
-	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/metadata"
-	"google.golang.org/grpc/status"
 )
 
 // ArduinoCoreServerImpl FIXMEDOC
@@ -69,7 +67,7 @@ func (s *ArduinoCoreServerImpl) BoardDetails(ctx context.Context, req *rpc.Board
 
 // BoardList FIXMEDOC
 func (s *ArduinoCoreServerImpl) BoardList(ctx context.Context, req *rpc.BoardListRequest) (*rpc.BoardListResponse, error) {
-	ports, err := board.List(req)
+	ports, _, err := board.List(req)
 	if err != nil {
 		return nil, convertErrorToRPCStatus(err)
 	}
@@ -109,42 +107,35 @@ func (s *ArduinoCoreServerImpl) BoardListWatch(stream rpc.ArduinoCoreService_Boa
 		return err
 	}
 
-	interrupt := make(chan bool, 1)
+	eventsChan, closeWatcher, err := board.Watch(msg.Instance.Id)
+	if err != nil {
+		return convertErrorToRPCStatus(err)
+	}
+
 	go func() {
-		defer close(interrupt)
+		defer closeWatcher()
 		for {
 			msg, err := stream.Recv()
 			// Handle client closing the stream and eventual errors
 			if err == io.EOF {
 				logrus.Info("boards watcher stream closed")
-				interrupt <- true
-				return
-			} else if st, ok := status.FromError(err); ok && st.Code() == codes.Canceled {
-				logrus.Info("boards watcher interrupted by host")
 				return
-			} else if err != nil {
+			}
+			if err != nil {
 				logrus.Infof("interrupting boards watcher: %v", err)
-				interrupt <- true
 				return
 			}
 
 			// Message received, does the client want to interrupt?
 			if msg != nil && msg.Interrupt {
 				logrus.Info("boards watcher interrupted by client")
-				interrupt <- msg.Interrupt
 				return
 			}
 		}
 	}()
 
-	eventsChan, err := board.Watch(msg.Instance.Id, interrupt)
-	if err != nil {
-		return convertErrorToRPCStatus(err)
-	}
-
 	for event := range eventsChan {
-		err = stream.Send(event)
-		if err != nil {
+		if err := stream.Send(event); err != nil {
 			logrus.Infof("sending board watch message: %v", err)
 		}
 	}