lib/model: Use a single lock (#9275)

I'm tired of the fmut/pmut shenanigans. This consolidates both under one
lock; I'm not convinced there are any significant performance
differences with this approach since we're literally just protecting map
juggling...

- The locking goes away when we were already under an appropriate fmut
lock.
- Where we had fmut.RLock()+pmut.Lock() it gets upgraded to an
fmut.Lock().
- Otherwise s/pmut/fmut/.

In order to avoid diff noise for an important change I did not do the
following cleanups, which will be filed in a PR after this one, if
accepted:

- Renaming fmut to just mut
- Renaming methods that refer to being "PRLocked" and stuff like that
- Removing the no longer relevant deadlock detector
- Comments referring to pmut and locking sequences...

2 files changed, 48 insertions, 71 deletions

diff --git a/lib/model/model.go b/lib/model/model.go
index bd87968d9..0b6fc24b8 100644
--- a/lib/model/model.go
+++ b/lib/model/model.go
@@ -157,8 +157,7 @@ type model struct {
 	folderEncryptionPasswordTokens map[string][]byte                                      // folder -> encryption token (may be missing, and only for encryption type folders)
 	folderEncryptionFailures       map[string]map[protocol.DeviceID]error                 // folder -> device -> error regarding encryption consistency (may be missing)
 
-	// fields protected by pmut
-	pmut                sync.RWMutex
+	// fields also protected by fmut
 	connections         map[string]protocol.Connection // connection ID -> connection
 	deviceConnIDs       map[protocol.DeviceID][]string // device -> connection IDs (invariant: if the key exists, the value is len >= 1, with the primary connection at the start of the slice)
 	promotedConnID      map[protocol.DeviceID]string   // device -> latest promoted connection ID
@@ -238,8 +237,7 @@ func NewModel(cfg config.Wrapper, id protocol.DeviceID, ldb *db.Lowlevel, protec
 		folderEncryptionPasswordTokens: make(map[string][]byte),
 		folderEncryptionFailures:       make(map[string]map[protocol.DeviceID]error),
 
-		// fields protected by pmut
-		pmut:                sync.NewRWMutex(),
+		// ditto
 		connections:         make(map[string]protocol.Connection),
 		deviceConnIDs:       make(map[protocol.DeviceID][]string),
 		promotedConnID:      make(map[protocol.DeviceID]string),
@@ -312,13 +310,13 @@ func (m *model) initFolders(cfg config.Configuration) error {
 }
 
 func (m *model) closeAllConnectionsAndWait() {
-	m.pmut.RLock()
+	m.fmut.RLock()
 	closed := make([]chan struct{}, 0, len(m.connections))
 	for connID, conn := range m.connections {
 		closed = append(closed, m.closed[connID])
 		go conn.Close(errStopped)
 	}
-	m.pmut.RUnlock()
+	m.fmut.RUnlock()
 	for _, c := range closed {
 		<-c
 	}
@@ -338,7 +336,6 @@ func (m *model) StartDeadlockDetector(timeout time.Duration) {
 	l.Infof("Starting deadlock detector with %v timeout", timeout)
 	detector := newDeadlockDetector(timeout, m.evLogger, m.fatal)
 	detector.Watch("fmut", m.fmut)
-	detector.Watch("pmut", m.pmut)
 }
 
 // Need to hold lock on m.fmut when calling this.
@@ -472,7 +469,6 @@ func (m *model) removeFolder(cfg config.FolderConfiguration) {
 	// We need to hold both fmut and pmut and must acquire locks in the same
 	// order always. (The locks can be *released* in any order.)
 	m.fmut.Lock()
-	m.pmut.RLock()
 
 	isPathUnique := true
 	for folderID, folderCfg := range m.folderCfgs {
@@ -498,7 +494,6 @@ func (m *model) removeFolder(cfg config.FolderConfiguration) {
 	})
 
 	m.fmut.Unlock()
-	m.pmut.RUnlock()
 
 	// Remove it from the database
 	db.DropFolder(m.db, cfg.ID)
@@ -563,15 +558,11 @@ func (m *model) restartFolder(from, to config.FolderConfiguration, cacheIgnoredF
 		m.addAndStartFolderLocked(to, fset, cacheIgnoredFiles)
 	}
 
-	// Care needs to be taken because we already hold fmut and the lock order
-	// must be the same everywhere. As fmut is acquired first, this is fine.
-	m.pmut.RLock()
 	runner, _ := m.folderRunners.Get(to.ID)
 	m.indexHandlers.Each(func(_ protocol.DeviceID, r *indexHandlerRegistry) error {
 		r.RegisterFolderState(to, fset, runner)
 		return nil
 	})
-	m.pmut.RUnlock()
 
 	var infoMsg string
 	switch {
@@ -603,15 +594,11 @@ func (m *model) newFolder(cfg config.FolderConfiguration, cacheIgnoredFiles bool
 	// Cluster configs might be received and processed before reaching this
 	// point, i.e. before the folder is started. If that's the case, start
 	// index senders here.
-	// Care needs to be taken because we already hold fmut and the lock order
-	// must be the same everywhere. As fmut is acquired first, this is fine.
-	m.pmut.RLock()
 	m.indexHandlers.Each(func(_ protocol.DeviceID, r *indexHandlerRegistry) error {
 		runner, _ := m.folderRunners.Get(cfg.ID)
 		r.RegisterFolderState(cfg, fset, runner)
 		return nil
 	})
-	m.pmut.RUnlock()
 
 	return nil
 }
@@ -645,11 +632,11 @@ func (m *model) UsageReportingStats(report *contract.Report, version int, previe
 		blockStatsMut.Unlock()
 
 		// Transport stats
-		m.pmut.RLock()
+		m.fmut.RLock()
 		for _, conn := range m.connections {
 			report.TransportStats[conn.Transport()]++
 		}
-		m.pmut.RUnlock()
+		m.fmut.RUnlock()
 
 		// Ignore stats
 		var seenPrefix [3]bool
@@ -736,8 +723,8 @@ type ConnectionInfo struct {
 
 // ConnectionStats returns a map with connection statistics for each device.
 func (m *model) ConnectionStats() map[string]interface{} {
-	m.pmut.RLock()
-	defer m.pmut.RUnlock()
+	m.fmut.RLock()
+	defer m.fmut.RUnlock()
 
 	res := make(map[string]interface{})
 	devs := m.cfg.Devices()
@@ -812,8 +799,6 @@ func (m *model) ConnectionStats() map[string]interface{} {
 func (m *model) DeviceStatistics() (map[protocol.DeviceID]stats.DeviceStatistics, error) {
 	m.fmut.RLock()
 	defer m.fmut.RUnlock()
-	m.pmut.RLock()
-	defer m.pmut.RUnlock()
 	res := make(map[protocol.DeviceID]stats.DeviceStatistics, len(m.deviceStatRefs))
 	for id, sr := range m.deviceStatRefs {
 		stats, err := sr.GetStatistics()
@@ -965,10 +950,10 @@ func (m *model) folderCompletion(device protocol.DeviceID, folder string) (Folde
 	}
 	defer snap.Release()
 
-	m.pmut.RLock()
+	m.fmut.RLock()
 	state := m.remoteFolderStates[device][folder]
 	downloaded := m.deviceDownloads[device].BytesDownloaded(folder)
-	m.pmut.RUnlock()
+	m.fmut.RUnlock()
 
 	need := snap.NeedSize(device)
 	need.Bytes -= downloaded
@@ -1191,9 +1176,9 @@ func (m *model) handleIndex(conn protocol.Connection, folder string, fs []protoc
 		return fmt.Errorf("%s: %w", folder, ErrFolderPaused)
 	}
 
-	m.pmut.RLock()
+	m.fmut.RLock()
 	indexHandler, ok := m.getIndexHandlerPRLocked(conn)
-	m.pmut.RUnlock()
+	m.fmut.RUnlock()
 	if !ok {
 		// This should be impossible, as an index handler is registered when
 		// we send a cluster config, and that is what triggers index
@@ -1306,9 +1291,9 @@ func (m *model) ClusterConfig(conn protocol.Connection, cm protocol.ClusterConfi
 		return err
 	}
 
-	m.pmut.Lock()
+	m.fmut.Lock()
 	m.remoteFolderStates[deviceID] = states
-	m.pmut.Unlock()
+	m.fmut.Unlock()
 
 	m.evLogger.Log(events.ClusterConfigReceived, ClusterConfigReceivedEventData{
 		Device: deviceID,
@@ -1317,11 +1302,11 @@ func (m *model) ClusterConfig(conn protocol.Connection, cm protocol.ClusterConfi
 	if len(tempIndexFolders) > 0 {
 		var connOK bool
 		var conn protocol.Connection
-		m.pmut.RLock()
+		m.fmut.RLock()
 		if connIDs, connIDOK := m.deviceConnIDs[deviceID]; connIDOK {
 			conn, connOK = m.connections[connIDs[0]]
 		}
-		m.pmut.RUnlock()
+		m.fmut.RUnlock()
 		// In case we've got ClusterConfig, and the connection disappeared
 		// from infront of our nose.
 		if connOK {
@@ -1354,11 +1339,8 @@ func (m *model) ensureIndexHandler(conn protocol.Connection) *indexHandlerRegist
 	deviceID := conn.DeviceID()
 	connID := conn.ConnectionID()
 
-	// We must acquire fmut first when acquiring both locks.
-	m.fmut.RLock()
-	defer m.fmut.RUnlock()
-	m.pmut.Lock()
-	defer m.pmut.Unlock()
+	m.fmut.Lock()
+	defer m.fmut.Unlock()
 
 	indexHandlerRegistry, ok := m.indexHandlers.Get(deviceID)
 	if ok && indexHandlerRegistry.conn.ConnectionID() == connID {
@@ -1650,13 +1632,13 @@ func (m *model) sendClusterConfig(ids []protocol.DeviceID) {
 		return
 	}
 	ccConns := make([]protocol.Connection, 0, len(ids))
-	m.pmut.RLock()
+	m.fmut.RLock()
 	for _, id := range ids {
 		if connIDs, ok := m.deviceConnIDs[id]; ok {
 			ccConns = append(ccConns, m.connections[connIDs[0]])
 		}
 	}
-	m.pmut.RUnlock()
+	m.fmut.RUnlock()
 	// Generating cluster-configs acquires fmut -> must happen outside of pmut.
 	for _, conn := range ccConns {
 		cm, passwords := m.generateClusterConfig(conn.DeviceID())
@@ -1893,10 +1875,10 @@ func (m *model) Closed(conn protocol.Connection, err error) {
 	connID := conn.ConnectionID()
 	deviceID := conn.DeviceID()
 
-	m.pmut.Lock()
+	m.fmut.Lock()
 	conn, ok := m.connections[connID]
 	if !ok {
-		m.pmut.Unlock()
+		m.fmut.Unlock()
 		return
 	}
 
@@ -1927,7 +1909,7 @@ func (m *model) Closed(conn protocol.Connection, err error) {
 		m.deviceConnIDs[deviceID] = remainingConns
 	}
 
-	m.pmut.Unlock()
+	m.fmut.Unlock()
 	if wait != nil {
 		<-wait
 	}
@@ -2029,9 +2011,9 @@ func (m *model) Request(conn protocol.Connection, folder, name string, _, size i
 
 	// Restrict parallel requests by connection/device
 
-	m.pmut.RLock()
+	m.fmut.RLock()
 	limiter := m.connRequestLimiters[deviceID]
-	m.pmut.RUnlock()
+	m.fmut.RUnlock()
 
 	// The requestResponse releases the bytes to the buffer pool and the
 	// limiters when its Close method is called.
@@ -2218,9 +2200,9 @@ func (m *model) GetMtimeMapping(folder string, file string) (fs.MtimeMapping, er
 
 // Connection returns if we are connected to the given device.
 func (m *model) ConnectedTo(deviceID protocol.DeviceID) bool {
-	m.pmut.RLock()
+	m.fmut.RLock()
 	_, ok := m.deviceConnIDs[deviceID]
-	m.pmut.RUnlock()
+	m.fmut.RUnlock()
 	return ok
 }
 
@@ -2353,7 +2335,7 @@ func (m *model) AddConnection(conn protocol.Connection, hello protocol.Hello) {
 	connID := conn.ConnectionID()
 	closed := make(chan struct{})
 
-	m.pmut.Lock()
+	m.fmut.Lock()
 
 	m.connections[connID] = conn
 	m.closed[connID] = closed
@@ -2384,7 +2366,7 @@ func (m *model) AddConnection(conn protocol.Connection, hello protocol.Hello) {
 		l.Infof(`Additional connection (+%d) for device %s at %s`, len(m.deviceConnIDs[deviceID])-1, deviceID.Short(), conn)
 	}
 
-	m.pmut.Unlock()
+	m.fmut.Unlock()
 
 	if (deviceCfg.Name == "" || m.cfg.Options().OverwriteRemoteDevNames) && hello.DeviceName != "" {
 		m.cfg.Modify(func(cfg *config.Configuration) {
@@ -2415,11 +2397,8 @@ func (m *model) scheduleConnectionPromotion() {
 // be called after adding new connections, and after closing a primary
 // device connection.
 func (m *model) promoteConnections() {
-	m.fmut.RLock() // for generateClusterConfigFRLocked
-	defer m.fmut.RUnlock()
-
-	m.pmut.Lock() // for most other things
-	defer m.pmut.Unlock()
+	m.fmut.Lock()
+	defer m.fmut.Unlock()
 
 	for deviceID, connIDs := range m.deviceConnIDs {
 		cm, passwords := m.generateClusterConfigFRLocked(deviceID)
@@ -2464,9 +2443,9 @@ func (m *model) DownloadProgress(conn protocol.Connection, folder string, update
 		return nil
 	}
 
-	m.pmut.RLock()
+	m.fmut.RLock()
 	downloads := m.deviceDownloads[deviceID]
-	m.pmut.RUnlock()
+	m.fmut.RUnlock()
 	downloads.Update(folder, updates)
 	state := downloads.GetBlockCounts(folder)
 
@@ -2511,8 +2490,8 @@ func (m *model) requestGlobal(ctx context.Context, deviceID protocol.DeviceID, f
 // ("primary") connection, which is dedicated to index data, and pick a
 // random one of the others.
 func (m *model) requestConnectionForDevice(deviceID protocol.DeviceID) (protocol.Connection, bool) {
-	m.pmut.RLock()
-	defer m.pmut.RUnlock()
+	m.fmut.RLock()
+	defer m.fmut.RUnlock()
 
 	connIDs, ok := m.deviceConnIDs[deviceID]
 	if !ok {
@@ -2903,12 +2882,10 @@ func (m *model) Availability(folder string, file protocol.FileInfo, block protoc
 	// get heavily modified on Close()), but also must acquire fmut before
 	// pmut. (The locks can be *released* in any order.)
 	m.fmut.RLock()
-	m.pmut.RLock()
-	defer m.pmut.RUnlock()
+	defer m.fmut.RUnlock()
 
 	fs, ok := m.folderFiles[folder]
 	cfg := m.folderCfgs[folder]
-	m.fmut.RUnlock()
 
 	if !ok {
 		return nil, ErrFolderMissing
@@ -2924,8 +2901,8 @@ func (m *model) Availability(folder string, file protocol.FileInfo, block protoc
 }
 
 func (m *model) availabilityInSnapshot(cfg config.FolderConfiguration, snap *db.Snapshot, file protocol.FileInfo, block protocol.BlockInfo) []Availability {
-	m.pmut.RLock()
-	defer m.pmut.RUnlock()
+	m.fmut.RLock()
+	defer m.fmut.RUnlock()
 	return m.availabilityInSnapshotPRlocked(cfg, snap, file, block)
 }
 
@@ -3113,9 +3090,9 @@ func (m *model) CommitConfiguration(from, to config.Configuration) bool {
 		}
 
 		if toCfg.MaxRequestKiB != fromCfg.MaxRequestKiB {
-			m.pmut.Lock()
+			m.fmut.Lock()
 			m.setConnRequestLimitersPLocked(toCfg)
-			m.pmut.Unlock()
+			m.fmut.Unlock()
 		}
 	}
 
@@ -3129,7 +3106,7 @@ func (m *model) CommitConfiguration(from, to config.Configuration) bool {
 	}
 	m.fmut.Unlock()
 
-	m.pmut.RLock()
+	m.fmut.RLock()
 	for _, id := range closeDevices {
 		delete(clusterConfigDevices, id)
 		if conns, ok := m.deviceConnIDs[id]; ok {
@@ -3146,7 +3123,7 @@ func (m *model) CommitConfiguration(from, to config.Configuration) bool {
 			}
 		}
 	}
-	m.pmut.RUnlock()
+	m.fmut.RUnlock()
 	// Generating cluster-configs acquires fmut -> must happen outside of pmut.
 	m.sendClusterConfig(clusterConfigDevices.AsSlice())
 
diff --git a/lib/model/model_test.go b/lib/model/model_test.go
index 3fbe60d57..5fa684521 100644
--- a/lib/model/model_test.go
+++ b/lib/model/model_test.go
@@ -902,13 +902,13 @@ func TestIssue5063(t *testing.T) {
 	defer cleanupModel(m)
 	defer cancel()
 
-	m.pmut.Lock()
+	m.fmut.Lock()
 	for _, c := range m.connections {
 		conn := c.(*fakeConnection)
 		conn.CloseCalls(func(_ error) {})
 		defer m.Closed(c, errStopped) // to unblock deferred m.Stop()
 	}
-	m.pmut.Unlock()
+	m.fmut.Unlock()
 
 	wg := sync.WaitGroup{}
 
@@ -2973,7 +2973,7 @@ func TestConnCloseOnRestart(t *testing.T) {
 	ci := &protocolmocks.ConnectionInfo{}
 	ci.ConnectionIDReturns(srand.String(16))
 	m.AddConnection(protocol.NewConnection(device1, br, nw, testutil.NoopCloser{}, m, ci, protocol.CompressionNever, nil, m.keyGen), protocol.Hello{})
-	m.pmut.RLock()
+	m.fmut.RLock()
 	if len(m.closed) != 1 {
 		t.Fatalf("Expected just one conn (len(m.closed) == %v)", len(m.closed))
 	}
@@ -2981,7 +2981,7 @@ func TestConnCloseOnRestart(t *testing.T) {
 	for _, c := range m.closed {
 		closed = c
 	}
-	m.pmut.RUnlock()
+	m.fmut.RUnlock()
 
 	waiter, err := w.RemoveDevice(device1)
 	if err != nil {
@@ -3074,12 +3074,12 @@ func TestDevicePause(t *testing.T) {
 	sub := m.evLogger.Subscribe(events.DevicePaused)
 	defer sub.Unsubscribe()
 
-	m.pmut.RLock()
+	m.fmut.RLock()
 	var closed chan struct{}
 	for _, c := range m.closed {
 		closed = c
 	}
-	m.pmut.RUnlock()
+	m.fmut.RUnlock()
 
 	pauseDevice(t, m.cfg, device1, true)