lib/model: Use a single lock (phase two: cleanup) (#9276)v1.27.2-rc.1

Cleanup after #9275.

This renames `fmut` -> `mut`, removes the deadlock detector and
associated plumbing, renames some things from `...PRLocked` to
`...RLocked` and similar, and updates comments.

Apart from the removal of the deadlock detection machinery, no
functional code changes... i.e. almost 100% diff noise, have fun
reviewing.

13 files changed, 210 insertions, 396 deletions

diff --git a/cmd/syncthing/main.go b/cmd/syncthing/main.go
index 7e2abcfdb..f321ebc8b 100644
--- a/cmd/syncthing/main.go
+++ b/cmd/syncthing/main.go
@@ -88,9 +88,6 @@ above.
  STTRACE           A comma separated string of facilities to trace. The valid
                    facility strings are listed below.
 
- STDEADLOCKTIMEOUT Used for debugging internal deadlocks; sets debug
-                   sensitivity. Use only under direction of a developer.
-
  STLOCKTHRESHOLD   Used for debugging internal deadlocks; sets debug
                    sensitivity.  Use only under direction of a developer.
 
@@ -173,7 +170,6 @@ type serveOptions struct {
 	// Debug options below
 	DebugDBIndirectGCInterval time.Duration `env:"STGCINDIRECTEVERY" help:"Database indirection GC interval"`
 	DebugDBRecheckInterval    time.Duration `env:"STRECHECKDBEVERY" help:"Database metadata recalculation interval"`
-	DebugDeadlockTimeout      int           `placeholder:"SECONDS" env:"STDEADLOCKTIMEOUT" help:"Used for debugging internal deadlocks"`
 	DebugGUIAssetsDir         string        `placeholder:"PATH" help:"Directory to load GUI assets from" env:"STGUIASSETS"`
 	DebugPerfStats            bool          `env:"STPERFSTATS" help:"Write running performance statistics to perf-$pid.csv (Unix only)"`
 	DebugProfileBlock         bool          `env:"STBLOCKPROFILE" help:"Write block profiles to block-$pid-$timestamp.pprof every 20 seconds"`
@@ -623,7 +619,6 @@ func syncthingMain(options serveOptions) {
 	}
 
 	appOpts := syncthing.Options{
-		DeadlockTimeoutS:     options.DebugDeadlockTimeout,
 		NoUpgrade:            options.NoUpgrade,
 		ProfilerAddr:         options.DebugProfilerListen,
 		ResetDeltaIdxs:       options.DebugResetDeltaIdxs,
@@ -634,10 +629,6 @@ func syncthingMain(options serveOptions) {
 	if options.Audit {
 		appOpts.AuditWriter = auditWriter(options.AuditFile)
 	}
-	if t := os.Getenv("STDEADLOCKTIMEOUT"); t != "" {
-		secs, _ := strconv.Atoi(t)
-		appOpts.DeadlockTimeoutS = secs
-	}
 	if dur, err := time.ParseDuration(os.Getenv("STRECHECKDBEVERY")); err == nil {
 		appOpts.DBRecheckInterval = dur
 	}
diff --git a/go.mod b/go.mod
index 4b32d7155..fcf52ae75 100644
--- a/go.mod
+++ b/go.mod
@@ -40,7 +40,6 @@ require (
 	github.com/prometheus/procfs v0.12.0 // indirect
 	github.com/quic-go/quic-go v0.40.0
 	github.com/rcrowley/go-metrics v0.0.0-20201227073835-cf1acfcdf475
-	github.com/sasha-s/go-deadlock v0.3.1
 	github.com/shirou/gopsutil/v3 v3.23.11
 	github.com/syncthing/notify v0.0.0-20210616190510-c6b7342338d2
 	github.com/syndtr/goleveldb v1.0.1-0.20220721030215-126854af5e6d
@@ -68,7 +67,6 @@ require (
 	github.com/matttproud/golang_protobuf_extensions/v2 v2.0.0 // indirect
 	github.com/onsi/ginkgo/v2 v2.13.2 // indirect
 	github.com/oschwald/maxminddb-golang v1.12.0 // indirect
-	github.com/petermattis/goid v0.0.0-20231126143041-f558c26febf5 // indirect
 	github.com/power-devops/perfstat v0.0.0-20221212215047-62379fc7944b // indirect
 	github.com/prometheus/client_model v0.5.0 // indirect
 	github.com/quic-go/qtls-go1-20 v0.4.1 // indirect
diff --git a/go.sum b/go.sum
index 6049fed19..c513fb137 100644
--- a/go.sum
+++ b/go.sum
@@ -139,9 +139,6 @@ github.com/oschwald/geoip2-golang v1.9.0 h1:uvD3O6fXAXs+usU+UGExshpdP13GAqp4GBrz
 github.com/oschwald/geoip2-golang v1.9.0/go.mod h1:BHK6TvDyATVQhKNbQBdrj9eAvuwOMi2zSFXizL3K81Y=
 github.com/oschwald/maxminddb-golang v1.12.0 h1:9FnTOD0YOhP7DGxGsq4glzpGy5+w7pq50AS6wALUMYs=
 github.com/oschwald/maxminddb-golang v1.12.0/go.mod h1:q0Nob5lTCqyQ8WT6FYgS1L7PXKVVbgiymefNwIjPzgY=
-github.com/petermattis/goid v0.0.0-20180202154549-b0b1615b78e5/go.mod h1:jvVRKCrJTQWu0XVbaOlby/2lO20uSCHEMzzplHXte1o=
-github.com/petermattis/goid v0.0.0-20231126143041-f558c26febf5 h1:+qIP3OMrT7SN5kLnTcVEISPOMB/97RyAKTg1UWA738E=
-github.com/petermattis/goid v0.0.0-20231126143041-f558c26febf5/go.mod h1:pxMtw7cyUw6B2bRH0ZBANSPg+AoSud1I1iyJHI69jH4=
 github.com/pierrec/lz4/v4 v4.1.18 h1:xaKrnTkyoqfh1YItXl56+6KJNVYWlEEPuAQW9xsplYQ=
 github.com/pierrec/lz4/v4 v4.1.18/go.mod h1:gZWDp/Ze/IJXGXf23ltt2EXimqmTUXEy0GFuRQyBid4=
 github.com/pkg/errors v0.8.1/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
@@ -168,8 +165,6 @@ github.com/rcrowley/go-metrics v0.0.0-20201227073835-cf1acfcdf475 h1:N/ElC8H3+5X
 github.com/rcrowley/go-metrics v0.0.0-20201227073835-cf1acfcdf475/go.mod h1:bCqnVzQkZxMG4s8nGwiZ5l3QUCyqpo9Y+/ZMZ9VjZe4=
 github.com/russross/blackfriday/v2 v2.1.0 h1:JIOH55/0cWyOuilr9/qlrm0BSXldqnqwMsf35Ld67mk=
 github.com/russross/blackfriday/v2 v2.1.0/go.mod h1:+Rmxgy9KzJVeS9/2gXHxylqXiyQDYRxCVz55jmeOWTM=
-github.com/sasha-s/go-deadlock v0.3.1 h1:sqv7fDNShgjcaxkO0JNcOAlr8B9+cV5Ey/OB71efZx0=
-github.com/sasha-s/go-deadlock v0.3.1/go.mod h1:F73l+cr82YSh10GxyRI6qZiCgK64VaZjwesgfQ1/iLM=
 github.com/sclevine/spec v1.4.0 h1:z/Q9idDcay5m5irkZ28M7PtQM4aOISzOpj4bUPkDee8=
 github.com/shirou/gopsutil/v3 v3.23.11 h1:i3jP9NjCPUz7FiZKxlMnODZkdSIp2gnzfrvsu9CuWEQ=
 github.com/shirou/gopsutil/v3 v3.23.11/go.mod h1:1FrWgea594Jp7qmjHUUPlJDTPgcsb9mGnXDxavtikzM=
diff --git a/lib/model/folder_recvonly_test.go b/lib/model/folder_recvonly_test.go
index b32f6dd33..83c2bcd15 100644
--- a/lib/model/folder_recvonly_test.go
+++ b/lib/model/folder_recvonly_test.go
@@ -535,8 +535,8 @@ func setupROFolder(t *testing.T) (*testModel, *receiveOnlyFolder, context.Cancel
 	<-m.started
 	must(t, m.ScanFolder("ro"))
 
-	m.fmut.RLock()
-	defer m.fmut.RUnlock()
+	m.mut.RLock()
+	defer m.mut.RUnlock()
 	r, _ := m.folderRunners.Get("ro")
 	f := r.(*receiveOnlyFolder)
 
diff --git a/lib/model/mocks/model.go b/lib/model/mocks/model.go
index bf29f47ec..bfb7a07a4 100644
--- a/lib/model/mocks/model.go
+++ b/lib/model/mocks/model.go
@@ -531,11 +531,6 @@ type Model struct {
 	setIgnoresReturnsOnCall map[int]struct {
 		result1 error
 	}
-	StartDeadlockDetectorStub        func(time.Duration)
-	startDeadlockDetectorMutex       sync.RWMutex
-	startDeadlockDetectorArgsForCall []struct {
-		arg1 time.Duration
-	}
 	StateStub        func(string) (string, time.Time, error)
 	stateMutex       sync.RWMutex
 	stateArgsForCall []struct {
@@ -3070,38 +3065,6 @@ func (fake *Model) SetIgnoresReturnsOnCall(i int, result1 error) {
 	}{result1}
 }
 
-func (fake *Model) StartDeadlockDetector(arg1 time.Duration) {
-	fake.startDeadlockDetectorMutex.Lock()
-	fake.startDeadlockDetectorArgsForCall = append(fake.startDeadlockDetectorArgsForCall, struct {
-		arg1 time.Duration
-	}{arg1})
-	stub := fake.StartDeadlockDetectorStub
-	fake.recordInvocation("StartDeadlockDetector", []interface{}{arg1})
-	fake.startDeadlockDetectorMutex.Unlock()
-	if stub != nil {
-		fake.StartDeadlockDetectorStub(arg1)
-	}
-}
-
-func (fake *Model) StartDeadlockDetectorCallCount() int {
-	fake.startDeadlockDetectorMutex.RLock()
-	defer fake.startDeadlockDetectorMutex.RUnlock()
-	return len(fake.startDeadlockDetectorArgsForCall)
-}
-
-func (fake *Model) StartDeadlockDetectorCalls(stub func(time.Duration)) {
-	fake.startDeadlockDetectorMutex.Lock()
-	defer fake.startDeadlockDetectorMutex.Unlock()
-	fake.StartDeadlockDetectorStub = stub
-}
-
-func (fake *Model) StartDeadlockDetectorArgsForCall(i int) time.Duration {
-	fake.startDeadlockDetectorMutex.RLock()
-	defer fake.startDeadlockDetectorMutex.RUnlock()
-	argsForCall := fake.startDeadlockDetectorArgsForCall[i]
-	return argsForCall.arg1
-}
-
 func (fake *Model) State(arg1 string) (string, time.Time, error) {
 	fake.stateMutex.Lock()
 	ret, specificReturn := fake.stateReturnsOnCall[len(fake.stateArgsForCall)]
@@ -3351,8 +3314,6 @@ func (fake *Model) Invocations() map[string][][]interface{} {
 	defer fake.serveMutex.RUnlock()
 	fake.setIgnoresMutex.RLock()
 	defer fake.setIgnoresMutex.RUnlock()
-	fake.startDeadlockDetectorMutex.RLock()
-	defer fake.startDeadlockDetectorMutex.RUnlock()
 	fake.stateMutex.RLock()
 	defer fake.stateMutex.RUnlock()
 	fake.usageReportingStatsMutex.RLock()
diff --git a/lib/model/model.go b/lib/model/model.go
index 0b6fc24b8..d2ea083d0 100644
--- a/lib/model/model.go
+++ b/lib/model/model.go
@@ -116,7 +116,6 @@ type Model interface {
 	DismissPendingDevice(device protocol.DeviceID) error
 	DismissPendingFolder(device protocol.DeviceID, folder string) error
 
-	StartDeadlockDetector(timeout time.Duration)
 	GlobalDirectoryTree(folder, prefix string, levels int, dirsOnly bool) ([]*TreeEntry, error)
 }
 
@@ -145,8 +144,8 @@ type model struct {
 	keyGen          *protocol.KeyGenerator
 	promotionTimer  *time.Timer
 
-	// fields protected by fmut
-	fmut                           sync.RWMutex
+	// fields protected by mut
+	mut                            sync.RWMutex
 	folderCfgs                     map[string]config.FolderConfiguration                  // folder -> cfg
 	folderFiles                    map[string]*db.FileSet                                 // folder -> files
 	deviceStatRefs                 map[protocol.DeviceID]*stats.DeviceStatisticsReference // deviceID -> statsRef
@@ -156,17 +155,15 @@ type model struct {
 	folderVersioners               map[string]versioner.Versioner                         // folder -> versioner (may be nil)
 	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 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
-	connRequestLimiters map[protocol.DeviceID]*semaphore.Semaphore
-	closed              map[string]chan struct{} // connection ID -> closed channel
-	helloMessages       map[protocol.DeviceID]protocol.Hello
-	deviceDownloads     map[protocol.DeviceID]*deviceDownloadState
-	remoteFolderStates  map[protocol.DeviceID]map[string]remoteFolderState // deviceID -> folders
-	indexHandlers       *serviceMap[protocol.DeviceID, *indexHandlerRegistry]
+	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
+	connRequestLimiters            map[protocol.DeviceID]*semaphore.Semaphore
+	closed                         map[string]chan struct{} // connection ID -> closed channel
+	helloMessages                  map[protocol.DeviceID]protocol.Hello
+	deviceDownloads                map[protocol.DeviceID]*deviceDownloadState
+	remoteFolderStates             map[protocol.DeviceID]map[string]remoteFolderState // deviceID -> folders
+	indexHandlers                  *serviceMap[protocol.DeviceID, *indexHandlerRegistry]
 
 	// for testing only
 	foldersRunning atomic.Int32
@@ -226,8 +223,8 @@ func NewModel(cfg config.Wrapper, id protocol.DeviceID, ldb *db.Lowlevel, protec
 		keyGen:               keyGen,
 		promotionTimer:       time.NewTimer(0),
 
-		// fields protected by fmut
-		fmut:                           sync.NewRWMutex(),
+		// fields protected by mut
+		mut:                            sync.NewRWMutex(),
 		folderCfgs:                     make(map[string]config.FolderConfiguration),
 		folderFiles:                    make(map[string]*db.FileSet),
 		deviceStatRefs:                 make(map[protocol.DeviceID]*stats.DeviceStatisticsReference),
@@ -236,21 +233,19 @@ func NewModel(cfg config.Wrapper, id protocol.DeviceID, ldb *db.Lowlevel, protec
 		folderVersioners:               make(map[string]versioner.Versioner),
 		folderEncryptionPasswordTokens: make(map[string][]byte),
 		folderEncryptionFailures:       make(map[string]map[protocol.DeviceID]error),
-
-		// ditto
-		connections:         make(map[string]protocol.Connection),
-		deviceConnIDs:       make(map[protocol.DeviceID][]string),
-		promotedConnID:      make(map[protocol.DeviceID]string),
-		connRequestLimiters: make(map[protocol.DeviceID]*semaphore.Semaphore),
-		closed:              make(map[string]chan struct{}),
-		helloMessages:       make(map[protocol.DeviceID]protocol.Hello),
-		deviceDownloads:     make(map[protocol.DeviceID]*deviceDownloadState),
-		remoteFolderStates:  make(map[protocol.DeviceID]map[string]remoteFolderState),
-		indexHandlers:       newServiceMap[protocol.DeviceID, *indexHandlerRegistry](evLogger),
+		connections:                    make(map[string]protocol.Connection),
+		deviceConnIDs:                  make(map[protocol.DeviceID][]string),
+		promotedConnID:                 make(map[protocol.DeviceID]string),
+		connRequestLimiters:            make(map[protocol.DeviceID]*semaphore.Semaphore),
+		closed:                         make(map[string]chan struct{}),
+		helloMessages:                  make(map[protocol.DeviceID]protocol.Hello),
+		deviceDownloads:                make(map[protocol.DeviceID]*deviceDownloadState),
+		remoteFolderStates:             make(map[protocol.DeviceID]map[string]remoteFolderState),
+		indexHandlers:                  newServiceMap[protocol.DeviceID, *indexHandlerRegistry](evLogger),
 	}
 	for devID, cfg := range cfg.Devices() {
 		m.deviceStatRefs[devID] = stats.NewDeviceStatisticsReference(m.db, devID)
-		m.setConnRequestLimitersPLocked(cfg)
+		m.setConnRequestLimitersLocked(cfg)
 	}
 	m.Add(m.folderRunners)
 	m.Add(m.progressEmitter)
@@ -310,13 +305,13 @@ func (m *model) initFolders(cfg config.Configuration) error {
 }
 
 func (m *model) closeAllConnectionsAndWait() {
-	m.fmut.RLock()
+	m.mut.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.fmut.RUnlock()
+	m.mut.RUnlock()
 	for _, c := range closed {
 		<-c
 	}
@@ -329,16 +324,7 @@ func (m *model) fatal(err error) {
 	}
 }
 
-// StartDeadlockDetector starts a deadlock detector on the models locks which
-// causes panics in case the locks cannot be acquired in the given timeout
-// period.
-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)
-}
-
-// Need to hold lock on m.fmut when calling this.
+// Need to hold lock on m.mut when calling this.
 func (m *model) addAndStartFolderLocked(cfg config.FolderConfiguration, fset *db.FileSet, cacheIgnoredFiles bool) {
 	ignores := ignore.New(cfg.Filesystem(nil), ignore.WithCache(cacheIgnoredFiles))
 	if cfg.Type != config.FolderTypeReceiveEncrypted {
@@ -461,14 +447,12 @@ func (m *model) warnAboutOverwritingProtectedFiles(cfg config.FolderConfiguratio
 }
 
 func (m *model) removeFolder(cfg config.FolderConfiguration) {
-	m.fmut.RLock()
+	m.mut.RLock()
 	wait := m.folderRunners.StopAndWaitChan(cfg.ID, 0)
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	<-wait
 
-	// 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.mut.Lock()
 
 	isPathUnique := true
 	for folderID, folderCfg := range m.folderCfgs {
@@ -493,13 +477,13 @@ func (m *model) removeFolder(cfg config.FolderConfiguration) {
 		return nil
 	})
 
-	m.fmut.Unlock()
+	m.mut.Unlock()
 
 	// Remove it from the database
 	db.DropFolder(m.db, cfg.ID)
 }
 
-// Need to hold lock on m.fmut when calling this.
+// Need to hold lock on m.mut when calling this.
 func (m *model) cleanupFolderLocked(cfg config.FolderConfiguration) {
 	// clear up our config maps
 	m.folderRunners.Remove(cfg.ID)
@@ -523,7 +507,7 @@ func (m *model) restartFolder(from, to config.FolderConfiguration, cacheIgnoredF
 
 	// This mutex protects the entirety of the restart operation, preventing
 	// there from being more than one folder restart operation in progress
-	// at any given time. The usual fmut/pmut stuff doesn't cover this,
+	// at any given time. The usual locking stuff doesn't cover this,
 	// because those locks are released while we are waiting for the folder
 	// to shut down (and must be so because the folder might need them as
 	// part of its operations before shutting down).
@@ -531,13 +515,13 @@ func (m *model) restartFolder(from, to config.FolderConfiguration, cacheIgnoredF
 	restartMut.Lock()
 	defer restartMut.Unlock()
 
-	m.fmut.RLock()
+	m.mut.RLock()
 	wait := m.folderRunners.StopAndWaitChan(from.ID, 0)
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	<-wait
 
-	m.fmut.Lock()
-	defer m.fmut.Unlock()
+	m.mut.Lock()
+	defer m.mut.Unlock()
 
 	// Cache the (maybe) existing fset before it's removed by cleanupFolderLocked
 	fset := m.folderFiles[folder]
@@ -586,8 +570,8 @@ func (m *model) newFolder(cfg config.FolderConfiguration, cacheIgnoredFiles bool
 		return fmt.Errorf("adding %v: %w", cfg.Description(), err)
 	}
 
-	m.fmut.Lock()
-	defer m.fmut.Unlock()
+	m.mut.Lock()
+	defer m.mut.Unlock()
 
 	m.addAndStartFolderLocked(cfg, fset, cacheIgnoredFiles)
 
@@ -632,11 +616,11 @@ func (m *model) UsageReportingStats(report *contract.Report, version int, previe
 		blockStatsMut.Unlock()
 
 		// Transport stats
-		m.fmut.RLock()
+		m.mut.RLock()
 		for _, conn := range m.connections {
 			report.TransportStats[conn.Transport()]++
 		}
-		m.fmut.RUnlock()
+		m.mut.RUnlock()
 
 		// Ignore stats
 		var seenPrefix [3]bool
@@ -723,8 +707,8 @@ type ConnectionInfo struct {
 
 // ConnectionStats returns a map with connection statistics for each device.
 func (m *model) ConnectionStats() map[string]interface{} {
-	m.fmut.RLock()
-	defer m.fmut.RUnlock()
+	m.mut.RLock()
+	defer m.mut.RUnlock()
 
 	res := make(map[string]interface{})
 	devs := m.cfg.Devices()
@@ -797,8 +781,8 @@ func (m *model) ConnectionStats() map[string]interface{} {
 
 // DeviceStatistics returns statistics about each device
 func (m *model) DeviceStatistics() (map[protocol.DeviceID]stats.DeviceStatistics, error) {
-	m.fmut.RLock()
-	defer m.fmut.RUnlock()
+	m.mut.RLock()
+	defer m.mut.RUnlock()
 	res := make(map[protocol.DeviceID]stats.DeviceStatistics, len(m.deviceStatRefs))
 	for id, sr := range m.deviceStatRefs {
 		stats, err := sr.GetStatistics()
@@ -818,8 +802,8 @@ func (m *model) DeviceStatistics() (map[protocol.DeviceID]stats.DeviceStatistics
 // FolderStatistics returns statistics about each folder
 func (m *model) FolderStatistics() (map[string]stats.FolderStatistics, error) {
 	res := make(map[string]stats.FolderStatistics)
-	m.fmut.RLock()
-	defer m.fmut.RUnlock()
+	m.mut.RLock()
+	defer m.mut.RUnlock()
 	err := m.folderRunners.Each(func(id string, runner service) error {
 		stats, err := runner.GetStatistics()
 		if err != nil {
@@ -936,10 +920,10 @@ func (m *model) Completion(device protocol.DeviceID, folder string) (FolderCompl
 }
 
 func (m *model) folderCompletion(device protocol.DeviceID, folder string) (FolderCompletion, error) {
-	m.fmut.RLock()
-	err := m.checkFolderRunningLocked(folder)
+	m.mut.RLock()
+	err := m.checkFolderRunningRLocked(folder)
 	rf := m.folderFiles[folder]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if err != nil {
 		return FolderCompletion{}, err
 	}
@@ -950,10 +934,10 @@ func (m *model) folderCompletion(device protocol.DeviceID, folder string) (Folde
 	}
 	defer snap.Release()
 
-	m.fmut.RLock()
+	m.mut.RLock()
 	state := m.remoteFolderStates[device][folder]
 	downloaded := m.deviceDownloads[device].BytesDownloaded(folder)
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 
 	need := snap.NeedSize(device)
 	need.Bytes -= downloaded
@@ -970,10 +954,10 @@ func (m *model) folderCompletion(device protocol.DeviceID, folder string) (Folde
 
 // DBSnapshot returns a snapshot of the database content relevant to the given folder.
 func (m *model) DBSnapshot(folder string) (*db.Snapshot, error) {
-	m.fmut.RLock()
-	err := m.checkFolderRunningLocked(folder)
+	m.mut.RLock()
+	err := m.checkFolderRunningRLocked(folder)
 	rf := m.folderFiles[folder]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if err != nil {
 		return nil, err
 	}
@@ -987,11 +971,11 @@ func (m *model) FolderProgressBytesCompleted(folder string) int64 {
 // NeedFolderFiles returns paginated list of currently needed files in
 // progress, queued, and to be queued on next puller iteration.
 func (m *model) NeedFolderFiles(folder string, page, perpage int) ([]db.FileInfoTruncated, []db.FileInfoTruncated, []db.FileInfoTruncated, error) {
-	m.fmut.RLock()
+	m.mut.RLock()
 	rf, rfOk := m.folderFiles[folder]
 	runner, runnerOk := m.folderRunners.Get(folder)
 	cfg := m.folderCfgs[folder]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 
 	if !rfOk {
 		return nil, nil, nil, ErrFolderMissing
@@ -1058,9 +1042,9 @@ func (m *model) NeedFolderFiles(folder string, page, perpage int) ([]db.FileInfo
 // RemoteNeedFolderFiles returns paginated list of currently needed files for a
 // remote device to become synced with a folder.
 func (m *model) RemoteNeedFolderFiles(folder string, device protocol.DeviceID, page, perpage int) ([]db.FileInfoTruncated, error) {
-	m.fmut.RLock()
+	m.mut.RLock()
 	rf, ok := m.folderFiles[folder]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 
 	if !ok {
 		return nil, ErrFolderMissing
@@ -1085,9 +1069,9 @@ func (m *model) RemoteNeedFolderFiles(folder string, device protocol.DeviceID, p
 }
 
 func (m *model) LocalChangedFolderFiles(folder string, page, perpage int) ([]db.FileInfoTruncated, error) {
-	m.fmut.RLock()
+	m.mut.RLock()
 	rf, ok := m.folderFiles[folder]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 
 	if !ok {
 		return nil, ErrFolderMissing
@@ -1176,9 +1160,9 @@ func (m *model) handleIndex(conn protocol.Connection, folder string, fs []protoc
 		return fmt.Errorf("%s: %w", folder, ErrFolderPaused)
 	}
 
-	m.fmut.RLock()
-	indexHandler, ok := m.getIndexHandlerPRLocked(conn)
-	m.fmut.RUnlock()
+	m.mut.RLock()
+	indexHandler, ok := m.getIndexHandlerRLocked(conn)
+	m.mut.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
@@ -1255,7 +1239,7 @@ func (m *model) ClusterConfig(conn protocol.Connection, cm protocol.ClusterConfi
 		break
 	}
 
-	// Needs to happen outside of the fmut, as can cause CommitConfiguration
+	// Needs to happen outside of the mut, as can cause CommitConfiguration
 	if deviceCfg.AutoAcceptFolders {
 		w, _ := m.cfg.Modify(func(cfg *config.Configuration) {
 			changedFcfg := make(map[string]config.FolderConfiguration)
@@ -1291,9 +1275,9 @@ func (m *model) ClusterConfig(conn protocol.Connection, cm protocol.ClusterConfi
 		return err
 	}
 
-	m.fmut.Lock()
+	m.mut.Lock()
 	m.remoteFolderStates[deviceID] = states
-	m.fmut.Unlock()
+	m.mut.Unlock()
 
 	m.evLogger.Log(events.ClusterConfigReceived, ClusterConfigReceivedEventData{
 		Device: deviceID,
@@ -1302,11 +1286,11 @@ func (m *model) ClusterConfig(conn protocol.Connection, cm protocol.ClusterConfi
 	if len(tempIndexFolders) > 0 {
 		var connOK bool
 		var conn protocol.Connection
-		m.fmut.RLock()
+		m.mut.RLock()
 		if connIDs, connIDOK := m.deviceConnIDs[deviceID]; connIDOK {
 			conn, connOK = m.connections[connIDs[0]]
 		}
-		m.fmut.RUnlock()
+		m.mut.RUnlock()
 		// In case we've got ClusterConfig, and the connection disappeared
 		// from infront of our nose.
 		if connOK {
@@ -1339,8 +1323,8 @@ func (m *model) ensureIndexHandler(conn protocol.Connection) *indexHandlerRegist
 	deviceID := conn.DeviceID()
 	connID := conn.ConnectionID()
 
-	m.fmut.Lock()
-	defer m.fmut.Unlock()
+	m.mut.Lock()
+	defer m.mut.Unlock()
 
 	indexHandlerRegistry, ok := m.indexHandlers.Get(deviceID)
 	if ok && indexHandlerRegistry.conn.ConnectionID() == connID {
@@ -1370,8 +1354,8 @@ func (m *model) ensureIndexHandler(conn protocol.Connection) *indexHandlerRegist
 	return indexHandlerRegistry
 }
 
-func (m *model) getIndexHandlerPRLocked(conn protocol.Connection) (*indexHandlerRegistry, bool) {
-	// Reads from index handlers, which requires pmut to be read locked
+func (m *model) getIndexHandlerRLocked(conn protocol.Connection) (*indexHandlerRegistry, bool) {
+	// Reads from index handlers, which requires the mutex to be read locked
 
 	deviceID := conn.DeviceID()
 	connID := conn.ConnectionID()
@@ -1450,14 +1434,14 @@ func (m *model) ccHandleFolders(folders []protocol.Folder, deviceCfg config.Devi
 
 		if err := m.ccCheckEncryption(cfg, folderDevice, ccDeviceInfos[folder.ID], deviceCfg.Untrusted); err != nil {
 			sameError := false
-			m.fmut.Lock()
+			m.mut.Lock()
 			if devs, ok := m.folderEncryptionFailures[folder.ID]; ok {
 				sameError = devs[deviceID] == err
 			} else {
 				m.folderEncryptionFailures[folder.ID] = make(map[protocol.DeviceID]error)
 			}
 			m.folderEncryptionFailures[folder.ID][deviceID] = err
-			m.fmut.Unlock()
+			m.mut.Unlock()
 			msg := fmt.Sprintf("Failure checking encryption consistency with device %v for folder %v: %v", deviceID, cfg.Description(), err)
 			if sameError {
 				l.Debugln(msg)
@@ -1470,7 +1454,7 @@ func (m *model) ccHandleFolders(folders []protocol.Folder, deviceCfg config.Devi
 			}
 			return tempIndexFolders, seenFolders, err
 		}
-		m.fmut.Lock()
+		m.mut.Lock()
 		if devErrs, ok := m.folderEncryptionFailures[folder.ID]; ok {
 			if len(devErrs) == 1 {
 				delete(m.folderEncryptionFailures, folder.ID)
@@ -1478,7 +1462,7 @@ func (m *model) ccHandleFolders(folders []protocol.Folder, deviceCfg config.Devi
 				delete(m.folderEncryptionFailures[folder.ID], deviceID)
 			}
 		}
-		m.fmut.Unlock()
+		m.mut.Unlock()
 
 		// Handle indexes
 
@@ -1582,9 +1566,9 @@ func (m *model) ccCheckEncryption(fcfg config.FolderConfiguration, folderDevice
 		// hasTokenRemote == true
 		ccToken = ccDeviceInfos.remote.EncryptionPasswordToken
 	}
-	m.fmut.RLock()
+	m.mut.RLock()
 	token, ok := m.folderEncryptionPasswordTokens[fcfg.ID]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if !ok {
 		var err error
 		token, err = readEncryptionToken(fcfg)
@@ -1598,9 +1582,9 @@ func (m *model) ccCheckEncryption(fcfg config.FolderConfiguration, folderDevice
 			}
 		}
 		if err == nil {
-			m.fmut.Lock()
+			m.mut.Lock()
 			m.folderEncryptionPasswordTokens[fcfg.ID] = token
-			m.fmut.Unlock()
+			m.mut.Unlock()
 		} else {
 			if err := writeEncryptionToken(ccToken, fcfg); err != nil {
 				if rerr, ok := redactPathError(err); ok {
@@ -1612,9 +1596,9 @@ func (m *model) ccCheckEncryption(fcfg config.FolderConfiguration, folderDevice
 					}
 				}
 			}
-			m.fmut.Lock()
+			m.mut.Lock()
 			m.folderEncryptionPasswordTokens[fcfg.ID] = ccToken
-			m.fmut.Unlock()
+			m.mut.Unlock()
 			// We can only announce ourselves once we have the token,
 			// thus we need to resend CCs now that we have it.
 			m.sendClusterConfig(fcfg.DeviceIDs())
@@ -1632,14 +1616,14 @@ func (m *model) sendClusterConfig(ids []protocol.DeviceID) {
 		return
 	}
 	ccConns := make([]protocol.Connection, 0, len(ids))
-	m.fmut.RLock()
+	m.mut.RLock()
 	for _, id := range ids {
 		if connIDs, ok := m.deviceConnIDs[id]; ok {
 			ccConns = append(ccConns, m.connections[connIDs[0]])
 		}
 	}
-	m.fmut.RUnlock()
-	// Generating cluster-configs acquires fmut -> must happen outside of pmut.
+	m.mut.RUnlock()
+	// Generating cluster-configs acquires the mutex.
 	for _, conn := range ccConns {
 		cm, passwords := m.generateClusterConfig(conn.DeviceID())
 		conn.SetFolderPasswords(passwords)
@@ -1875,10 +1859,10 @@ func (m *model) Closed(conn protocol.Connection, err error) {
 	connID := conn.ConnectionID()
 	deviceID := conn.DeviceID()
 
-	m.fmut.Lock()
+	m.mut.Lock()
 	conn, ok := m.connections[connID]
 	if !ok {
-		m.fmut.Unlock()
+		m.mut.Unlock()
 		return
 	}
 
@@ -1909,14 +1893,14 @@ func (m *model) Closed(conn protocol.Connection, err error) {
 		m.deviceConnIDs[deviceID] = remainingConns
 	}
 
-	m.fmut.Unlock()
+	m.mut.Unlock()
 	if wait != nil {
 		<-wait
 	}
 
-	m.fmut.RLock()
-	m.deviceDidCloseFRLocked(deviceID, time.Since(conn.EstablishedAt()))
-	m.fmut.RUnlock()
+	m.mut.RLock()
+	m.deviceDidCloseRLocked(deviceID, time.Since(conn.EstablishedAt()))
+	m.mut.RUnlock()
 
 	k := map[bool]string{false: "secondary", true: "primary"}[removedIsPrimary]
 	l.Infof("Lost %s connection to %s at %s: %v (%d remain)", k, deviceID.Short(), conn, err, len(remainingConns))
@@ -1969,10 +1953,10 @@ func (m *model) Request(conn protocol.Connection, folder, name string, _, size i
 
 	deviceID := conn.DeviceID()
 
-	m.fmut.RLock()
+	m.mut.RLock()
 	folderCfg, ok := m.folderCfgs[folder]
 	folderIgnores := m.folderIgnores[folder]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if !ok {
 		// The folder might be already unpaused in the config, but not yet
 		// in the model.
@@ -2011,9 +1995,9 @@ func (m *model) Request(conn protocol.Connection, folder, name string, _, size i
 
 	// Restrict parallel requests by connection/device
 
-	m.fmut.RLock()
+	m.mut.RLock()
 	limiter := m.connRequestLimiters[deviceID]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 
 	// The requestResponse releases the bytes to the buffer pool and the
 	// limiters when its Close method is called.
@@ -2142,9 +2126,9 @@ func (m *model) recheckFile(deviceID protocol.DeviceID, folder, name string, off
 	// to what we have in the database, yet the content we've read off the filesystem doesn't
 	// Something is fishy, invalidate the file and rescan it.
 	// The file will temporarily become invalid, which is ok as the content is messed up.
-	m.fmut.RLock()
+	m.mut.RLock()
 	runner, ok := m.folderRunners.Get(folder)
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if !ok {
 		l.Debugf("%v recheckFile: %s: %q / %q: Folder stopped before rescan could be scheduled", m, deviceID, folder, name)
 		return
@@ -2156,9 +2140,9 @@ func (m *model) recheckFile(deviceID protocol.DeviceID, folder, name string, off
 }
 
 func (m *model) CurrentFolderFile(folder string, file string) (protocol.FileInfo, bool, error) {
-	m.fmut.RLock()
+	m.mut.RLock()
 	fs, ok := m.folderFiles[folder]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if !ok {
 		return protocol.FileInfo{}, false, ErrFolderMissing
 	}
@@ -2172,9 +2156,9 @@ func (m *model) CurrentFolderFile(folder string, file string) (protocol.FileInfo
 }
 
 func (m *model) CurrentGlobalFile(folder string, file string) (protocol.FileInfo, bool, error) {
-	m.fmut.RLock()
+	m.mut.RLock()
 	ffs, ok := m.folderFiles[folder]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if !ok {
 		return protocol.FileInfo{}, false, ErrFolderMissing
 	}
@@ -2188,10 +2172,10 @@ func (m *model) CurrentGlobalFile(folder string, file string) (protocol.FileInfo
 }
 
 func (m *model) GetMtimeMapping(folder string, file string) (fs.MtimeMapping, error) {
-	m.fmut.RLock()
+	m.mut.RLock()
 	ffs, ok := m.folderFiles[folder]
 	fcfg := m.folderCfgs[folder]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	if !ok {
 		return fs.MtimeMapping{}, ErrFolderMissing
 	}
@@ -2200,19 +2184,19 @@ 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.fmut.RLock()
+	m.mut.RLock()
 	_, ok := m.deviceConnIDs[deviceID]
-	m.fmut.RUnlock()
+	m.mut.RUnlock()
 	return ok
 }
 
 // LoadIgnores loads or refreshes the ignore patterns from disk, if the
 // folder is healthy, and returns the refreshed lines and patterns.
 func (m *model) LoadIgnores(folder string) ([]string, []string, error) {
-	m.fmut.RLock()