Unify DB and Handle types into just DB

db.go

@@ -82,6 +82,12 @@ import (
 //  6. Periodically garbage collect the graveyard by finding
 //     the lowest revision of all delete trackers.
 type DB struct {
+	handleName string
+	*dbState
+}
+
+// dbState is the underlying state of the database shared by all [DB] handles.
+type dbState struct {
 	mu                  sync.Mutex // protects 'tables' and sequences modifications to the root tree
 	ctx                 context.Context
 	cancel              context.CancelFunc
@@ -90,7 +96,6 @@ type DB struct {
 	gcExited            chan struct{}
 	gcRateLimitInterval time.Duration
 	metrics             Metrics
-	defaultHandle       Handle
 }
 
 type dbRoot []tableEntry
@@ -121,10 +126,12 @@ func New(options ...Option) *DB {
 	}
 
 	db := &DB{
-		metrics:             opts.metrics,
-		gcRateLimitInterval: defaultGCRateLimitInterval,
+		dbState: &dbState{
+			metrics:             opts.metrics,
+			gcRateLimitInterval: defaultGCRateLimitInterval,
+		},
 	}
-	db.defaultHandle = Handle{db, "DB"}
+	db.handleName = "DB"
 	root := dbRoot{}
 	db.root.Store(&root)
 	return db
@@ -177,7 +184,10 @@ func (db *DB) registerTable(table TableMeta, root *dbRoot) error {
 //
 // The returned ReadTxn is not thread-safe.
 func (db *DB) ReadTxn() ReadTxn {
-	return db.defaultHandle.ReadTxn()
+	return &txn{
+		db:   db,
+		root: *db.root.Load(),
+	}
 }
 
 // WriteTxn constructs a new write transaction against the given set of tables.
@@ -187,64 +197,6 @@ func (db *DB) ReadTxn() ReadTxn {
 //
 // The returned WriteTxn is not thread-safe.
 func (db *DB) WriteTxn(table TableMeta, tables ...TableMeta) WriteTxn {
-	return db.defaultHandle.WriteTxn(table, tables...)
-}
-
-// Start the background workers for the database.
-//
-// This starts the graveyard worker that deals with garbage collecting
-// deleted objects that are no longer necessary for Changes().
-func (db *DB) Start() error {
-	db.gcTrigger = make(chan struct{}, 1)
-	db.gcExited = make(chan struct{})
-	db.ctx, db.cancel = context.WithCancel(context.Background())
-	go graveyardWorker(db, db.ctx, db.gcRateLimitInterval)
-	return nil
-}
-
-// Stop the background workers.
-func (db *DB) Stop() error {
-	db.cancel()
-	<-db.gcExited
-	return nil
-}
-
-// ServeHTTP is an HTTP handler for dumping StateDB as JSON.
-//
-// Example usage:
-//
-//	var db *statedb.DB
-//
-//	http.Handle("/db", db)
-//	http.ListenAndServe(":8080", nil)
-func (db *DB) ServeHTTP(w http.ResponseWriter, r *http.Request) {
-	w.Header().Add("Content-Type", "application/json")
-	w.WriteHeader(http.StatusOK)
-	db.ReadTxn().WriteJSON(w)
-}
-
-// setGCRateLimitInterval can set the graveyard GC interval before DB is started.
-// Used by tests.
-func (db *DB) setGCRateLimitInterval(interval time.Duration) {
-	db.gcRateLimitInterval = interval
-}
-
-// NewHandle returns a named handle to the DB. The handle has the same ReadTxn and
-// WriteTxn methods as DB, but annotated with the given name for more accurate
-// cost accounting in e.g. metrics.
-func (db *DB) NewHandle(name string) Handle {
-	return Handle{db, name}
-}
-
-// Handle is a named handle to the database for constructing read or write
-// transactions.
-type Handle struct {
-	db   *DB
-	name string
-}
-
-func (h Handle) WriteTxn(table TableMeta, tables ...TableMeta) WriteTxn {
-	db := h.db
 	allTables := append(tables, table)
 	smus := internal.SortableMutexes{}
 	for _, table := range allTables {
@@ -267,7 +219,7 @@ func (h Handle) WriteTxn(table TableMeta, tables ...TableMeta) WriteTxn {
 		tableNames = append(tableNames, table.Name())
 
 		db.metrics.WriteTxnTableAcquisition(
-			h.name,
+			db.handleName,
 			table.Name(),
 			table.sortableMutex().AcquireDuration(),
 		)
@@ -277,7 +229,7 @@ func (h Handle) WriteTxn(table TableMeta, tables ...TableMeta) WriteTxn {
 	sort.Strings(tableNames)
 
 	db.metrics.WriteTxnTotalAcquisition(
-		h.name,
+		db.handleName,
 		tableNames,
 		acquiredAt.Sub(lockAt),
 	)
@@ -289,19 +241,56 @@ func (h Handle) WriteTxn(table TableMeta, tables ...TableMeta) WriteTxn {
 		smus:           smus,
 		acquiredAt:     acquiredAt,
 		tableNames:     tableNames,
-		handle:         h.name,
+		handle:         db.handleName,
 	}
 	runtime.SetFinalizer(txn, txnFinalizer)
 	return txn
 }
 
-// ReadTxn constructs a new read transaction for performing reads against
-// a snapshot of the database.
+// Start the background workers for the database.
 //
-// The returned ReadTxn is not thread-safe.
-func (h Handle) ReadTxn() ReadTxn {
-	return &txn{
-		db:   h.db,
-		root: *h.db.root.Load(),
+// This starts the graveyard worker that deals with garbage collecting
+// deleted objects that are no longer necessary for Changes().
+func (db *DB) Start() error {
+	db.gcTrigger = make(chan struct{}, 1)
+	db.gcExited = make(chan struct{})
+	db.ctx, db.cancel = context.WithCancel(context.Background())
+	go graveyardWorker(db, db.ctx, db.gcRateLimitInterval)
+	return nil
+}
+
+// Stop the background workers.
+func (db *DB) Stop() error {
+	db.cancel()
+	<-db.gcExited
+	return nil
+}
+
+// ServeHTTP is an HTTP handler for dumping StateDB as JSON.
+//
+// Example usage:
+//
+//	var db *statedb.DB
+//
+//	http.Handle("/db", db)
+//	http.ListenAndServe(":8080", nil)
+func (db *DB) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	w.Header().Add("Content-Type", "application/json")
+	w.WriteHeader(http.StatusOK)
+	db.ReadTxn().WriteJSON(w)
+}
+
+// setGCRateLimitInterval can set the graveyard GC interval before DB is started.
+// Used by tests.
+func (db *DB) setGCRateLimitInterval(interval time.Duration) {
+	db.gcRateLimitInterval = interval
+}
+
+// NewHandle returns a new named handle to the DB. The given name is used to annotate
+// metrics.
+func (db *DB) NewHandle(name string) *DB {
+	return &DB{
+		handleName: name,
+		dbState:    db.dbState,
 	}
 }

reconciler/example/main.go

@@ -97,7 +97,14 @@ func main() {
 	cmd.Execute()
 }
 
-var Hive = hive.New(
+var Hive = hive.NewWithOptions(
+	hive.Options{
+		// Create a named DB handle for each module.
+		ModuleDecorator: func(db *statedb.DB, id cell.ModuleID) *statedb.DB {
+			return db.NewHandle(string(id))
+		},
+	},
+
 	statedb.Cell,
 	job.Cell,
 
@@ -162,6 +169,8 @@ func registerHTTPServer(
 
 	mux := http.NewServeMux()
 
+	// To dump the metrics:
+	// curl -s http://localhost:8080/expvar
 	mux.Handle("/expvar", expvar.Handler())
 
 	// For dumping the database:

reconciler/metrics.go

@@ -41,10 +41,10 @@ func (m *ExpVarMetrics) PruneDuration(moduleID cell.FullModuleID, duration time.
 
 func (m *ExpVarMetrics) PruneError(moduleID cell.FullModuleID, err error) {
 	m.PruneCountVar.Add(moduleID.String(), 1)
-	m.PruneTotalErrorsVar.Add(moduleID.String(), 1)
 
 	var intVar expvar.Int
 	if err != nil {
+		m.PruneTotalErrorsVar.Add(moduleID.String(), 1)
 		intVar.Set(1)
 	}
 	m.PruneCurrentErrorsVar.Set(moduleID.String(), &intVar)