walletdb: add new Batch package-level function

In this commit, we add a new package-level function to emulate the
existing Batch interface for bbolt. We do this via a new super-set
interface which is then checked against in the main implementation of
the Batch method.

go.sum

@@ -89,6 +89,7 @@ golang.org/x/oauth2 v0.0.0-20180821212333-d2e6202438be/go.mod h1:N/0e6XlmueqKjAG
 golang.org/x/sync v0.0.0-20180314180146-1d60e4601c6f/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sync v0.0.0-20181221193216-37e7f081c4d4 h1:YUO/7uOKsKeq9UokNS62b8FYywz3ker1l1vDZRCRefw=
 golang.org/x/sync v0.0.0-20181221193216-37e7f081c4d4/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+golang.org/x/sync v0.0.0-20190911185100-cd5d95a43a6e h1:vcxGaoTs7kV8m5Np9uUNQin4BrLOthgV7252N8V+FwY=
 golang.org/x/sync v0.0.0-20190911185100-cd5d95a43a6e/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sys v0.0.0-20180830151530-49385e6e1522/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20180909124046-d0be0721c37e/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=

walletdb/bdb/db.go

@@ -342,6 +342,19 @@ func (db *db) Close() error {
 	return convertErr((*bbolt.DB)(db).Close())
 }
 
+// Batch is similar to the package-level Update method, but it will attempt to
+// optismitcally combine the invocation of several transaction functions into a
+// single db write transaction.
+//
+// This function is part of the walletdb.Db interface implementation.
+func (db *db) Batch(f func(tx walletdb.ReadWriteTx) error) error {
+	return (*bbolt.DB)(db).Batch(func(btx *bbolt.Tx) error {
+		interfaceTx := transaction{btx}
+
+		return f(&interfaceTx)
+	})
+}
+
 // filesExists reports whether the named file or directory exists.
 func fileExists(name string) bool {
 	if _, err := os.Stat(name); err != nil {

walletdb/go.mod

@@ -7,5 +7,6 @@ require (
 	github.com/coreos/bbolt v1.3.3
 	github.com/davecgh/go-spew v1.1.1
 	go.etcd.io/bbolt v1.3.3 // indirect
+	golang.org/x/sync v0.0.0-20190911185100-cd5d95a43a6e
 	golang.org/x/sys v0.0.0-20190904154756-749cb33beabd // indirect
 )

walletdb/go.sum

@@ -6,5 +6,7 @@ github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 go.etcd.io/bbolt v1.3.3 h1:MUGmc65QhB3pIlaQ5bB4LwqSj6GIonVJXpZiaKNyaKk=
 go.etcd.io/bbolt v1.3.3/go.mod h1:IbVyRI1SCnLcuJnV2u8VeU0CEYM7e686BmAb1XKL+uU=
+golang.org/x/sync v0.0.0-20190911185100-cd5d95a43a6e h1:vcxGaoTs7kV8m5Np9uUNQin4BrLOthgV7252N8V+FwY=
+golang.org/x/sync v0.0.0-20190911185100-cd5d95a43a6e/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sys v0.0.0-20190904154756-749cb33beabd h1:DBH9mDw0zluJT/R+nGuV3jWFWLFaHyYZWD4tOT+cjn0=
 golang.org/x/sys v0.0.0-20190904154756-749cb33beabd/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=

walletdb/interface.go

@@ -7,7 +7,10 @@
 
 package walletdb
 
-import "io"
+import (
+	"fmt"
+	"io"
+)
 
 // ReadTx represents a database transaction that can only be used for reads.  If
 // a database update must occur, use a ReadWriteTx.
@@ -201,6 +204,18 @@ type DB interface {
 	Close() error
 }
 
+// BatchDB is a special version of the main DB interface that allos the caller
+// to specify write transactions that should be combine dtoegether if multiple
+// goroutines are calling the Batch method.
+type BatchDB interface {
+	DB
+
+	// Batch is similar to the package-level Update method, but it will
+	// attempt to optismitcally combine the invocation of several
+	// transaction functions into a single db write transaction.
+	Batch(func(tx ReadWriteTx) error) error
+}
+
 // View opens a database read transaction and executes the function f with the
 // transaction passed as a parameter.  After f exits, the transaction is rolled
 // back.  If f errors, its error is returned, not a rollback error (if any
@@ -260,6 +275,23 @@ func Update(db DB, f func(tx ReadWriteTx) error) error {
 	return tx.Commit()
 }
 
+// Batch opens a database read/write transaction and executes the function f
+// with the transaction passed as a parameter.  After f exits, if f did not
+// error, the transaction is committed.  Otherwise, if f did error, the
+// transaction is rolled back.  If the rollback fails, the original error
+// returned by f is still returned.  If the commit fails, the commit error is
+// returned.
+//
+// Batch is only useful when there are multiple goroutines calling it.
+func Batch(db DB, f func(tx ReadWriteTx) error) error {
+	batchDB, ok := db.(BatchDB)
+	if !ok {
+		return fmt.Errorf("need batch")
+	}
+
+	return batchDB.Batch(f)
+}
+
 // Driver defines a structure for backend drivers to use when they registered
 // themselves as a backend which implements the Db interface.
 type Driver struct {

walletdb/walletdbtest/interface.go

@@ -5,9 +5,11 @@
 package walletdbtest
 
 import (
+	"bytes"
 	"fmt"
 	"os"
 	"reflect"
+	"sync"
 
 	"github.com/btcsuite/btcwallet/walletdb"
 )
@@ -146,7 +148,7 @@ func testSequence(tc *testContext, testBucket walletdb.ReadWriteBucket) bool {
 		return false
 	}
 
-	return false
+	return true
 }
 
 // testReadWriteBucketInterface ensures the bucket interface is working properly by
@@ -726,6 +728,70 @@ func testAdditionalErrors(tc *testContext) bool {
 	return true
 }
 
+// testBatchInterface tests that if the target database implements the batch
+// method, then the method functions as expected.
+func testBatchInterface(tc *testContext) bool {
+	// If the database doesn't support the batch super-set of the
+	// interface, then we're done here.
+	batchDB, ok := tc.db.(walletdb.BatchDB)
+	if !ok {
+		return true
+	}
+
+	const numGoroutines = 5
+	errChan := make(chan error, numGoroutines)
+
+	var wg sync.WaitGroup
+	for i := 0; i < numGoroutines; i++ {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			err := walletdb.Batch(batchDB, func(tx walletdb.ReadWriteTx) error {
+				b, err := tx.CreateTopLevelBucket([]byte("test"))
+				if err != nil {
+					return err
+				}
+
+				byteI := []byte{byte(i)}
+				return b.Put(byteI, byteI)
+			})
+			errChan <- err
+		}(i)
+	}
+
+	wg.Wait()
+	close(errChan)
+
+	for err := range errChan {
+		if err != nil {
+			tc.t.Errorf("Batch: unexpected error: %v", err)
+			return false
+		}
+	}
+
+	err := walletdb.View(batchDB, func(tx walletdb.ReadTx) error {
+		b := tx.ReadBucket([]byte("test"))
+
+		for i := 0; i < numGoroutines; i++ {
+			byteI := []byte{byte(i)}
+			if v := b.Get(byteI); v == nil {
+				return fmt.Errorf("key %v not present", byteI)
+			} else if !bytes.Equal(v, byteI) {
+				return fmt.Errorf("key %v not equal to value: "+
+					"%v", byteI, v)
+			}
+		}
+
+		return nil
+	})
+	if err != nil {
+		tc.t.Errorf("Batch: unexpected error: %v", err)
+		return false
+	}
+
+	return true
+}
+
 // TestInterface performs all interfaces tests for this database driver.
 func TestInterface(t Tester, dbType, dbPath string) {
 	db, err := walletdb.Create(dbType, dbPath, true)
@@ -754,4 +820,9 @@ func TestInterface(t Tester, dbType, dbPath string) {
 	if !testAdditionalErrors(&context) {
 		return
 	}
+
+	// If applicable, also test the behavior of the Batch call.
+	if !testBatchInterface(&context) {
+		return
+	}
 }