channeldb/kvdb: create new package for an abstract kv store

In this commit, we create a new package `kvdb`, which is meant to serve
as the basis for any future database abstractions within `lnd`. Rather
than directly use the `walletdb` package (which we base off of), we
instead use a series of type-aliases to re-type the fundamental
types/interfaces of the `walletdb` package. This lets us type
`kvdb.RwTx` instead of `walletdb.ReadWriteTransaction` everywhere.
Additionally, our usage of type-aliases is also intended to create an
easy pathway in the future wherein we can gradually re-defined or
re-implement these types to wean off of the `walletdb` package.

channeldb/db.go

@@ -13,6 +13,7 @@ import (
 	"github.com/btcsuite/btcd/wire"
 	"github.com/coreos/bbolt"
 	"github.com/go-errors/errors"
+	"github.com/lightningnetwork/lnd/channeldb/kvdb"
 	"github.com/lightningnetwork/lnd/channeldb/migration12"
 	"github.com/lightningnetwork/lnd/channeldb/migration13"
 	"github.com/lightningnetwork/lnd/channeldb/migration_01_to_11"
@@ -28,7 +29,7 @@ const (
 // migration is a function which takes a prior outdated version of the database
 // instances and mutates the key/bucket structure to arrive at a more
 // up-to-date version of the database.
-type migration func(tx *bbolt.Tx) error
+type migration func(tx kvdb.RwTx) error
 
 type version struct {
 	number    uint32

channeldb/kvdb/bbolt.go

@@ -0,0 +1,10 @@
+package kvdb
+
+import (
+	_ "github.com/btcsuite/btcwallet/walletdb/bdb" // Import to register backend.
+)
+
+// BoltBackendName is the name of the backend that should be passed into
+// kvdb.Create to initialize a new instance of kvdb.Backend backed by a live
+// instance of bbolt.
+const BoltBackendName = "bdb"

channeldb/kvdb/interface.go

@@ -0,0 +1,90 @@
+package kvdb
+
+import (
+	"github.com/btcsuite/btcwallet/walletdb"
+	_ "github.com/btcsuite/btcwallet/walletdb/bdb" // Import to register backend.
+)
+
+// Update 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.
+var Update = walletdb.Update
+
+// 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
+// occur).
+var View = walletdb.View
+
+// Batch is identical to the Update call, but it attempts to combine several
+// individual Update transactions into a single write database transaction on
+// an optimistic basis. This only has benefits if multiple goroutines call
+// Batch.
+var Batch = walletdb.Batch
+
+// Create initializes and opens a database for the specified type. The
+// arguments are specific to the database type driver. See the documentation
+// for the database driver for further details.
+//
+// ErrDbUnknownType will be returned if the database type is not registered.
+var Create = walletdb.Create
+
+// Backend represents an ACID database. All database access is performed
+// through read or read+write transactions.
+type Backend = walletdb.DB
+
+// Open opens an existing database for the specified type. The arguments are
+// specific to the database type driver. See the documentation for the database
+// driver for further details.
+//
+// ErrDbUnknownType will be returned if the database type is not registered.
+var Open = walletdb.Open
+
+// Driver defines a structure for backend drivers to use when they registered
+// themselves as a backend which implements the Backend interface.
+type Driver = walletdb.Driver
+
+// ReadBucket represents a bucket (a hierarchical structure within the
+// database) that is only allowed to perform read operations.
+type ReadBucket = walletdb.ReadBucket
+
+// ReadCursor represents a bucket cursor that can be positioned at the start or
+// end of the bucket's key/value pairs and iterate over pairs in the bucket.
+// This type is only allowed to perform database read operations.
+type ReadCursor = walletdb.ReadCursor
+
+// ReadTx represents a database transaction that can only be used for reads. If
+// a database update must occur, use a RwTx.
+type ReadTx = walletdb.ReadTx
+
+// RwBucket represents a bucket (a hierarchical structure within the database)
+// that is allowed to perform both read and write operations.
+type RwBucket = walletdb.ReadWriteBucket
+
+// RwCursor represents a bucket cursor that can be positioned at the start or
+// end of the bucket's key/value pairs and iterate over pairs in the bucket.
+// This abstraction is allowed to perform both database read and write
+// operations.
+type RwCursor = walletdb.ReadWriteCursor
+
+// ReadWriteTx represents a database transaction that can be used for both
+// reads and writes. When only reads are necessary, consider using a ReadTx
+// instead.
+type RwTx = walletdb.ReadWriteTx
+
+var (
+	// ErrBucketNotFound is returned when trying to access a bucket that
+	// has not been created yet.
+	ErrBucketNotFound = walletdb.ErrBucketNotFound
+
+	// ErrBucketExists is returned when creating a bucket that already
+	// exists.
+	ErrBucketExists = walletdb.ErrBucketExists
+
+	// ErrDatabaseNotOpen is returned when a database instance is accessed
+	// before it is opened or after it is closed.
+	ErrDatabaseNotOpen = walletdb.ErrDbNotOpen
+)