support for accessing the underlying index/store impls

now you can access the underlying index/store implementations
using the Advanced() method.  this is intedned for advanced
usage only, and can lead to problems if misused.

also, there is a new method NewUsing(...) which allows callers
of the top-level API to choose which underlying k/v store they
want to use.

index.go

@@ -11,6 +11,8 @@ package bleve
 
 import (
 	"github.com/blevesearch/bleve/document"
+	"github.com/blevesearch/bleve/index"
+	"github.com/blevesearch/bleve/index/store"
 )
 
 // A Batch groups together multiple Index and Delete
@@ -90,6 +92,8 @@ type Index interface {
 	GetInternal(key []byte) ([]byte, error)
 	SetInternal(key, val []byte) error
 	DeleteInternal(key []byte) error
+
+	Advanced() (index.Index, store.KVStore, error)
 }
 
 // A Classifier is an interface describing any object
@@ -105,6 +109,17 @@ func New(path string, mapping *IndexMapping) (Index, error) {
 	return newIndex(path, mapping)
 }
 
+// NewUsing creates index at the specified path,
+// which must not already exist.
+// The provided mapping will be used for all
+// Index/Search operations.
+// The specified kvstore implemenation will be used
+// and the provided kvconfig will be passed to its
+// constructor.
+func NewUsing(path string, mapping *IndexMapping, kvstore string, kvconfig map[string]interface{}) (Index, error) {
+	return newIndexUsing(path, mapping, kvstore, kvconfig)
+}
+
 // Open index at the specified path, must exist.
 // The mapping used when it was created will be used for all Index/Search operations.
 func Open(path string) (Index, error) {

index_alias_impl.go

@@ -14,6 +14,8 @@ import (
 	"sync"
 
 	"github.com/blevesearch/bleve/document"
+	"github.com/blevesearch/bleve/index"
+	"github.com/blevesearch/bleve/index/store"
 	"github.com/blevesearch/bleve/search"
 )
 
@@ -298,6 +300,22 @@ func (i *indexAliasImpl) DeleteInternal(key []byte) error {
 	return i.indexes[0].DeleteInternal(key)
 }
 
+func (i *indexAliasImpl) Advanced() (index.Index, store.KVStore, error) {
+	i.mutex.RLock()
+	defer i.mutex.RUnlock()
+
+	if !i.open {
+		return nil, nil, ErrorIndexClosed
+	}
+
+	err := i.isAliasToSingleIndex()
+	if err != nil {
+		return nil, nil, err
+	}
+
+	return i.indexes[0].Advanced()
+}
+
 func (i *indexAliasImpl) Add(indexes ...Index) {
 	i.mutex.Lock()
 	defer i.mutex.Unlock()

index_alias_impl_test.go

@@ -7,6 +7,8 @@ import (
 	"time"
 
 	"github.com/blevesearch/bleve/document"
+	"github.com/blevesearch/bleve/index"
+	"github.com/blevesearch/bleve/index/store"
 	"github.com/blevesearch/bleve/search"
 )
 
@@ -725,3 +727,7 @@ func (i *stubIndex) SetInternal(key, val []byte) error {
 func (i *stubIndex) DeleteInternal(key []byte) error {
 	return i.err
 }
+
+func (i *stubIndex) Advanced() (index.Index, store.KVStore, error) {
+	return nil, nil, nil
+}

index_impl.go

@@ -90,7 +90,7 @@ func newMemIndex(mapping *IndexMapping) (*indexImpl, error) {
 	return &rv, nil
 }
 
-func newIndex(path string, mapping *IndexMapping) (*indexImpl, error) {
+func newIndexUsing(path string, mapping *IndexMapping, kvstore string, kvconfig map[string]interface{}) (*indexImpl, error) {
 	// first validate the mapping
 	err := mapping.validate()
 	if err != nil {
@@ -104,7 +104,7 @@ func newIndex(path string, mapping *IndexMapping) (*indexImpl, error) {
 	rv := indexImpl{
 		path:  path,
 		m:     mapping,
-		meta:  newIndexMeta(Config.DefaultKVStore),
+		meta:  newIndexMeta(kvstore),
 		stats: &IndexStat{},
 	}
 	storeConstructor := registry.KVStoreConstructorByName(rv.meta.Storage)
@@ -116,14 +116,13 @@ func newIndex(path string, mapping *IndexMapping) (*indexImpl, error) {
 	if err != nil {
 		return nil, err
 	}
-	storeConfig := map[string]interface{}{
-		"path":              indexStorePath(path),
-		"create_if_missing": true,
-		"error_if_exists":   true,
+	if kvconfig == nil {
+		kvconfig = map[string]interface{}{}
 	}
+	kvconfig["path"] = indexStorePath(path)
 
 	// now open the store
-	rv.s, err = storeConstructor(storeConfig)
+	rv.s, err = storeConstructor(kvconfig)
 	if err != nil {
 		return nil, err
 	}
@@ -153,6 +152,14 @@ func newIndex(path string, mapping *IndexMapping) (*indexImpl, error) {
 	return &rv, nil
 }
 
+func newIndex(path string, mapping *IndexMapping) (*indexImpl, error) {
+	defaultKVConfig := map[string]interface{}{
+		"create_if_missing": true,
+		"error_if_exists":   true,
+	}
+	return newIndexUsing(path, mapping, Config.DefaultKVStore, defaultKVConfig)
+}
+
 func openIndex(path string) (*indexImpl, error) {
 
 	rv := indexImpl{
@@ -224,6 +231,12 @@ func openIndex(path string) (*indexImpl, error) {
 	return &rv, nil
 }
 
+// Advanced returns implementation internals
+// necessary ONLY for advanced usage.
+func (i *indexImpl) Advanced() (index.Index, store.KVStore, error) {
+	return i.i, i.s, nil
+}
+
 // Mapping returns the IndexMapping in use by this
 // Index.
 func (i *indexImpl) Mapping() *IndexMapping {