enable read_only mode for boltdb indexes

fixes #405

index/store/boltdb/store.go

@@ -59,19 +59,27 @@ func New(mo store.MergeOperator, config map[string]interface{}) (store.KVStore,
 		fillPercent = bolt.DefaultFillPercent
 	}
 
-	db, err := bolt.Open(path, 0600, nil)
+	bo := &bolt.Options{}
+	ro, ok := config["read_only"].(bool)
+	if ok {
+		bo.ReadOnly = ro
+	}
+
+	db, err := bolt.Open(path, 0600, bo)
 	if err != nil {
 		return nil, err
 	}
 	db.NoSync = noSync
 
-	err = db.Update(func(tx *bolt.Tx) error {
-		_, err := tx.CreateBucketIfNotExists([]byte(bucket))
+	if !bo.ReadOnly {
+		err = db.Update(func(tx *bolt.Tx) error {
+			_, err := tx.CreateBucketIfNotExists([]byte(bucket))
 
-		return err
-	})
-	if err != nil {
-		return nil, err
+			return err
+		})
+		if err != nil {
+			return nil, err
+		}
 	}
 
 	rv := Store{

index_test.go

@@ -1564,3 +1564,59 @@ func BenchmarkBatchOverhead(b *testing.B) {
 		batch.Reset()
 	}
 }
+
+func TestOpenReadonlyMultiple(t *testing.T) {
+	defer func() {
+		err := os.RemoveAll("testidx")
+		if err != nil {
+			t.Fatal(err)
+		}
+	}()
+
+	// build an index and close it
+	index, err := New("testidx", NewIndexMapping())
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	doca := map[string]interface{}{
+		"name": "marty",
+		"desc": "gophercon india",
+	}
+	err = index.Index("a", doca)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = index.Close()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// now open it read-only
+	index, err = OpenUsing("testidx", map[string]interface{}{
+		"read_only": true,
+	})
+
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// now open it again
+	index2, err := OpenUsing("testidx", map[string]interface{}{
+		"read_only": true,
+	})
+
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = index.Close()
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = index2.Close()
+	if err != nil {
+		t.Fatal(err)
+	}
+}