fix race condition in incorrectly shared state in MultiSearch

When performing a MultiSearch, we create child SearchRequests
from the original SearchRequest.  In doing so we copy many fields.
But, copying of the SortOrder was incorrect, as this contains
state, and distint SortOrder objects must be used.  This change
introduces a Copy() method to the SearchSort interface, and
to the SortOrder types.  MultiSearch now creates a new copy of
the SortOrder for each child request.

index_alias_impl.go

@@ -432,7 +432,7 @@ func createChildSearchRequest(req *SearchRequest) *SearchRequest {
 		Fields:           req.Fields,
 		Facets:           req.Facets,
 		Explain:          req.Explain,
-		Sort:             req.Sort,
+		Sort:             req.Sort.Copy(),
 		IncludeLocations: req.IncludeLocations,
 	}
 	return &rv

search/sort.go

@@ -36,6 +36,8 @@ type SearchSort interface {
 	RequiresDocID() bool
 	RequiresScoring() bool
 	RequiresFields() []string
+
+	Copy() SearchSort
 }
 
 func ParseSearchSortObj(input map[string]interface{}) (SearchSort, error) {
@@ -205,6 +207,14 @@ func (so SortOrder) UpdateVisitor(field string, term []byte) {
 	}
 }
 
+func (so SortOrder) Copy() SortOrder {
+	rv := make(SortOrder, len(so))
+	for i, soi := range so {
+		rv[i] = soi.Copy()
+	}
+	return rv
+}
+
 // Compare will compare two document matches using the specified sort order
 // if both are numbers, we avoid converting back to term
 func (so SortOrder) Compare(cachedScoring, cachedDesc []bool, i, j *DocumentMatch) int {
@@ -475,6 +485,12 @@ func (s *SortField) MarshalJSON() ([]byte, error) {
 	return json.Marshal(sfm)
 }
 
+func (s *SortField) Copy() SearchSort {
+	var rv SortField
+	rv = *s
+	return &rv
+}
+
 // SortDocID will sort results by the document identifier
 type SortDocID struct {
 	Desc bool
@@ -512,6 +528,12 @@ func (s *SortDocID) MarshalJSON() ([]byte, error) {
 	return json.Marshal("_id")
 }
 
+func (s *SortDocID) Copy() SearchSort {
+	var rv SortDocID
+	rv = *s
+	return &rv
+}
+
 // SortScore will sort results by the document match score
 type SortScore struct {
 	Desc bool
@@ -549,6 +571,12 @@ func (s *SortScore) MarshalJSON() ([]byte, error) {
 	return json.Marshal("_score")
 }
 
+func (s *SortScore) Copy() SearchSort {
+	var rv SortScore
+	rv = *s
+	return &rv
+}
+
 var maxDistance = string(numeric.MustNewPrefixCodedInt64(math.MaxInt64, 0))
 
 // NewSortGeoDistance creates SearchSort instance for sorting documents by
@@ -675,3 +703,9 @@ func (s *SortGeoDistance) MarshalJSON() ([]byte, error) {
 
 	return json.Marshal(sfm)
 }
+
+func (s *SortGeoDistance) Copy() SearchSort {
+	var rv SortGeoDistance
+	rv = *s
+	return &rv
+}