Add JSON Path expression evaluation support (#7315)

- Includes support for FROM clause JSON path

pkg/s3select/sql/jsonpath.go

@@ -0,0 +1,100 @@
+/*
+ * Minio Cloud Storage, (C) 2019 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package sql
+
+import (
+	"errors"
+
+	"github.com/bcicen/jstream"
+)
+
+var (
+	errKeyLookup                 = errors.New("Cannot look up key in non-object value")
+	errIndexLookup               = errors.New("Cannot look up array index in non-array value")
+	errWildcardObjectLookup      = errors.New("Object wildcard used on non-object value")
+	errWildcardArrayLookup       = errors.New("Array wildcard used on non-array value")
+	errWilcardObjectUsageInvalid = errors.New("Invalid usage of object wildcard")
+)
+
+func jsonpathEval(p []*JSONPathElement, v interface{}) (r interface{}, err error) {
+	// fmt.Printf("JPATHexpr: %v jsonobj: %v\n\n", p, v)
+	if len(p) == 0 || v == nil {
+		return v, nil
+	}
+
+	switch {
+	case p[0].Key != nil:
+		key := p[0].Key.keyString()
+
+		kvs, ok := v.(jstream.KVS)
+		if !ok {
+			return nil, errKeyLookup
+		}
+		for _, kv := range kvs {
+			if kv.Key == key {
+				return jsonpathEval(p[1:], kv.Value)
+			}
+		}
+		// Key not found - return nil result
+		return nil, nil
+
+	case p[0].Index != nil:
+		idx := *p[0].Index
+
+		arr, ok := v.([]interface{})
+		if !ok {
+			return nil, errIndexLookup
+		}
+
+		if idx >= len(arr) {
+			return nil, nil
+		}
+		return jsonpathEval(p[1:], arr[idx])
+
+	case p[0].ObjectWildcard:
+		kvs, ok := v.(jstream.KVS)
+		if !ok {
+			return nil, errWildcardObjectLookup
+		}
+
+		if len(p[1:]) > 0 {
+			return nil, errWilcardObjectUsageInvalid
+		}
+
+		return kvs, nil
+
+	case p[0].ArrayWildcard:
+		arr, ok := v.([]interface{})
+		if !ok {
+			return nil, errWildcardArrayLookup
+		}
+
+		// Lookup remainder of path in each array element and
+		// make result array.
+		var result []interface{}
+		for _, a := range arr {
+			rval, err := jsonpathEval(p[1:], a)
+			if err != nil {
+				return nil, err
+			}
+
+			result = append(result, rval)
+		}
+		return result, nil
+	}
+	panic("cannot reach here")
+}