Fix issue where path in walk would get mutated (#7657)

I'm still not sure where this mutation occurs, as the append operation
done in `walk` returns a copy and shouldn't mutate. But at least creating
a shallow copy rather than a slice fixes the issue, and is still quite
performant compared to the previous implementation that did a deep-copy
of the path in each iteration.

Fixes #7656

Signed-off-by: Anders Eknert <[email protected]>
(cherry picked from commit 9d0557b)

Author: anderseknert

v1/test/cases/testdata/v1/walkbuiltin/test-walkbuiltin-issue-7656.yaml

@@ -0,0 +1,26 @@
+---
+cases:
+  # verify fix for issue 7656
+  - note: walkbuiltin/array path not overwritten
+    query: data.generated.p = x
+    modules:
+      - |
+        package generated
+
+        x := {"a": [{"b": [{"c": [
+            {"aa": "AA"},
+            {"bb": "BB"},
+            {"cc": "CC"},
+            {"dd": "DDD"},
+        ]}]}]}
+
+        p := {path: value |
+            [path, value] := walk(x)
+            count(value) == 2
+        }
+    data: {}
+    want_result:
+      - x:
+          "[\"a\",0,\"b\",0,\"c\",0,\"aa\"]": "AA"
+          "[\"a\",0,\"b\",0,\"c\",1,\"bb\"]": "BB"
+          "[\"a\",0,\"b\",0,\"c\",2,\"cc\"]": "CC"

v1/topdown/walk.go

@@ -30,7 +30,7 @@ func walk(filter, path *ast.Array, input *ast.Term, iter func(*ast.Term) error)
 			pathCopy = ast.InternedEmptyArrayValue
 		} else {
 			// Shallow copy, as while the array is modified, the elements are not
-			pathCopy = path.Slice(0, path.Len())
+			pathCopy = copyShallow(path)
 		}
 
 		// TODO(ae): I'd *really* like these terms to be retrieved from a sync.Pool, and
@@ -48,8 +48,7 @@ func walk(filter, path *ast.Array, input *ast.Term, iter func(*ast.Term) error)
 		filter = filter.Slice(1, -1)
 		if key.IsGround() {
 			if term := input.Get(key); term != nil {
-				path = pathAppend(path, key)
-				return walk(filter, path, term, iter)
+				return walk(filter, pathAppend(path, key), term, iter)
 			}
 			return nil
 		}
@@ -149,6 +148,16 @@ func pathIsWildcard(operands []*ast.Term) bool {
 	return false
 }
 
+func copyShallow(arr *ast.Array) *ast.Array {
+	cpy := make([]*ast.Term, 0, arr.Len())
+
+	arr.Foreach(func(elem *ast.Term) {
+		cpy = append(cpy, elem)
+	})
+
+	return ast.NewArray(cpy...)
+}
+
 func init() {
 	RegisterBuiltinFunc(ast.WalkBuiltin.Name, evalWalk)
 }