Adding acceptance test for #1257, fixed panic

acceptance_tests/basic.sh

@@ -331,4 +331,26 @@ EOM
   assertEquals "$expected" "$X"
 }
 
+testBasicMultiplyAssignMultiDoc() {
+      cat >test.yml <<EOL
+a: 1
+---
+b: 2
+EOL
+
+read -r -d '' expected << EOM
+a: 1
+c: 3
+---
+b: 2
+c: 3
+EOM
+
+
+  X=$(./yq '. *= {"c":3}' test.yml)
+  assertEquals "$expected" "$X"
+}
+
+
+
 source ./scripts/shunit2

pkg/yqlib/operator_assign.go

@@ -22,6 +22,8 @@ func assignUpdateOperator(d *dataTreeNavigator, context Context, expressionNode
 	}
 
 	prefs := assignPreferences{}
+	// they way *= (multipleAssign) is handled, we set the multiplePrefs
+	// on the node, not assignPrefs. Long story.
 	if p, ok := expressionNode.Operation.Preferences.(assignPreferences); ok {
 		prefs = p
 	}

release_notes.txt

@@ -1,3 +1,6 @@
+4.25.4:
+  - Fixed panic when using multiply assign on multiple documents #1256 Thanks @care0717
+
 4.25.3:
   - xml decoder now maintains namespaces by default. Use new flags to disable if required. Thanks @rndmit
   - Length and other similar operators no longer return comments (#1231)