Add androidmk error for multiple assignments

Identify variables that are cleared or reassigned, add an error and
prevent reassignment which caused a parser error.

Test: go test androidmk_test
Bug: 112653593
Change-Id: I0db3372b60812ff4cdaebb38a56ed0af0dbdb27e

androidmk/androidmk/androidmk.go

@@ -458,6 +458,9 @@ func setVariable(file *bpFile, plusequals bool, prefix, name string, value bppar
 			}
 			file.defs = append(file.defs, a)
 		} else {
+			if _, ok := file.globalAssignments[name]; ok {
+				return fmt.Errorf("cannot assign a variable multiple times: \"%s\"", name)
+			}
 			a := &bpparser.Assignment{
 				Name:      name,
 				NamePos:   pos,

androidmk/androidmk/androidmk_test.go

@@ -1386,6 +1386,35 @@ cc_binary {
     name: "test",
     srcs: dashed_dash_variable,
 }
+`,
+	},
+	{
+		desc: "variableReassigned",
+		in: `
+include $(CLEAR_VARS)
+
+src_files:= a.cpp
+
+LOCAL_SRC_FILES:= $(src_files)
+LOCAL_MODULE:= test
+include $(BUILD_EXECUTABLE)
+
+# clear locally used variable
+src_files:=
+`,
+		expected: `
+
+
+src_files = ["a.cpp"]
+cc_binary {
+    name: "test",
+
+    srcs: src_files,
+}
+
+// clear locally used variable
+// ANDROIDMK TRANSLATION ERROR: cannot assign a variable multiple times: "src_files"
+// src_files :=
 `,
 	},
 	{