check if running `go mod vendor` has an effect on vendor directory and if so fail

.circleci/config.yml

@@ -10,3 +10,16 @@ jobs:
     steps:
       - checkout
       - run: make ci
+  check-vendor-vs-mod:
+    docker:
+      - image: circleci/golang:1.12
+    working_directory: /go/src/github.com/hashicorp/packer
+    steps:
+      - checkout
+      - run: make check-vendor-vs-mod
+workflows:
+  version: 2
+  build_and_check_vendor_vs_module:
+    jobs:
+      - build
+      - check-vendor-vs-mod

Makefile

@@ -111,6 +111,13 @@ testacc: deps generate ## Run acceptance tests
 testrace: fmt-check mode-check vet ## Test with race detection enabled
 	@GO111MODULE=off go test -race $(TEST) $(TESTARGS) -timeout=3m -p=8
 
+check-vendor-vs-mod:
+	@GO111MODULE=on go mod vendor 
+	@git diff --exit-code --ignore-space-change --ignore-space-at-eol -- vendor ; if [ $$? -eq 1 ]; then \
+		echo "ERROR: vendor dir is not on par with go modules definition." && \
+		exit 1; \
+	fi
+
 updatedeps:
 	@echo "INFO: Packer deps are managed by go modules. See .github/CONTRIBUTING.md"