Makefile: add vet target

Add a vet target in order to catch suspicious constructs
reported by go vet.

Vet has successfully detected problems in the past,
for example, see

  482460c4c8
  fc36b1cd94
  68a41035a9
  7b704fb77d
  4f3f85b165
  95fa353ee9
  4bfe18b40d

Some vet flags are noisy. In particular, the following flags
reports a large amount of generally unharmful constructs:

  -assign: check for useless assignments
  -composites: check that composite literals used field-keyed
               elements
  -shadow: check for shadowed variables
  -shadowstrict: whether to be strict about shadowing
  -unreachable: check for unreachable code

In order to skip running the flags mentioned above, vet is
invoked on a directory basis with 'go tool vet .' since package-
level type-checking with 'go vet' doesn't accept flags.

Hence, each file is vetted in isolation, which is weaker than
package-level type-checking. But nevertheless, it might catch
suspicious constructs that pose a real problem.

The vet target runs the following flags on the entire repo:

  -asmdecl: check assembly against Go declarations
  -atomic: check for common mistaken usages of the
           sync/atomic package
  -bool: check for mistakes involving boolean operators
  -buildtags: check that +build tags are valid
  -copylocks: check that locks are not passed by value
  -methods: check that canonically named methods are canonically
            defined
  -nilfunc: check for comparisons between functions and nil
  -printf: check printf-like invocations
  -rangeloops: check that range loop variables are used correctly
  -shift: check for useless shifts
  -structtags: check that struct field tags have canonical format
               and apply to exported fields as needed
  -unsafeptr: check for misuse of unsafe.Pointer

Now and then, it might make sense to check the output of

VETARGS=-unreachable make vet

manually, just in case it detects several lines of dead code etc.

Makefile

@@ -1,4 +1,5 @@
 TEST?=./...
+VETARGS?=-asmdecl -atomic -bool -buildtags -copylocks -methods -nilfunc -printf -rangeloops -shift -structtags -unsafeptr
 
 default: test
 
@@ -10,6 +11,7 @@ dev: generate
 
 test: generate
 	TF_ACC= go test $(TEST) $(TESTARGS) -timeout=10s -parallel=4
+	@$(MAKE) vet
 
 testacc: generate
 	@if [ "$(TEST)" = "./..." ]; then \
@@ -26,10 +28,22 @@ updatedeps:
 		git symbolic-ref --short HEAD 2>/dev/null \
 		|| git rev-parse HEAD"))
 	go get -u golang.org/x/tools/cmd/stringer
+	go get -u golang.org/x/tools/cmd/vet
 	go get -f -u -v ./...
 	git checkout $(REF)
 
+vet:
+	@go tool vet 2>/dev/null ; if [ $$? -eq 3 ]; then \
+		go get golang.org/x/tools/cmd/vet; \
+	fi
+	@echo "go tool vet $(VETARGS) ."
+	@go tool vet $(VETARGS) . ; if [ $$? -eq 1 ]; then \
+		echo ""; \
+		echo "Vet found suspicious constructs. Please check the reported constructs"; \
+		echo "and fix them if necessary before submitting the code for reviewal."; \
+	fi
+
 generate:
 	go generate ./...
 
-.PHONY: bin default generate test updatedeps
+.PHONY: bin default generate test updatedeps vet