Enforce go lint check and fix several lint issues (#570)

This fix updates the Makefile to add the `go lint` check
for the build. This fix also fixes several go lint issues.

NOTE: This fix does not enforce `go lint` (suggestion only).
This fix also ignores the `go lint` error:
```
middleware/middleware.go:72:1: context.Context should be the first parameter of a function
```
as it requires too many changes in API.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

Makefile

@@ -7,23 +7,27 @@ all: coredns
 # Phony this to ensure we always build the binary.
 # TODO: Add .go file dependencies.
 .PHONY: coredns
-coredns: deps core/zmiddleware.go core/dnsserver/zdirectives.go
+coredns: check core/zmiddleware.go core/dnsserver/zdirectives.go
 	go build $(BUILD_VERBOSE) -ldflags="-s -w"
 
 .PHONY: deps
-deps: fmt
+deps:
 	go get ${BUILD_VERBOSE}
+	go get -u github.com/golang/lint/golint
+
+.PHONY: check
+check: fmt deps
 
 .PHONY: test
-test: deps
+test: check
 	go test -race $(TEST_VERBOSE) ./test ./middleware/...
 
 .PHONY: testk8s
-testk8s: deps
+testk8s: check
 	go test -race $(TEST_VERBOSE) -tags=k8s -run 'TestKubernetes' ./test ./middleware/kubernetes/...
 
 .PHONY: coverage
-coverage: deps
+coverage: check
 	set -e -x
 	echo "" > coverage.txt
 	for d in `go list ./... | grep -v vendor`; do \
@@ -52,6 +56,11 @@ fmt:
 	@test -z "$$(gofmt -s -l . | grep -v vendor/ | tee /dev/stderr)" || \
 		(echo "please format Go code with 'gofmt -s -w'" && false)
 
+.PHONY: lint
+lint: deps
+	## run go lint, suggestion only (not enforced)
+	@test -z "$$(golint ./... | grep -v vendor/ | grep -v ".pb.go:" | grep -vE "context\.Context should be the first parameter of a function" | tee /dev/stderr)"
+
 .PHONY: distclean
 distclean: clean
 	# Clean all dependencies and build artifacts

core/dnsserver/server.go

@@ -155,10 +155,10 @@ func (s *Server) Address() string { return s.Addr }
 // defined in the request so that the correct zone
 // (configuration and middleware stack) will handle the request.
 func (s *Server) ServeDNS(w dns.ResponseWriter, r *dns.Msg) {
-	s.ServeDNSWithContext(context.Background(), w, r)
+	s.serveDNSWithContext(context.Background(), w, r)
 }
 
-func (s *Server) ServeDNSWithContext(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) {
+func (s *Server) serveDNSWithContext(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) {
 	defer func() {
 		// In case the user doesn't enable error middleware, we still
 		// need to make sure that we stay alive up here

middleware/trace/trace_test.go

@@ -6,15 +6,15 @@ import (
 	"github.com/mholt/caddy"
 )
 
-// CreateTestTrace creates a trace middleware to be used in tests
+// createTestTrace creates a trace middleware to be used in tests
-func CreateTestTrace(config string) (*caddy.Controller, *trace, error) {
+func createTestTrace(config string) (*caddy.Controller, *trace, error) {
 	c := caddy.NewTestController("dns", config)
 	m, err := traceParse(c)
 	return c, m, err
 }
 
 func TestTrace(t *testing.T) {
-	_, m, err := CreateTestTrace(`trace`)
+	_, m, err := createTestTrace(`trace`)
 	if err != nil {
 		t.Errorf("Error parsing test input: %s", err)
 		return