refactor: rewrite auth handler code (!89)

* Rewrite the code handling basic auth to make it easier to extend for other types of auth.
* The behaviour of the existing code is maintained.
* No changes to how basic auth is configured from a user's perspective.

https://gitlab.com/hectorjsmith/fail2ban-prometheus-exporter/-/merge_requests/89

server/middleware.go

@@ -0,0 +1,17 @@
+package server
+
+import (
+	"net/http"
+
+	"gitlab.com/hectorjsmith/fail2ban-prometheus-exporter/auth"
+)
+
+func BasicAuthMiddleware(handlerFunc http.HandlerFunc, authProvider auth.AuthProvider) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		if authProvider.IsAllowed(r) {
+			handlerFunc.ServeHTTP(w, r)
+		} else {
+			w.WriteHeader(http.StatusUnauthorized)
+		}
+	}
+}

server/middleware_test.go

@@ -0,0 +1,46 @@
+package server
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+type testAuthProvider struct {
+	match bool
+}
+
+func (p testAuthProvider) IsAllowed(request *http.Request) bool {
+	return p.match
+}
+
+func newTestRequest() *http.Request {
+	return httptest.NewRequest(http.MethodGet, "http://example.com", nil)
+}
+
+func executeBasicAuthMiddlewareTest(t *testing.T, authMatches bool, expectedCode int, expectedCallCount int) {
+	callCount := 0
+	testHandler := func(w http.ResponseWriter, r *http.Request) {
+		callCount++
+	}
+
+	handler := BasicAuthMiddleware(testHandler, testAuthProvider{match: authMatches})
+	recorder := httptest.NewRecorder()
+	request := newTestRequest()
+	handler.ServeHTTP(recorder, request)
+
+	if recorder.Code != expectedCode {
+		t.Errorf("statusCode = %v, want %v", recorder.Code, expectedCode)
+	}
+	if callCount != expectedCallCount {
+		t.Errorf("callCount = %v, want %v", callCount, expectedCallCount)
+	}
+}
+
+func Test_GIVEN_MatchingBasicAuth_WHEN_MethodCalled_THEN_RequestProcessed(t *testing.T) {
+	executeBasicAuthMiddlewareTest(t, true, http.StatusOK, 1)
+}
+
+func Test_GIVEN_NonMatchingBasicAuth_WHEN_MethodCalled_THEN_RequestRejected(t *testing.T) {
+	executeBasicAuthMiddlewareTest(t, false, http.StatusUnauthorized, 0)
+}