Adding first version of HealthCheck

Added a expvar style handler for the debug http server to allow health checks (/debug/health).

Signed-off-by: Diogo Monica <diogo@docker.com>

health/checks/checks.go

@@ -0,0 +1,35 @@
+package checks
+
+import (
+	"errors"
+	"github.com/docker/distribution/health"
+	"net/http"
+	"os"
+)
+
+// FileChecker checks the existence of a file and returns and error
+// if the file exists, taking the application out of rotation
+func FileChecker(f string) health.Checker {
+	return health.CheckFunc(func() error {
+		if _, err := os.Stat(f); err == nil {
+			return errors.New("file exists")
+		}
+		return nil
+	})
+}
+
+// HTTPChecker does a HEAD request and verifies if the HTTP status
+// code return is a 200, taking the application out of rotation if
+// otherwise
+func HTTPChecker(r string) health.Checker {
+	return health.CheckFunc(func() error {
+		response, err := http.Head(r)
+		if err != nil {
+			return errors.New("error while checking: " + r)
+		}
+		if response.StatusCode != http.StatusOK {
+			return errors.New("downstream service returned unexpected status: " + string(response.StatusCode))
+		}
+		return nil
+	})
+}