fix filechecker in health with precondition check

Signed-off-by: andy xie <andy.xning@gmail.com>

health/checks/checks.go

@@ -2,9 +2,11 @@ package checks
 
 import (
 	"errors"
+	"fmt"
 	"net"
 	"net/http"
 	"os"
+	"path/filepath"
 	"strconv"
 	"time"
 
@@ -15,10 +17,19 @@ import (
 // if the file exists.
 func FileChecker(f string) health.Checker {
 	return health.CheckFunc(func() error {
-		if _, err := os.Stat(f); err == nil {
-			return errors.New("file exists")
+		absoluteFilePath, err := filepath.Abs(f)
+		if err != nil {
+			return fmt.Errorf("failed to get absolute path for %q: %v", f, err)
 		}
-		return nil
+
+		_, err = os.Stat(absoluteFilePath)
+		if err == nil {
+			return errors.New("file exists")
+		} else if os.IsNotExist(err) {
+			return nil
+		}
+
+		return err
 	})
 }
 

health/doc.go

@@ -122,6 +122,11 @@
 //  # curl localhost:5001/debug/health
 //  {"fileChecker":"file exists"}
 //
+// FileChecker only accepts absolute or relative file path. It does not work properly with tilde(~).
+// You should make sure that the application has proper permission(read and execute permission
+// for directory along with the specified file path). Otherwise, the FileChecker will report error
+// and file health check is not ok.
+//
 // You could also test the connectivity to a downstream service by using a
 // "HTTPChecker", but ensure that you only mark the test unhealthy if there
 // are a minimum of two failures in a row: