Disallow writeback caching with cache_after (#11002)

fixes #10974

cmd/config/cache/lookup.go

@@ -222,6 +222,10 @@ func LookupConfig(kvs config.KVS) (Config, error) {
 		if err != nil {
 			return cfg, err
 		}
+		if cfg.After > 0 && cfg.CommitWriteback {
+			err := errors.New("cache after cannot be used with commit writeback")
+			return cfg, config.ErrInvalidCacheSetting(err)
+		}
 	}
 
 	return cfg, nil

cmd/config/errors.go

@@ -108,6 +108,11 @@ var (
 		"MINIO_CACHE_COMMIT: Valid expected value is `writeback` or `writethrough`",
 	)
 
+	ErrInvalidCacheSetting = newErrFn(
+		"Incompatible cache setting",
+		"Please check the passed value",
+		"MINIO_CACHE_AFTER cannot be used with MINIO_CACHE_COMMIT setting",
+	)
 	ErrInvalidRotatingCredentialsBackendEncrypted = newErrFn(
 		"Invalid rotating credentials",
 		"Please set correct rotating credentials in the environment for decryption",