Add a check for metrics directory existence.

Previously on startup the program would just quit without stating
explicitly why.

Change-Id: I833b85eb74d2dd27cdc3f0f2e65d7bb1c42caa39

storage/metric/tiered.go

@@ -112,8 +112,12 @@ const (
 
 const watermarkCacheLimit = 1024 * 1024
 
-func NewTieredStorage(appendToDiskQueueDepth, viewQueueDepth uint, flushMemoryInterval, memoryTTL time.Duration, root string) (*TieredStorage, error) {
-	diskStorage, err := NewLevelDBMetricPersistence(root)
+func NewTieredStorage(appendToDiskQueueDepth, viewQueueDepth uint, flushMemoryInterval, memoryTTL time.Duration, rootDirectory string) (*TieredStorage, error) {
+	if isDir, _ := utility.IsDir(rootDirectory); !isDir {
+		return nil, fmt.Errorf("Could not find metrics directory %s", rootDirectory)
+	}
+
+	diskStorage, err := NewLevelDBMetricPersistence(rootDirectory)
 	if err != nil {
 		return nil, err
 	}

utility/file_ext.go

@@ -0,0 +1,33 @@
+// Copyright 2013 Prometheus Team
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Useful file/filesystem related functions.
+
+package utility
+
+import (
+	"fmt"
+	"os"
+)
+
+// Returns true iff dirPath is a valid directory path.
+func IsDir(dirPath string) (bool, error) {
+	finfo, err := os.Stat(dirPath)
+	if err != nil {
+		return false, err
+	}
+	if !finfo.IsDir() {
+		return false, fmt.Errorf("%s not a directory", dirPath)
+	}
+	return true, nil
+}