Code restructuring, assigning proper subsystems to each

- Drivers contain
    * donut/*
    * file/*
    * memory/*

 - Storage format contains
    * donut/*
      - GetObject() --> renamed to GetObjectReader()
      - Deleted stale objectwriter.go, renamed donutwriter.go to object_writer.go

Simplifying, and documenting codebase further

pkg/drivers/file/file_bucket.go

@@ -0,0 +1,133 @@
+/*
+ * Minimalist Object File, (C) 2015 Minio, Inc.
+ *
+ * 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.
+ */
+
+package file
+
+import (
+	"os"
+	"path"
+	"sort"
+	"strings"
+
+	"io/ioutil"
+	"path/filepath"
+
+	"github.com/minio-io/minio/pkg/drivers"
+)
+
+/// Bucket Operations
+
+// ListBuckets - Get service
+func (file *fileDriver) ListBuckets() ([]drivers.BucketMetadata, error) {
+	files, err := ioutil.ReadDir(file.root)
+	if err != nil {
+		return []drivers.BucketMetadata{}, drivers.EmbedError("bucket", "", err)
+	}
+
+	var metadataList []drivers.BucketMetadata
+	for _, fileName := range files {
+		// Skip policy files
+		if strings.HasSuffix(fileName.Name(), "_policy.json") {
+			continue
+		}
+		if !fileName.IsDir() {
+			return []drivers.BucketMetadata{}, drivers.BackendCorrupted{Path: file.root}
+		}
+		metadata := drivers.BucketMetadata{
+			Name:    fileName.Name(),
+			Created: fileName.ModTime(), // TODO - provide real created time
+		}
+		metadataList = append(metadataList, metadata)
+	}
+	return metadataList, nil
+}
+
+// CreateBucket - PUT Bucket
+func (file *fileDriver) CreateBucket(bucket string) error {
+	file.lock.Lock()
+	defer file.lock.Unlock()
+
+	// verify bucket path legal
+	if drivers.IsValidBucket(bucket) == false {
+		return drivers.BucketNameInvalid{Bucket: bucket}
+	}
+
+	// get bucket path
+	bucketDir := path.Join(file.root, bucket)
+
+	// check if bucket exists
+	if _, err := os.Stat(bucketDir); err == nil {
+		return drivers.BucketExists{
+			Bucket: bucket,
+		}
+	}
+
+	// make bucket
+	err := os.Mkdir(bucketDir, 0700)
+	if err != nil {
+		return drivers.EmbedError(bucket, "", err)
+	}
+	return nil
+}
+
+// ListObjects - GET bucket (list objects)
+func (file *fileDriver) ListObjects(bucket string, resources drivers.BucketResourcesMetadata) ([]drivers.ObjectMetadata, drivers.BucketResourcesMetadata, error) {
+	p := bucketDir{}
+	p.files = make(map[string]os.FileInfo)
+
+	if drivers.IsValidBucket(bucket) == false {
+		return []drivers.ObjectMetadata{}, resources, drivers.BucketNameInvalid{Bucket: bucket}
+	}
+	if resources.Prefix != "" && drivers.IsValidObject(resources.Prefix) == false {
+		return []drivers.ObjectMetadata{}, resources, drivers.ObjectNameInvalid{Bucket: bucket, Object: resources.Prefix}
+	}
+
+	rootPrefix := path.Join(file.root, bucket)
+	// check bucket exists
+	if _, err := os.Stat(rootPrefix); os.IsNotExist(err) {
+		return []drivers.ObjectMetadata{}, resources, drivers.BucketNotFound{Bucket: bucket}
+	}
+
+	p.root = rootPrefix
+	err := filepath.Walk(rootPrefix, p.getAllFiles)
+	if err != nil {
+		return []drivers.ObjectMetadata{}, resources, drivers.EmbedError(bucket, "", err)
+	}
+
+	var metadataList []drivers.ObjectMetadata
+	var metadata drivers.ObjectMetadata
+
+	// Populate filtering mode
+	resources.Mode = drivers.GetMode(resources)
+
+	for name, f := range p.files {
+		if len(metadataList) >= resources.Maxkeys {
+			resources.IsTruncated = true
+			goto ret
+		}
+		metadata, resources, err = file.filter(bucket, name, f, resources)
+		if err != nil {
+			return []drivers.ObjectMetadata{}, resources, drivers.EmbedError(bucket, "", err)
+		}
+		if metadata.Bucket != "" {
+			metadataList = append(metadataList, metadata)
+		}
+	}
+
+ret:
+	sort.Sort(byObjectKey(metadataList))
+	return metadataList, resources, nil
+}