accessPolicy: Implement Put, Get, Delete access policy.

This patch implements Get,Put,Delete bucket policies

Supporting - http://docs.aws.amazon.com/AmazonS3/latest/dev/access-policy-language-overview.html

Currently supports following actions.

   "*":                             true,
   "s3:*":                          true,
   "s3:GetObject":                  true,
   "s3:ListBucket":                 true,
   "s3:PutObject":                  true,
   "s3:CreateBucket":               true,
   "s3:GetBucketLocation":          true,
   "s3:DeleteBucket":               true,
   "s3:DeleteObject":               true,
   "s3:AbortMultipartUpload":       true,
   "s3:ListBucketMultipartUploads": true,
   "s3:ListMultipartUploadParts":   true,

following conditions for "StringEquals" and "StringNotEquals"

   "s3:prefix", "s3:max-keys"

bucket-policy.go

@@ -0,0 +1,147 @@
+/*
+ * Minio Cloud Storage, (C) 2015, 2016 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 main
+
+import (
+	"io/ioutil"
+	"os"
+	"path/filepath"
+
+	"github.com/minio/minio/pkg/fs"
+	"github.com/minio/minio/pkg/probe"
+)
+
+// getBucketsConfigPath - get buckets path.
+func getBucketsConfigPath() (string, *probe.Error) {
+	configPath, err := getConfigPath()
+	if err != nil {
+		return "", err.Trace()
+	}
+	return filepath.Join(configPath, "buckets"), nil
+}
+
+// createBucketsConfigPath - create buckets directory.
+func createBucketsConfigPath() *probe.Error {
+	bucketsConfigPath, err := getBucketsConfigPath()
+	if err != nil {
+		return err
+	}
+	if e := os.MkdirAll(bucketsConfigPath, 0700); e != nil {
+		return probe.NewError(e)
+	}
+	return nil
+}
+
+// getBucketConfigPath - get bucket path.
+func getBucketConfigPath(bucket string) (string, *probe.Error) {
+	bucketsConfigPath, err := getBucketsConfigPath()
+	if err != nil {
+		return "", err.Trace()
+	}
+	return filepath.Join(bucketsConfigPath, bucket), nil
+}
+
+// createBucketConfigPath - create bucket directory.
+func createBucketConfigPath(bucket string) *probe.Error {
+	bucketConfigPath, err := getBucketConfigPath(bucket)
+	if err != nil {
+		return err
+	}
+	if e := os.MkdirAll(bucketConfigPath, 0700); e != nil {
+		return probe.NewError(e)
+	}
+	return nil
+}
+
+// readBucketPolicy - read bucket policy.
+func readBucketPolicy(bucket string) ([]byte, *probe.Error) {
+	// Verify bucket is valid.
+	if !fs.IsValidBucketName(bucket) {
+		return nil, probe.NewError(fs.BucketNameInvalid{Bucket: bucket})
+	}
+
+	bucketConfigPath, err := getBucketConfigPath(bucket)
+	if err != nil {
+		return nil, err.Trace()
+	}
+
+	// Get policy file.
+	bucketPolicyFile := filepath.Join(bucketConfigPath, "access-policy.json")
+	if _, e := os.Stat(bucketPolicyFile); e != nil {
+		if os.IsNotExist(e) {
+			return nil, probe.NewError(fs.BucketPolicyNotFound{Bucket: bucket})
+		}
+		return nil, probe.NewError(e)
+	}
+
+	accessPolicyBytes, e := ioutil.ReadFile(bucketPolicyFile)
+	if e != nil {
+		return nil, probe.NewError(e)
+	}
+	return accessPolicyBytes, nil
+}
+
+// removeBucketPolicy - remove bucket policy.
+func removeBucketPolicy(bucket string) *probe.Error {
+	// Verify bucket is valid.
+	if !fs.IsValidBucketName(bucket) {
+		return probe.NewError(fs.BucketNameInvalid{Bucket: bucket})
+	}
+
+	bucketConfigPath, err := getBucketConfigPath(bucket)
+	if err != nil {
+		return err.Trace(bucket)
+	}
+
+	// Get policy file.
+	bucketPolicyFile := filepath.Join(bucketConfigPath, "access-policy.json")
+	if _, e := os.Stat(bucketPolicyFile); e != nil {
+		if os.IsNotExist(e) {
+			return probe.NewError(fs.BucketPolicyNotFound{Bucket: bucket})
+		}
+		return probe.NewError(e)
+	}
+	return nil
+}
+
+// writeBucketPolicy - save bucket policy.
+func writeBucketPolicy(bucket string, accessPolicyBytes []byte) *probe.Error {
+	// Verify if bucket path legal
+	if !fs.IsValidBucketName(bucket) {
+		return probe.NewError(fs.BucketNameInvalid{Bucket: bucket})
+	}
+
+	bucketConfigPath, err := getBucketConfigPath(bucket)
+	if err != nil {
+		return err.Trace()
+	}
+
+	// Get policy file.
+	bucketPolicyFile := filepath.Join(bucketConfigPath, "access-policy.json")
+	if _, e := os.Stat(bucketPolicyFile); e != nil {
+		if !os.IsNotExist(e) {
+			return probe.NewError(e)
+		}
+	}
+
+	// Write bucket policy.
+	if e := ioutil.WriteFile(bucketPolicyFile, accessPolicyBytes, 0600); e != nil {
+		return probe.NewError(e)
+	}
+
+	return nil
+}