Ensure that we use constants everywhere (#7845)

This allows for canonicalization of the strings
throughout our code and provides a common space
for all these constants to reside.

This list is rather non-exhaustive but captures
all the headers used in AWS S3 API operations

cmd/http/headers.go

@@ -0,0 +1,81 @@
+/*
+ * MinIO Cloud Storage, (C) 2019 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 http
+
+// Standard S3 HTTP response constants
+const (
+	LastModified       = "Last-Modified"
+	Date               = "Date"
+	ETag               = "ETag"
+	ContentType        = "Content-Type"
+	ContentMD5         = "Content-Md5"
+	ContentEncoding    = "Content-Encoding"
+	Expires            = "Expires"
+	ContentLength      = "Content-Length"
+	ContentLanguage    = "Content-Language"
+	ContentRange       = "Content-Range"
+	Connection         = "Connection"
+	AcceptRanges       = "Accept-Ranges"
+	AmzBucketRegion    = "X-Amz-Bucket-Region"
+	ServerInfo         = "Server"
+	RetryAfter         = "Retry-After"
+	Location           = "Location"
+	CacheControl       = "Cache-Control"
+	ContentDisposition = "Content-Disposition"
+	Authorization      = "Authorization"
+	Action             = "Action"
+)
+
+// Standard S3 HTTP request constants
+const (
+	IfModifiedSince   = "If-Modified-Since"
+	IfUnmodifiedSince = "If-Unmodified-Since"
+	IfMatch           = "If-Match"
+	IfNoneMatch       = "If-None-Match"
+
+	// S3 extensions
+	AmzCopySourceIfModifiedSince   = "x-amz-copy-source-if-modified-since"
+	AmzCopySourceIfUnmodifiedSince = "x-amz-copy-source-if-unmodified-since"
+
+	AmzCopySourceIfNoneMatch = "x-amz-copy-source-if-none-match"
+	AmzCopySourceIfMatch     = "x-amz-copy-source-if-match"
+
+	AmzCopySource          = "X-Amz-Copy-Source"
+	AmzCopySourceVersionID = "X-Amz-Copy-Source-Version-Id"
+	AmzCopySourceRange     = "X-Amz-Copy-Source-Range"
+
+	// Signature V4 related contants.
+	AmzContentSha256        = "X-Amz-Content-Sha256"
+	AmzDate                 = "X-Amz-Date"
+	AmzAlgorithm            = "X-Amz-Algorithm"
+	AmzExpires              = "X-Amz-Expires"
+	AmzSignedHeaders        = "X-Amz-SignedHeaders"
+	AmzSignature            = "X-Amz-Signature"
+	AmzCredential           = "X-Amz-Credential"
+	AmzSecurityToken        = "X-Amz-Security-Token"
+	AmzDecodedContentLength = "X-Amz-Decoded-Content-Length"
+
+	// Signature v2 related constants
+	AmzSignatureV2 = "Signature"
+	AmzAccessKeyID = "AWSAccessKeyId"
+
+	// Response request id.
+	AmzRequestID = "x-amz-request-id"
+
+	// Deployment id.
+	MinioDeploymentID = "x-minio-deployment-id"
+)