Preserve ETag case for S3 compatibility (minio#7498)

Most hadoop distributions hortonworks, cloudera all
depend on aws-sdk-java 1.7.x to 1.10.x - the releases
which have bugs related case sensitive check for
ETag header. Go changes the case of the headers set
to be canonical but only preserves them when set
through a direct map.

This fixes most compatibility issues we have had
in the past supporting older hadoop distributions.

cmd/api-headers.go

@@ -76,7 +76,7 @@ func setObjectHeaders(w http.ResponseWriter, objInfo ObjectInfo, rs *HTTPRangeSp
 
 	// Set Etag if available.
 	if objInfo.ETag != "" {
-		w.Header().Set("ETag", "\""+objInfo.ETag+"\"")
+		w.Header()["ETag"] = []string{"\"" + objInfo.ETag + "\""}
 	}
 
 	if objInfo.ContentType != "" {
@@ -90,6 +90,7 @@ func setObjectHeaders(w http.ResponseWriter, objInfo ObjectInfo, rs *HTTPRangeSp
 	if !objInfo.Expires.IsZero() {
 		w.Header().Set("Expires", objInfo.Expires.UTC().Format(http.TimeFormat))
 	}
+
 	// Set all other user defined metadata.
 	for k, v := range objInfo.UserDefined {
 		if hasPrefix(k, ReservedMetadataPrefix) {

cmd/bucket-handlers.go

@@ -669,7 +669,7 @@ func (api objectAPIHandlers) PostPolicyBucketHandler(w http.ResponseWriter, r *h
 	}
 
 	location := getObjectLocation(r, globalDomainNames, bucket, object)
-	w.Header().Set("ETag", `"`+objInfo.ETag+`"`)
+	w.Header()["ETag"] = []string{`"` + objInfo.ETag + `"`}
 	w.Header().Set("Location", location)
 
 	// Notify object created event.

cmd/object-handlers-common.go

@@ -71,7 +71,7 @@ func checkCopyObjectPreconditions(ctx context.Context, w http.ResponseWriter, r
 		w.Header().Set("Last-Modified", objInfo.ModTime.UTC().Format(http.TimeFormat))
 
 		if objInfo.ETag != "" {
-			w.Header().Set("ETag", "\""+objInfo.ETag+"\"")
+			w.Header()["ETag"] = []string{"\"" + objInfo.ETag + "\""}
 		}
 	}
 	// x-amz-copy-source-if-modified-since: Return the object only if it has been modified
@@ -166,7 +166,7 @@ func checkPreconditions(ctx context.Context, w http.ResponseWriter, r *http.Requ
 		w.Header().Set("Last-Modified", objInfo.ModTime.UTC().Format(http.TimeFormat))
 
 		if objInfo.ETag != "" {
-			w.Header().Set("ETag", "\""+objInfo.ETag+"\"")
+			w.Header()["ETag"] = []string{"\"" + objInfo.ETag + "\""}
 		}
 	}
 	// If-Modified-Since : Return the object only if it has been modified since the specified time,

cmd/object-handlers.go

@@ -1258,7 +1258,7 @@ func (api objectAPIHandlers) PutObjectHandler(w http.ResponseWriter, r *http.Req
 	} else if hasServerSideEncryptionHeader(r.Header) {
 		etag = getDecryptedETag(r.Header, objInfo, false)
 	}
-	w.Header().Set("ETag", "\""+etag+"\"")
+	w.Header()["ETag"] = []string{"\"" + etag + "\""}
 
 	if objectAPI.IsEncryptionSupported() {
 		if crypto.IsEncrypted(objInfo.UserDefined) {
@@ -1941,7 +1941,7 @@ func (api objectAPIHandlers) PutObjectPartHandler(w http.ResponseWriter, r *http
 	} else if isEncrypted {
 		etag = tryDecryptETag(objectEncryptionKey, partInfo.ETag, crypto.SSEC.IsRequested(r.Header))
 	}
-	w.Header().Set("ETag", "\""+etag+"\"")
+	w.Header()["ETag"] = []string{"\"" + etag + "\""}
 
 	writeSuccessResponseHeadersOnly(w)
 }
@@ -2304,7 +2304,7 @@ func (api objectAPIHandlers) CompleteMultipartUploadHandler(w http.ResponseWrite
 	}
 
 	// Set etag.
-	w.Header().Set("ETag", "\""+objInfo.ETag+"\"")
+	w.Header()["ETag"] = []string{"\"" + objInfo.ETag + "\""}
 
 	// Write success response.
 	writeSuccessResponseXML(w, encodedSuccessResponse)

cmd/test-utils_test.go

@@ -2413,7 +2413,7 @@ func uploadTestObject(t *testing.T, apiRouter http.Handler, creds auth.Credentia
 			rec = httptest.NewRecorder()
 			apiRouter.ServeHTTP(rec, req)
 			checkRespErr(rec, http.StatusOK)
-			etag := rec.Header().Get("ETag")
+			etag := rec.Header()["ETag"][0]
 			if etag == "" {
 				t.Fatalf("Unexpected empty etag")
 			}

staticcheck.conf

@@ -1 +1 @@
-checks = ["all", "-ST1005", "-ST1000", "-SA4000", "-SA9004", "-SA1019"]
+checks = ["all", "-ST1005", "-ST1000", "-SA4000", "-SA9004", "-SA1019", "-SA1008"]