api, core: Add new Core API {Get,Stat}Object with pre-conditions.

Implements a new API to provide a way to set headers
for GetObject(), StatObject() request such as to

 - read partial data starting at offsets.
 - read only if etag matches.
 - read only if modtime matches.
 - read only if etag doesn't match.
 - read only if modtime doesn't match.

Fixes #669

api-get-object-file.go

@@ -78,8 +78,13 @@ func (c Client) FGetObject(bucketName, objectName, filePath string) error {
 		return err
 	}
 
+	// Initialize get object request headers to set the
+	// appropriate range offsets to read from.
+	reqHeaders := NewGetReqHeaders()
+	reqHeaders.SetRange(st.Size(), 0)
+
 	// Seek to current position for incoming reader.
-	objectReader, objectStat, err := c.getObject(bucketName, objectName, st.Size(), 0)
+	objectReader, objectStat, err := c.getObject(bucketName, objectName, reqHeaders)
 	if err != nil {
 		return err
 	}

api-get-object.go

@@ -1,5 +1,5 @@
 /*
- * Minio Go Library for Amazon S3 Compatible Cloud Storage (C) 2015, 2016 Minio, Inc.
+ * Minio Go Library for Amazon S3 Compatible Cloud Storage (C) 2015, 2016, 2017 Minio, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -97,16 +97,19 @@ func (c Client) GetObject(bucketName, objectName string) (*Object, error) {
 				if req.isFirstReq {
 					// First request is a Read/ReadAt.
 					if req.isReadOp {
+						reqHeaders := NewGetReqHeaders()
 						// Differentiate between wanting the whole object and just a range.
 						if req.isReadAt {
 							// If this is a ReadAt request only get the specified range.
 							// Range is set with respect to the offset and length of the buffer requested.
 							// Do not set objectInfo from the first readAt request because it will not get
 							// the whole object.
-							httpReader, _, err = c.getObject(bucketName, objectName, req.Offset, int64(len(req.Buffer)))
+							reqHeaders.SetRange(req.Offset, req.Offset+int64(len(req.Buffer))-1)
+							httpReader, _, err = c.getObject(bucketName, objectName, reqHeaders)
 						} else {
+							reqHeaders.SetRange(req.Offset, 0)
 							// First request is a Read request.
-							httpReader, objectInfo, err = c.getObject(bucketName, objectName, req.Offset, 0)
+							httpReader, objectInfo, err = c.getObject(bucketName, objectName, reqHeaders)
 						}
 						if err != nil {
 							resCh <- getResponse{
@@ -166,16 +169,20 @@ func (c Client) GetObject(bucketName, objectName string) (*Object, error) {
 					// new ones when they haven't been already.
 					// All readAt requests are new requests.
 					if req.DidOffsetChange || !req.beenRead {
+						reqHeaders := NewGetReqHeaders()
 						if httpReader != nil {
 							// Close previously opened http reader.
 							httpReader.Close()
 						}
 						// If this request is a readAt only get the specified range.
 						if req.isReadAt {
 							// Range is set with respect to the offset and length of the buffer requested.
-							httpReader, _, err = c.getObject(bucketName, objectName, req.Offset, int64(len(req.Buffer)))
+							reqHeaders.SetRange(req.Offset, req.Offset+int64(len(req.Buffer))-1)
+							httpReader, _, err = c.getObject(bucketName, objectName, reqHeaders)
 						} else {
-							httpReader, objectInfo, err = c.getObject(bucketName, objectName, req.Offset, 0)
+							// Range is set with respect to the offset.
+							reqHeaders.SetRange(req.Offset, 0)
+							httpReader, objectInfo, err = c.getObject(bucketName, objectName, reqHeaders)
 						}
 						if err != nil {
 							resCh <- getResponse{
@@ -230,8 +237,8 @@ type getResponse struct {
 	objectInfo ObjectInfo // Used for the first request.
 }
 
-// Object represents an open object. It implements Read, ReadAt,
-// Seeker, Close for a HTTP stream.
+// Object represents an open object. It implements
+// Reader, ReaderAt, Seeker, Closer for a HTTP stream.
 type Object struct {
 	// Mutex.
 	mutex *sync.Mutex
@@ -594,7 +601,7 @@ func newObject(reqCh chan<- getRequest, resCh <-chan getResponse, doneCh chan<-
 //
 // For more information about the HTTP Range header.
 // go to http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.35.
-func (c Client) getObject(bucketName, objectName string, offset, length int64) (io.ReadCloser, ObjectInfo, error) {
+func (c Client) getObject(bucketName, objectName string, reqHeaders RequestHeaders) (io.ReadCloser, ObjectInfo, error) {
 	// Validate input arguments.
 	if err := isValidBucketName(bucketName); err != nil {
 		return nil, ObjectInfo{}, err
@@ -603,15 +610,10 @@ func (c Client) getObject(bucketName, objectName string, offset, length int64) (
 		return nil, ObjectInfo{}, err
 	}
 
+	// Set all the necessary reqHeaders.
 	customHeader := make(http.Header)
-	// Set ranges if length and offset are valid.
-	// See  https://tools.ietf.org/html/rfc7233#section-3.1 for reference.
-	if length > 0 && offset >= 0 {
-		customHeader.Set("Range", fmt.Sprintf("bytes=%d-%d", offset, offset+length-1))
-	} else if offset > 0 && length == 0 {
-		customHeader.Set("Range", fmt.Sprintf("bytes=%d-", offset))
-	} else if length < 0 && offset == 0 {
-		customHeader.Set("Range", fmt.Sprintf("bytes=%d", length))
+	for key, value := range reqHeaders.Header {
+		customHeader[key] = value
 	}
 
 	// Execute GET on objectName.
@@ -646,6 +648,7 @@ func (c Client) getObject(bucketName, objectName string, offset, length int64) (
 			Region:    resp.Header.Get("x-amz-bucket-region"),
 		}
 	}
+
 	// Get content-type.
 	contentType := strings.TrimSpace(resp.Header.Get("Content-Type"))
 	if contentType == "" {

api-stat.go

@@ -86,12 +86,31 @@ func (c Client) StatObject(bucketName, objectName string) (ObjectInfo, error) {
 	if err := isValidObjectName(objectName); err != nil {
 		return ObjectInfo{}, err
 	}
+	reqHeaders := NewHeadReqHeaders()
+	return c.statObject(bucketName, objectName, reqHeaders)
+}
+
+// Lower level API for statObject supporting pre-conditions and range headers.
+func (c Client) statObject(bucketName, objectName string, reqHeaders RequestHeaders) (ObjectInfo, error) {
+	// Input validation.
+	if err := isValidBucketName(bucketName); err != nil {
+		return ObjectInfo{}, err
+	}
+	if err := isValidObjectName(objectName); err != nil {
+		return ObjectInfo{}, err
+	}
+
+	customHeader := make(http.Header)
+	for k, v := range reqHeaders.Header {
+		customHeader[k] = v
+	}
 
 	// Execute HEAD on objectName.
 	resp, err := c.executeMethod("HEAD", requestMetadata{
 		bucketName:         bucketName,
 		objectName:         objectName,
 		contentSHA256Bytes: emptySHA256,
+		customHeader:       customHeader,
 	})
 	defer closeResponse(resp)
 	if err != nil {
@@ -124,6 +143,7 @@ func (c Client) StatObject(bucketName, objectName string) (ObjectInfo, error) {
 			}
 		}
 	}
+
 	// Parse Last-Modified has http time format.
 	date, err := time.Parse(http.TimeFormat, resp.Header.Get("Last-Modified"))
 	if err != nil {
@@ -137,6 +157,7 @@ func (c Client) StatObject(bucketName, objectName string) (ObjectInfo, error) {
 			Region:     resp.Header.Get("x-amz-bucket-region"),
 		}
 	}
+
 	// Fetch content type if any present.
 	contentType := strings.TrimSpace(resp.Header.Get("Content-Type"))
 	if contentType == "" {

core.go

@@ -98,3 +98,16 @@ func (c Core) GetBucketPolicy(bucket string) (policy.BucketAccessPolicy, error)
 func (c Core) PutBucketPolicy(bucket string, bucketPolicy policy.BucketAccessPolicy) error {
 	return c.putBucketPolicy(bucket, bucketPolicy)
 }
+
+// GetObject is a lower level API implemented to support reading
+// partial objects and also downloading objects with special conditions
+// matching etag, modtime etc.
+func (c Core) GetObject(bucketName, objectName string, reqHeaders RequestHeaders) (io.ReadCloser, ObjectInfo, error) {
+	return c.getObject(bucketName, objectName, reqHeaders)
+}
+
+// StatObject is a lower level API implemented to support special
+// conditions matching etag, modtime on a request.
+func (c Core) StatObject(bucketName, objectName string, reqHeaders RequestHeaders) (ObjectInfo, error) {
+	return c.statObject(bucketName, objectName, reqHeaders)
+}

core_test.go

@@ -29,6 +29,165 @@ import (
 	"time"
 )
 
+// Tests for Core GetObject() function.
+func TestGetObjectCore(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping functional tests for the short runs")
+	}
+
+	// Seed random based on current time.
+	rand.Seed(time.Now().Unix())
+
+	// Instantiate new minio core client object.
+	c, err := NewCore(
+		os.Getenv("S3_ADDRESS"),
+		os.Getenv("ACCESS_KEY"),
+		os.Getenv("SECRET_KEY"),
+		mustParseBool(os.Getenv("S3_SECURE")),
+	)
+	if err != nil {
+		t.Fatal("Error:", err)
+	}
+
+	// Enable tracing, write to stderr.
+	// c.TraceOn(os.Stderr)
+
+	// Set user agent.
+	c.SetAppInfo("Minio-go-FunctionalTest", "0.1.0")
+
+	// Generate a new random bucket name.
+	bucketName := randString(60, rand.NewSource(time.Now().UnixNano()), "minio-go-test")
+
+	// Make a new bucket.
+	err = c.MakeBucket(bucketName, "us-east-1")
+	if err != nil {
+		t.Fatal("Error:", err, bucketName)
+	}
+
+	// Generate data more than 32K
+	buf := bytes.Repeat([]byte("3"), rand.Intn(1<<20)+32*1024)
+
+	// Save the data
+	objectName := randString(60, rand.NewSource(time.Now().UnixNano()), "")
+	n, err := c.Client.PutObject(bucketName, objectName, bytes.NewReader(buf), "binary/octet-stream")
+	if err != nil {
+		t.Fatal("Error:", err, bucketName, objectName)
+	}
+
+	if n != int64(len(buf)) {
+		t.Fatalf("Error: number of bytes does not match, want %v, got %v\n", len(buf), n)
+	}
+
+	reqHeaders := NewGetReqHeaders()
+
+	offset := int64(2048)
+
+	// read directly
+	buf1 := make([]byte, 512)
+	buf2 := make([]byte, 512)
+	buf3 := make([]byte, n)
+
+	reqHeaders.SetRange(offset, offset+int64(len(buf1))-1)
+	reader, objectInfo, err := c.GetObject(bucketName, objectName, reqHeaders)
+	if err != nil {
+		t.Fatal(err)
+	}
+	m, err := io.ReadFull(reader, buf1)
+	if err != nil {
+		reader.Close()
+		t.Fatal(err)
+	}
+	reader.Close()
+
+	if objectInfo.Size != int64(m) {
+		t.Fatalf("Error: GetObject read shorter bytes before reaching EOF, want %v, got %v\n", objectInfo.Size, m)
+	}
+	if !bytes.Equal(buf1, buf[offset:offset+512]) {
+		t.Fatal("Error: Incorrect read between two GetObject from same offset.")
+	}
+	offset += 512
+
+	reqHeaders.SetRange(offset, offset+int64(len(buf2))-1)
+	reader, objectInfo, err = c.GetObject(bucketName, objectName, reqHeaders)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	m, err = io.ReadFull(reader, buf2)
+	if err != nil {
+		reader.Close()
+		t.Fatal(err)
+	}
+	reader.Close()
+
+	if objectInfo.Size != int64(m) {
+		t.Fatalf("Error: GetObject read shorter bytes before reaching EOF, want %v, got %v\n", objectInfo.Size, m)
+	}
+	if !bytes.Equal(buf2, buf[offset:offset+512]) {
+		t.Fatal("Error: Incorrect read between two GetObject from same offset.")
+	}
+
+	reqHeaders.SetRange(0, int64(len(buf3)))
+	reader, objectInfo, err = c.GetObject(bucketName, objectName, reqHeaders)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	m, err = io.ReadFull(reader, buf3)
+	if err != nil {
+		reader.Close()
+		t.Fatal(err)
+	}
+	reader.Close()
+
+	if objectInfo.Size != int64(m) {
+		t.Fatalf("Error: GetObject read shorter bytes before reaching EOF, want %v, got %v\n", objectInfo.Size, m)
+	}
+	if !bytes.Equal(buf3, buf) {
+		t.Fatal("Error: Incorrect data read in GetObject, than what was previously upoaded.")
+	}
+
+	reqHeaders = NewGetReqHeaders()
+	reqHeaders.SetMatchETag("etag")
+	_, _, err = c.GetObject(bucketName, objectName, reqHeaders)
+	if err == nil {
+		t.Fatal("Unexpected GetObject should fail with mismatching etags")
+	}
+	if errResp := ToErrorResponse(err); errResp.Code != "PreconditionFailed" {
+		t.Fatalf("Expected \"PreconditionFailed\" as code, got %s instead", errResp.Code)
+	}
+
+	reqHeaders = NewGetReqHeaders()
+	reqHeaders.SetMatchETagExcept("etag")
+	reader, objectInfo, err = c.GetObject(bucketName, objectName, reqHeaders)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	m, err = io.ReadFull(reader, buf3)
+	if err != nil {
+		reader.Close()
+		t.Fatal(err)
+	}
+	reader.Close()
+
+	if objectInfo.Size != int64(m) {
+		t.Fatalf("Error: GetObject read shorter bytes before reaching EOF, want %v, got %v\n", objectInfo.Size, m)
+	}
+	if !bytes.Equal(buf3, buf) {
+		t.Fatal("Error: Incorrect data read in GetObject, than what was previously upoaded.")
+	}
+
+	err = c.RemoveObject(bucketName, objectName)
+	if err != nil {
+		t.Fatal("Error: ", err)
+	}
+	err = c.RemoveBucket(bucketName)
+	if err != nil {
+		t.Fatal("Error:", err)
+	}
+}
+
 // Tests get bucket policy core API.
 func TestGetBucketPolicy(t *testing.T) {
 	if testing.Short() {