objcache: Return io.ReaderAt to avoid Seeking and Reading. (#3735)

cmd/xl-v1-object.go

@@ -202,19 +202,22 @@ func (xl xlObjects) GetObject(bucket, object string, startOffset int64, length i
 	// Object cache enabled block.
 	if xlMeta.Stat.Size > 0 && xl.objCacheEnabled {
 		// Validate if we have previous cache.
-		var cachedBuffer io.ReadSeeker
+		var cachedBuffer io.ReaderAt
 		cachedBuffer, err = xl.objCache.Open(path.Join(bucket, object), modTime)
-		if err == nil { // Cache hit.
-			// Advance the buffer to offset as if it was read.
-			if _, err = cachedBuffer.Seek(startOffset, 0); err != nil { // Seek to the offset.
-				return traceError(err)
-			}
-			// Write the requested length.
-			if _, err = io.CopyN(writer, cachedBuffer, length); err != nil {
+		if err == nil { // Cache hit
+			// Create a new section reader, starting at an offset with length.
+			reader := io.NewSectionReader(cachedBuffer, startOffset, length)
+
+			// Copy the data out.
+			if _, err = io.Copy(writer, reader); err != nil {
 				return traceError(err)
 			}
+
+			// Success.
 			return nil
+
 		} // Cache miss.
+
 		// For unknown error, return and error out.
 		if err != objcache.ErrKeyNotFoundInCache {
 			return traceError(err)

pkg/objcache/objcache.go

@@ -217,7 +217,7 @@ func (c *Cache) Create(key string, size int64) (w io.WriteCloser, err error) {
 // returns an error ErrNotFoundInCache, if the key does not exist.
 // Returns ErrKeyNotFoundInCache if entry's lastAccessedTime is older
 // than objModTime.
-func (c *Cache) Open(key string, objModTime time.Time) (io.ReadSeeker, error) {
+func (c *Cache) Open(key string, objModTime time.Time) (io.ReaderAt, error) {
 	// Entry exists, return the readable buffer.
 	c.mutex.Lock()
 	defer c.mutex.Unlock()

pkg/objcache/objcache_test.go

@@ -20,7 +20,6 @@ package objcache
 import (
 	"bytes"
 	"io"
-	"io/ioutil"
 	"testing"
 	"time"
 )
@@ -173,7 +172,8 @@ func TestObjCache(t *testing.T) {
 		t.Errorf("Test case 4 expected to pass, failed instead %s", err)
 	}
 	// Reads everything stored for key "test".
-	cbytes, err := ioutil.ReadAll(r)
+	cbytes := make([]byte, 5)
+	_, err = r.ReadAt(cbytes, 0)
 	if err != nil {
 		t.Errorf("Test case 4 expected to pass, failed instead %s", err)
 	}