Fix accessing the Etag causing panic in Download method of blockcache

component/block_cache/block_cache.go

@@ -938,12 +938,18 @@ func (bc *BlockCache) refreshBlock(handle *handlemap.Handle, index uint64, prefe
 
 // lineupDownload : Create a work item and schedule the download
 func (bc *BlockCache) lineupDownload(handle *handlemap.Handle, block *Block, prefetch bool) {
+	etagInterface, found := handle.GetValue("ETAG")
+	etag := ""
+	if found {
+		etag = etagInterface.(string)
+	}
 	item := &workItem{
 		handle:   handle,
 		block:    block,
 		prefetch: prefetch,
 		failCnt:  0,
 		upload:   false,
+		ETag:     etag,
 	}
 
 	// Remove this block from free block list and add to in-process list
@@ -1055,10 +1061,7 @@ func (bc *BlockCache) download(item *workItem) {
 
 	// Compare the ETAG value and fail download if blob has changed
 	if etag != "" {
-		item.handle.Lock()
-		etagVal, found := item.handle.GetValue("ETAG")
-		item.handle.Unlock()
-		if found && etagVal != etag {
+		if item.ETag != "" && item.ETag != etag {
 			log.Err("BlockCache::download : Blob has changed for %v=>%s (index %v, offset %v)", item.handle.ID, item.handle.Path, item.block.id, item.block.offset)
 			item.block.Failed()
 			item.block.Ready(BlockStatusDownloadFailed)

component/block_cache/threadpool.go

@@ -69,8 +69,18 @@ type workItem struct {
 	failCnt  int32             // How many times this item has failed to download
 	upload   bool              // Flag marking this is a upload request or not
 	blockId  string            // BlockId of the block
+	ETag     string            // Etag of the file before scheduling.
 }
 
+// Reason for storing Etag in workitem struct
+// here getting the value of ETag inside upload/download methods
+// from the handle is somewhat tricker.
+// firstly we need to aquire a lock to read it from the handle.
+// In these methods the handle may be locked/maynotbe locked by
+// other go routine hence acquring would cause a deadlock.
+// It is already locked if the call came from the readInBuffer.
+// It is may be locked if the call come from the prefetch.
+
 // newThreadPool creates a new thread pool
 func newThreadPool(count uint32, reader func(*workItem), writer func(*workItem)) *ThreadPool {
 	if count == 0 || reader == nil {

test/e2e_tests/file_test.go

@@ -117,7 +117,9 @@ func (suite *fileTestSuite) TestOpenFlag_O_TRUNC() {
 	fileName := suite.testPath + "/test_on_open"
 	buf := "foo"
 	srcFile, err := os.OpenFile(fileName, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, 0666)
-	srcFile.Write([]byte(buf))
+	suite.Nil(err)
+	bytesWritten, err := srcFile.Write([]byte(buf))
+	suite.Equal(len(buf), bytesWritten)
 	suite.Nil(err)
 	srcFile.Close()