File open call in preload

[souravgupta-msft]

✅ What

Preload operation downloads the blobs in parallel. If open call comes for a file and it has not been downloaded yet, then it will be scheduled for download on priority.

🤔 Why

The open call for a file downloads it on priority, if not yet scheduled for download. This is done so that the users will not have to wait till the scheduler picks it up for download.

NOTE

The UT will come in the next PR

[jainakanksha-msft]

optimised version

`
// Do is the core task to be executed by each worker thread
func (threadPool *ThreadPool) Do(priority bool) {
defer threadPool.waitGroup.Done()

for {
	select {
	case item := <-threadPool.priorityItems:
		threadPool.process(item)

	case item := <-threadPool.workItems:
		if !priority { // Only process workItems if it's not a priority thread
			threadPool.process(item)
		}

	case <-threadPool.ctx.Done(): // Listen to cancellation signal
		return
	}
}

}
`
Please check the correctness if I understood your point correctly.
If possible write down the unit test for this.
So this use case are covered for this. And we don't miss any scenario

[jainakanksha-msft]

`
return openFile(localPath, options, flock)
}

func openFile(localPath string, options internal.OpenFileOptions, flock *common.LockMapItem) (*handlemap.Handle, error) {
fh, err := os.OpenFile(localPath, options.Flags, options.Mode)
if err != nil {
log.Err("Xload::OpenFile : error opening cached file %s [%s]", options.Name, err.Error())
return nil, err
}

// Increment the handle count in this lock item as there is one handle open for this now
flock.Inc()

handle := handlemap.NewHandle(options.Name)
info, err := fh.Stat()
if err == nil {
	handle.Size = info.Size()
}

handle.UnixFD = uint64(fh.Fd())
handle.Flags.Set(handlemap.HandleFlagCached)

log.Info("Xload::OpenFile : file=%s, fd=%d", options.Name, fh.Fd())
handle.SetFileObject(fh)

return handle, nil

}
`

Extract separate method to segregate out validations and proper openFile implementation