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

[vibhansa-msft]

If we allocate a new block here then release shall also check if queue is full or not, and if so it shall release the memory otherwise this one block will linger for the lifespan of blobfuse.

[souravgupta-msft]

Already doing that, https://github.com/Azure/azure-storage-fuse/blob/sourav/prefetchOpen/component/xload/blockpool.go#L144-L151
The release of block back to the pool is non-blocking and if the pool is full, the block will be deleted, and its memory will be released.

func (pool *BlockPool) Release(b *Block) {
	select {
	case pool.blocksCh <- b:
		break
	default:
		_ = b.Delete()
	}
}