Reduce the use of unnecessary buffers around cache

[ktock]

Though we currently use bytes.Buffers in many places around the cache, some of these uses seem to be able to reduce.
This commit reduces the use of buffers around the cache, which leads to reduce memory usage of containerd-stargz-grpc.

We are currently using buffer during cache.Add() but we can reduce it by providing direct access to *os.File of each cache file to the cache client.

stargz-snapshotter/cache/cache.go

Lines 226 to 229 in 17b648d

	// Cache the passed data to disk.
	b2 := dc.bufPool.Get().(*bytes.Buffer)
	b2.Reset()
	b2.Write(p)

This commit modifies the interface to provide the writer of that file directly to the client when adding contents to the cache.

type BlobCache interface {
	Add(key string, opts ...Option) (Writer, error)

// omit...
}

type Writer interface {
	io.WriteCloser
	Commit() error
	Abort() error
}

Another buffer we can reduce is the one in blob.ReadAt(). We are using this buffer for trimming the data got from the registry. Thus we are buffering the downloaded data first into this buffer then trimming off the unnecessary range.

stargz-snapshotter/fs/remote/blob.go

Lines 204 to 209 in 17b648d

	// Use temporally buffer for aligning this chunk
	bf := b.resolver.bufPool.Get().(*bytes.Buffer)
	putBufs = append(putBufs, bf)
	bf.Reset()
	bf.Grow(int(chunk.size()))
	allData[chunk] = bf

But instead, we can remove this by just discarding the data out of the necessary range.

Both of the above buffers are on the code path of prefetch and background-fetch, where the entire layer blob is cached and loaded to/from the cache. So the above fixes reduced the memory usage, especially during rpull.

Memory usage during ctr-remote i rpull ghcr.io/stargz-containers/python:3.7-esgz

master	PR