Library name and version

Azure.Storage.Blobs 12.23.0

Describe the bug

OpenReadAsync, if not provided a buffer size, defaults to 4MB. OpenReadAsync, as part of it's initialization, pulls the blob properties to fail fast if the blob does not exist. The blob length in these properties is used to initialize the initialLenght (note: misspelled) of the LazyLoadingReadonlyStream.

There is an inefficiency here though. In the case where allowModifications = false and initialLength < Constants.DefaultStreamingDownloadSize, the stream will pre-initialize a buffer of 4MB (the default download size), but will never use the entire buffer. the math for the buffer initialization should be

int maxBufferSize = allowModifications ? Constants.DefaultStreamingDownloadSize : Math.Min(initialLength, Constants.DefaultStreamingDownloadSize);
_bufferSize = bufferSize ?? maxBufferSize;
_buffer = ArrayPool<byte>.Shared.Rent(_bufferSize);

In the case that a user requests a larger buffer (by specifying bufferSize in the read options), the client will honor that and create that buffer. Otherwise the default should allocate the buffer size wisely.

Expected behavior

LazyLoadingReadonlyStream with a blob that does not allow modifications should create the smallest buffer necessary

Actual behavior

LazyLoadingReadonlyStream always creates a 4 MB buffer (if buffer size not specified)

Reproduction Steps

Create 2KB blob
BlobOpenReadOptions options = new BlobOpenReadOptions(allowModifications: false)
var stream = await blobClient.OpenReadAsync(options: options)

stream._buffer is a byte array is 4MB in size.

Environment

No response