[FEATURE REQ] OpenWriteAsync to leave existing blob content alone until Flush() is called on the stream

[danielearwicker]

Library name

Azure.Storage.Blobs

Please describe the feature.

BlockBlobClient's OpenWriteInternal method returns BlockBlobWriteStream. This stages blocks and adds their IDs to a list _blockIds.

The CommitInternal method actually commits the blocks to become the new content of the blob. I only see one call to that, from FlushInternal, which is called from Flush and FlushAsync.

This seems excellent because it suggests I can attempt to write an updated version to the same blob name, and if there is a failure part way through so I never call Flush, the blocks will be abandoned and the previous version of the blob's contents are unharmed.

However, before returning the BlockBlobWriteStream, the OpenWriteInternal does something mystifying!:

// Create Block Blob
await UploadInternal(
    content: new MemoryStream(Array.Empty<byte>()),
    ...

That is, it immediately truncates the blob, destroying its current contents before anything to replace it has been sent.

Is this necessary? Is it possible to add a switch to BlockBlobOpenWriteOptions to make it not do that?

[github-actions]

Thanks for the feedback! We are routing this to the appropriate team for follow-up. cc @xgithubtriage.

[danielearwicker]

Wondering whether this question has reached the appropriate team! 🛩️