ContentHash when using BlockBlobClient.OpenWriteAsync(true)

Describe the bug I am upgrading from storage v11 to v12. Looking at Issue 17676 I understand that there are two modes depending on file size: Put Blob and Put Block / Put Blob List.

A test is failing which creates a small Blob using OpenWriteAsync(true), because the ContentHash is not being set. When I try and set ContentHash I am not allowed to, because it doesn’t match what the server calculates. If the server can calculate it, why isn’t it set?

Expected behavior When I set the BlobHttpHeaders explicity, then it should accept whatever I give it, or set it itself:

await using var outputStream = await blockBlobClient.OpenWriteAsync(true, new BlockBlobOpenWriteOptions
{
    HttpHeaders = new BlobHttpHeaders
    {
        ContentHash = content.ToMd5()
    }
});

I get:

The MD5 value specified in the request did not match with the MD5 value calculated by the server.
RequestId:6a06f4b4-a243-438a-bb83-f7fc23531ee1
Time:2021-03-04T17:08:10.0157094Z
Status: 400 (The MD5 value specified in the request did not match with the MD5 value calculated by the server.)
ErrorCode: Md5Mismatch

Additional Information:
UserSpecifiedMd5: mJNTIjPK/5jNCDoRawE8Cw==
ServerCalculatedMd5: 1B2M2Y8AsgTpgAmY7PhCfg==

Now the MD5 I calculate is wrong - it is the MD5 of the string rather than the gzipped string - my concern is as follows:

• If the server can calculate the MD5, then why doesn’t it set the value into ContentMD5? If you run this code without the headers then the ContentMD5 property is not set.
• How do I make all files sizes act consistently using OpenWriteAsync? I can’t see any obvious way to prevent the server calculting the MD5 and letting me set what I want - I think with UploadAsync I would use InitialTransferLength = 0, which would then use Put Block / Put Blob List for everything?
• If the server doesn’t set ContentMD5, then why isn’t this user settable to anything? I haven’t tested yet but presumably if the blob size was greater > 256MB, it would use Put Block / Put Blob List and the server couldn’t/wouldn’t calculate the Hash and I could set whatever I wanted? In this case the behaviour would be inconsistent.
• I don’t want to refactor to use UploadAsync if I can help it. The way streams are passed around in this app makes this undesirable.

Actual behavior (include Exception or Stack Trace) A small blob doesn’t get a ContentMD5 set, but knows what it should be and doesn’t let me set it to what I want.

To Reproduce

var content = "something small";

await using var outputStream = await blockBlobClient.OpenWriteAsync(true, new BlockBlobOpenWriteOptions
{
    HttpHeaders = new BlobHttpHeaders
    {
        ContentHash = content.ToMd5()
    }
});

await using var gZipStream = new GZipStream(outputStream, CompressionMode.Compress);
await using var streamWriter = new StreamWriter(gZipStream);

await streamWriter.WriteAsync(content);

Environment:

• Name and version of the Library package used: Azure.Storage.Blobs 12.8.0
• Hosting platform or OS and .NET runtime version (dotnet --info output for .NET Core projects): Windows 10 .NET Core 3.1
• IDE and version : Visual Studio 16.8.6

[seanmcc-msft]

Yes, you can set it to any value.

[michaeldtaylor]

I meant any value to ContentHash/x-ms-blob-content-md5. I assume the answer is yes.