new feature: allow inject a customized signer for aws s3

[flaneur2020]

Feature Description

background: apache/iceberg-rust#506

It seems we need to implement this signer in iceberg-rust and pass it as a customized signer to opendal.

Problem and Solution

but currently the signer is a struct AwsV4Signer.

we need make the signer in S3Core a trait before allowing it customizable.

maybe we can defined a trait called S3Signer in opendal, and make AwsV4Signer as it's implementation by default.

Additional Context

No response

Are you willing to contribute to the development of this feature?

• Yes, I am willing to contribute to the development of this feature.

[flaneur2020]

to make the Signer customizable, i've got some difficulties to solely add it in the opendal part.

adding a trait called S3Signer in opendal requires a signature to be object safe, but a signature like fn sign<T>(&self, req: &mut Request<T>, cred: &AwsCredential) -> Result<()> is
not object safe because it contains a generic parameter inside it.

so i think there might have two ways to make it possible:

1: add a GenericSigner trait with fn sign(&self, req: &mut dyn SignableRequest, cred: &AwsCredential) -> Result<()> in reqsign.

but as Signer accepts &mut impl SignableRequest, we may have to change the api of Signer to allow it accept trait objects.

2: change Signer in reqsign into an enum like:

pub enum Signer {
  V4(V4Signer),
  Custom(Box<dyn CustomSigner>)
}

this approach can make the api of Signer unchanged, but would introduce a small dispatch cost on every sign.

(i'd prefer the second approach, it seems less intrusive)

@Xuanwo any suggestion about this?

[Xuanwo]

Hi, S3::customized_credential_load should allow you to implement your own logic.

[flaneur2020]

Hi, S3::customized_credential_load should allow you to implement your own logic.

imho customized_credential_load is to customize the credential logic, but here we need customize is the signing logic.

the Signer currently is not a trait like dyn AwsCredentialLoad yet.

[Xuanwo]

Thank you for the clarification. I now understand what the problem is.

My current ideas:

• Add an Sign trait for entire reqsign

   use http;
   
   #[async_trait]
   trait Sign: 'static {
   	type Credential;
   
       async fn sign(&self, req: &mut http::request::Parts, cred: &Self::Credential) -> Result<()>
   }

• Expose a Signer from the reqsign

  reqsign will expose a type AwsSigner = Arc<dyn Sign<Credential=AwsCredential>>.

• OpenDAL can accept any impl Sign<Credential=AwsCredential> with AwsV4Signer as default impl.