Get Request Precondition Support

[tustvold]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

There are 4 broadly supported request preconditions:

• If-Match
• If-None-Match
• If-Modified-Since
• If-Unmodified-Since

https://cloud.google.com/storage/docs/request-preconditions
https://docs.microsoft.com/en-us/rest/api/storageservices/specifying-conditional-headers-for-blob-service-operations
https://docs.aws.amazon.com/AmazonS3/latest/API/API_GetObject.html#API_GetObject_RequestSyntax

Describe the solution you'd like

I would like to propose adding

#[derive(Debug, Default)]
struct GetOptions {
    pub if_match: Option<String>,
    pub if_none_match: Option<String>,
    pub if_modified_since: Option<DateTime<Utc>>,
    pub if_unmodified_since: Option<DateTime<Utc>>,
    pub range: Option<usize>
}

pub trait ObjectStore {
     async fn get_opts(&self, location: &Path, options: GetOptions) -> Result<GetResult>
     ...
}

pub enum Error {
    #[snafu(display("Precondition failed: {}", source))]
    Precondition {
        source: Box<dyn std::error::Error + Send + Sync + 'static>,
    }

    #[snafu(display("Not modified: {}", source))]
    NotModified {
        source: Box<dyn std::error::Error + Send + Sync + 'static>,
    }
}

Describe alternatives you've considered

We could not add support for this

Additional context

#2230
#2240

[crepererum]

How do you model stores that don't support these checks? Do they just return "unsupported" errors?

[tustvold]

All stores support these for get requests.

[crepererum]

All stores support these for get requests.

I think this crate also provides some extendable interface, i.e. users can implement their own stores and use for example etcd as a storage backend. But I suppose every store could implement this check client-side if the "other end" doesn't support it, so this is OK then.

[tustvold]

Yeah, I would expect such stores to return Error::NotSupported if the feature cannot be supported, and Error::NotImplemented if it simply has not been implemented