More detailled reporting of errors from flavors

[sirhcel]

The Flavor trait reports errors as postcard::Error. This enum has no variant for indicating an error specifically originating from data handling from a flavor. There are variants like SerializeBufferFull and SerdeSerCustom but they might be misleading.

To illustrate this with an example for adding a MAC to postcard frames:

impl<B> Flavor for Mac<B>
where
    B: Flavor
{
    // ...

    fn try_extend(&mut self, data: &[u8]) -> postcard::Result<()> {
        self.mac
            .update(data)
            .map_err(|_| postcard::Error:::SerializeBufferFull)?;
        self.base.try_extend(data)
    }

    fn finalize(mut self) ->- postcard::Result<Self::Output> {
        let mac = self
            .mac
            .finish()
            .map_err(|_| postcard::Error::SerializeBufferFull)?
            .as_bytes();
        self.base.try_extend(mac)?;
        self.base.finalize()
    }
}

Today I spent some time debugging this because I fell for my own red herring SerializeBufferFull when I was looking into the TX buffer in the first place where actually the mac computation failed. Strictly, the latter is neither SerialializeBufferFull, SerializeSeqLengthUnknown, or SerdeSerCustom.

Logging the error with inspect_err, will provide at least a clue to the friendly reader. But this needs to be taken care of for every potential souce while converting the original error needs to be done anyways and gets checked by the compiler.

[sirhcel]

The original error could be chained if postcard::Error implemented core::Error. What's the reason for not implementing the latter for postcard::Error in the first place?

Are there any plans for supporting custom errors from Flavors?