Option for trailer etag

[kurtextrem]

Prerequisites

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🚀 Feature Proposal

Add an option to set the etag header as trailer header instead of normal header.

Motivation

The fastify docs mention the etag header as an example for trailer headers: https://www.fastify.io/docs/latest/Reference/Reply/#trailerkey-function. So adding an option to making the etag header a trailer header inside of this fastify plugin might make sense.

However, I'm a little bit confused how a trailer etag is used by browsers. If the etag header is only sent after the body has been sent, what is the purpose of it? (e.g. status code can't change anymore, body transfer can't be skipped, ...). Maybe we should answer this first.

Example

No response

[mcollina]

However, I'm a little bit confused how a trailer etag is used by browsers. If the etag header is only sent after the body has been sent, what is the purpose of it? (e.g. status code can't change anymore, body transfer can't be skipped, ...). Maybe we should answer this first.

The etag will be stored by the browser and sent again in the If-None-Match HTTP header when requesting the same resource twice.

[mcollina]

Thanks for reporting! Would you like to send a Pull Request to address this issue? Remember to add unit tests.