> The BodyBytes hypothetical isn't particularly convincing because you usually don't actually have the bytes before reading them, they're queued up on a socket.

Ah, sorry, we were talking about two different 'http.Request.Body's. For some weird reason both the `http.Client.Do`'s request and `http.Server`'s request are the same type.

You're right that you usually don't have the bytes for the server, but for the client, like a huge fraction of client requests are `http.NewRequestWithContext(context.TODO(), "POST", "api.foo.com", bytes.NewReader(jsonBytesForAPI))`. You clearly have the bytes in that case.

Anyway, another example of the wisdom of the stdlib, you can save on structs by re-using one struct, and then having a bunch of comments like "For server requests, this field means X, for client requests, this is ignored or means Y".

Thinking about that more though, http.Client.Do is going to take that io.Reader and pipe it out to a socket. What would it do differently if you handed it a []byte? I suppose you could reduce some copying. Maybe worth it but I think Go already has other ways to avoid unnecessary copies when piping readers and writers together (e.g. using `WriterTo` instead of doing Read+Write.)

> If body is of type *bytes.Buffer, *bytes.Reader, or *strings.Reader, the returned request's ContentLength is set to its exact value

Servers like to know Content-Length, and the package already special-cases certain readers to effectively treat them like a `[]byte`.

Clearly it does something differently already.

Also, following redirects only works if you can send the body multiple times, so currently whether the client follows redirects or not depends on the type of the reader you pass in... if you add a logging intercepter to your reader to debug something, suddenly your code compiles but breaks because it stops following redirects, ask me how I know.

In this case, there is not any functionality you can't get through other means: You can set GetBody and the content length header manually, which is what you probably wound up doing if I had to guess (been there too, same hat.) I think Go does this mainly to make basic usage more convenient. Unfortunately though, it makes this unnecessarily subtle footgun in return.

Maybe Go 2 will finally do something about this. I would really like some (hopefully efficient) way to make "transparent" wrapper types that can magically forward methods.

The request body on the client do a lot of other things than reading the body once (an io.Reader can only be read once).

There's Content-Length, and there's also the need to read it multiple times in case a redirect happens (so the same body need to be sent again when being redirected).

As a result, the implementation in stdlib would check a few common io.Reader implementations (bytes.Buffer, bytes.Reader, strings.Reader) and make sure it stores something that can be read multiple times (if it's none of the 3, it's read fully into memory and stored instead).

This is the same basic reply as the other one but my thoughts are roughly the same. The only comment I have aside from what I replied on the sibling comment (this just being another case of wrappers not being transparent biting us in the ass) is that they could've done this in a more generic way than they did, at the downside of requiring more interfaces.

Yea I saw your other reply later and agree on most of it. But I'd say there's a balance between simplicity of the API and more specific cases. For example they can make an optional api to io.Reader to provide size info, and maybe another optional api to io.Reader to make it able to be read more than once, etc.. But at the same time, if you have all those info, that _usually_ means you already have either a []byte or string, and you would most likely use one of the 3 types to convert that into an io.Reader, so that special handling is enough without adding more public apis, and the go team is notoriously conservative when adding new public apis.