Pinging @elastic/es-core-infra

We have discussed this in the past, and if I remember correctly we said we are not in favor of modifying the request by adding /. That sounds like a subtle change to what users request, as users are providing the requests and can prepend them with / if they need to (see #30119 (comment) for this discussion). We did encounter this problem though with proxies and we went for a slightly different solution, which allows to set pathPrefix to / in case we want to add a / prefix to every request when missing. See #30119.

That said I am closing this PR, but thanks a lot for opening it, and hopefully you will find some other improvement to work on ;)

I've done some more digging, and I would like to discuss this further if that's OK? I believe the current position is inconsistent.

In RestClientBuilder.setPathPrefix, there is the following code:

if (cleanPathPrefix.startsWith("/") == false) {
    cleanPathPrefix = "/" + cleanPathPrefix;
}

ie paths are forced to have a leading slash.

There is also this:

if (cleanPathPrefix.isEmpty() || "/".equals(cleanPathPrefix)) {
    throw new IllegalArgumentException("pathPrefix must not be empty or '/': [" + pathPrefix + "]");
}

ie you cannot simply handle the trivial case of "always prepend a slash" because the code actually doesn't let you.

As far as I can tell, there is no correct use case for the user providing a relative path URI without a pathPrefix. If we don't automatically fix this error (per the fix I suggested) then we should throw an exception with advice that either a pathPrefix or a leading slash is required.