usePreemptive is dropped when httpConfiguration blocks are merged

[slachiewicz]

Affected version

3.5.3 and master.

Bug description

usePreemptive has no effect whenever a <server> configures both <all> and a block for the method being used, which is the shape most people write. The effective value is always false.

HttpMethodConfiguration.copy() copies everything except that one field:

HttpMethodConfiguration copy() {
    HttpMethodConfiguration copy = new HttpMethodConfiguration();
    copy.setConnectionTimeout(getConnectionTimeout());
    copy.setReadTimeout(getReadTimeout());
    if (getHeaders() != null) {
        copy.getHeaders().putAll(getHeaders());
    }
    if (getParams() != null) {
        copy.getParams().putAll(getParams());
    }
    copy.setUseDefaultHeaders(isUseDefaultHeaders());
    return copy;
}

and ConfigurationUtils contains no reference to usePreemptive at all, so merge() never carries it over either. The merged configuration therefore keeps the field's default of false no matter what was configured.

Why it is not caught

HttpWagonPreemptiveTest configures <all> and nothing else. With only one block there is nothing to merge, so getMethodConfiguration returns it directly and the flag survives. Add a <get> block alongside and it stops working.

Workaround

Configure usePreemptive in a block that is not merged — in practice, use <all> on its own for that server.

Note

There is a second, related oddity nearby: AbstractHttpClientWagon.put primes the auth cache whenever credentials exist, regardless of this setting, and the source says so — // FIXME Perform only when preemptive has been configured. So PUT is preemptive even when the flag is honoured and false. Worth deciding together, since fixing the merge without looking at that would still leave the observable behaviour surprising.

Found while writing the HTTP configuration guide in #905, which documents the current behaviour and the workaround.