Avoid large reallocations in webserver's header response

[d-a-v]

No description provided.

[earlephilhower]

CI seems to be aborting (not code related) frequently right about now. Let's try to re-run later...

[mcspr]

minor nit

at a glance, we have a bunch of typed pairs that generate multiple emplace_... for every sendHeader variant
I'd consider using perfect forward and a specific type we already have

using Pair = std::pair<String, String>;

auto foo(Pair pair, bool condition) {
    if (condition) {
        foo(std::move(pair));
    } else {
        bar(std::move(pair));
    }
}

template <typename S1, typename S2>
auto foo(S1&& name, S2&& value, bool condition) {
    foo(std::make_pair(
        String(std::forward<S1>(name)),
        String(std::forward<S2>(value)), condition);
}

[hmueller01]

@d-a-v I have pulled your branch and did a quick test. Looks good. Thanks for the quick fix!

What I found when more that one client is connecting to the WebServer is this:

[String] 'User-Agent ... ome/110.0.': Reallocating large String(143 -> 144 bytes)
[String] 'User-Agent ... 53 Mobile ': Reallocating large String(159 -> 160 bytes)

It's surely not releated to this change and I even do not know if it comes from ESPWebServer or our code ...

[TD-er]

When will these be cleared if streaming failed?

N.B. I got the impression the current implementation of the webserver does hold some data way longer than needed as opening a web page may sometimes free up quite a bit of memory. (maybe POST data of a previous request???)

[hmueller01]

I think the same happens in the ESP8266HTTPClient.cpp#addHeader ...

        if (first) {
            _headers = headerLine + _headers;
        } else {
            _headers += headerLine;
        }

No one knows in advance how big _headers grows. Resulting in:

[String] 'x-ESP8266- ...  1478656
': Reallocating large String(134 -> 165 bytes)
[String] 'x-ESP8266- ... : 548896
': Reallocating large String(165 -> 221 bytes)
[String] 'x-ESP8266- ... afd6bfc5
': Reallocating large String(221 -> 251 bytes)
[String] 'x-ESP8266- ...  2097152
': Reallocating large String(251 -> 290 bytes)
[String] 'x-ESP8266- ... b29dcd3)
': Reallocating large String(290 -> 315 bytes)
[String] 'x-ESP8266- ...  version
': Reallocating large String(315 -> 338 bytes)
[String] 'x-ESP8266- ... sion: 41
': Reallocating large String(338 -> 357 bytes)

[hmueller01]

@d-a-v Is there any progress on this PR / issue #8872?