Reduce allocations in CookieContainer.GetCookieHeader

[stephentoub]

HttpClient defaults to UseCookies=true, which means by default every request ends up asking the CookieContainer to get the cookie header that should be used. Today, even if there aren't any cookies for the given Uri, this ends up doing a lot of allocation. And if there are cookies for the Uri, the serialization of each cookie results in a handful of strings, char[]s, and string[] allocations. This commit tweaks the implementation to avoid a bunch of these, including taking advantage of StringBuilderCache. (There are still some more that could be cleaned up with further work, but it becomes much more invasive.)

For this sample program:

CookieContainer cc = new CookieContainer();

Uri exists = new Uri("http://example.com/exists");
Uri notExists = new Uri("http://example.com/notExists");

cc.Add(exists, new Cookie("test1", "value1"));
cc.Add(exists, new Cookie("test2", "value2"));

for (int i = 0; i < 1000; i++)
{
    cc.GetCookieHeader(exists);
}

before these changes, I see the following allocations:

and after thes changes:

If I then change the GetCookieHeader line to be:

cc.GetCookieHeader(notExists);

before these changes, I see the following allocations:

and after these changes:

cc: @davidsh, @CIPop, @ericeil

[davidsh]

You should consider making equivalent changes to the copy of Cookie.cs here:
https://github.com/dotnet/corefx/blob/master/src/System.Net.Http/src/netcore50/System/Net/cookie.cs

Long term....there won't be a copy of the file. But due to the way that netcore50 needs cookie functionality, it needed to duplicate the file for now....could potentially be refactored to common code, etc.

[stephentoub]

Thanks, @davidsh. I ported the Cookie changes over to cookie.cs. I decided not to use StringBuilderCache (yet), since it's not currently used in the Http library, but that's something we likely want to do for other reasons soon.

[davidsh]

LGTM

[stephentoub]

Thanks, @davidsh. Do you want to run any .NET Native tests before this is merged?

[davidsh]

I'm ok w/ merging now.