TagHelperExecutionContext must be smart about new TagHelper Content allocations.

[sornaks]

Quoting Taylor's comment:

You'll have to add a bit more to make the copying work as expected/performantly. The generated code that calls into this method looks like:

if (__tagHelperExecutionContext.Output.IsContentModified)
{
    Write(__tagHelperExecutionContext.Output.GenerateContent());
}
else if (__tagHelperExecutionContext.ChildContentRetrieved)
{
    Write(__tagHelperExecutionContext.GetChildContentAsync().Result);
}
else
{
    __tagHelperExecutionContext.ExecuteChildContentAsync().Wait();
}

So in the case that the user retrieved the child content: __tagHelperExecutionContext.ChildContentRetrieved you will end up copying the result again before writing it (no bueno).

Now it's also important to re-copy each time the GetChildContentAsync method is called so multiple TagHelper authors can see the original content (without worrying about someone else modifying it); you have this part covered.

You'll need to add a capability to retrieve the original, uncopied version for the page to use in the else if (__tagHelperExecutionContext.ChildContentRetrieved) clause.

[rynowak]

The work item here is really to profile our new changes and see what still lights up as a hotspot for allocations and copying. The comments linked here called out a case that we might want to optimize, but the work that we need to do is much broader than what's called out above.

[Eilon]

Covered by #358.