Async timer cannot be start from sync timer handler

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Description

Class TProcessEventTimer is async timer and used inside TTreePlayer::Process() method to enable events processing during TTree::Draw(). Main problem - all data handling in the THttpServer performed inside THttpTimer handler - including TTree::Draw() invoked from RBrowser. So to say one cannot initiate async timer inside sync timer handler.

There is simple solution. In TUnixSystem::DispatchTimers(Bool_t mode) one should change line 2947:
https://github.com/root-project/root/blob/master/core/unix/src/TUnixSystem.cxx#L2947

fInsideNotify = !mode; 

Means handling of "normal" sync timers is not dangerous and do not block functionality to reset async timer inside
TUnixSystem::ResetTimer which implemented as:

void TUnixSystem::ResetTimer(TTimer *ti)
{
   if (!fInsideNotify && ti && ti->IsAsync())
      UnixSetitimer(NextTimeOut(kFALSE));
}

Another solution - let process THttpServer events differently - not in the THttpTimer handler.
But this means extension of TSystem::DispatchOneEvent method. Like adding generic event handlers called every time gSystem->ProcessEvents() is called.

Reproducer

Not provided

ROOT version

master

Installation method

build from source

Operating system

OpenSUSE Linux

Additional context

No response