Fixes when accessing fgn_maxgen_percent

[ghost]

PR #25350 changed fgn_maxgen_percent to be a per-heap property when
MULTIPLE_HEAPS is set. A few uses need to be updated.

• In full_gc_wait, must re-read fgn_maxgen_percent before the
  second test of maxgen_percent == 0.
  (Otherwise the second test is statically unreachable.)
• In RegisterForFullGCNotification, must set fgn_maxgen_percent when
  MULTIPLE_HEAPS is not set
• In CancelFullGCNotification, must set fgn_maxgen_percent for each
  heap separately when MULTIPLE_HEAPS is set.

Fix dotnet/corefx#39374

[Maoni0]

maxgen_percent = g_heaps[0]->fgn_maxgen_percent; [](start = 7, length = 49)

maxgen_percent was already read at the beginning of this method.

[Maoni0]

oh I see your comment from above - is it because the 1st time we read the value we observe its not set yet?

[ghost]

The operation may be canceled while we're waiting, and CancelFullGCNotification works by setting fgn_maxgen_percent.

[Maoni0]

ahh right; we still wanna return the right status so we need to re-read. makes sense.

[Maoni0]

btw we could also write the code this way (like what the other places are doing) so you don't have to duplicate the code:

#ifdef MULTIPLE_HEAPS
    gc_heap* hp = gc_heap::g_heaps[0];
#else
    gc_heap* hp = pGenGCHeap;
#endif //MULTIPLE_HEAPS

if (hp->fgn_maxgen_percent == 0)

[ghost]

Simplified test case:

using System;
using System.Threading;

namespace cs
{
    class Program
    {
        public static int Main(string[] args)
        {
        	Console.WriteLine($"Server gc? {System.Runtime.GCSettings.IsServerGC}");
            GC.RegisterForFullGCNotification(20, 20);
            Thread cancelProc = new Thread(new ThreadStart(CancelProc));
            cancelProc.Start();
            GCNotificationStatus result = GC.WaitForFullGCComplete(-1);
            cancelProc.Join();
            if (result == GCNotificationStatus.Canceled)
            {
            	Console.WriteLine("success");
            	return 0;
            }
            else
            {
                Console.Error.WriteLine($"Error - WaitForFullGCComplete result not Cancelled. Instead result is: {result}");
                return 1;
            }
        }

        private static void CancelProc()
        {
            Thread.Sleep(500);
            GC.CancelFullGCNotification();
        }
    }
}

Tested with both server and workstation GCs.

[MeiChin-Tsai]

approved to merge.