Crossgen outerloop regression: WeakReference/IsAlive/IsAlive.cmd

[kg]

WeakReference/IsAlive/IsAlive.cmd fails on the crossgen outerloop lanes:

Return code:      1
Raw output file:      C:\h\w\A5E609B0\w\A9D4091E\uploads\API\WeakReference\IsAlive\output.txt
Raw output:
BEGIN EXECUTION
IsAlive.dll
        1 file(s) copied.
 7:04:11.39
Response file: C:\h\w\A5E609B0\w\A9D4091E\e\GC\API\WeakReference\IsAlive\\composite-r2r.dll.rsp
C:\h\w\A5E609B0\w\A9D4091E\e\GC\API\WeakReference\IsAlive\IL-CG2\*.dll
-o:C:\h\w\A5E609B0\w\A9D4091E\e\GC\API\WeakReference\IsAlive\\composite-r2r.dll
--targetarch:x64
--targetos:windows
--verify-type-and-field-layout
--method-layout:random
-r:C:\h\w\A5E609B0\p\System.*.dll
-r:C:\h\w\A5E609B0\p\Microsoft.*.dll
-r:C:\h\w\A5E609B0\p\xunit.*.dll
-r:C:\h\w\A5E609B0\p\mscorlib.dll
-r:C:\h\w\A5E609B0\p\netstandard.dll
"Running CrossGen2:  "C:\h\w\A5E609B0\p\crossgen2\crossgen2.exe" @"C:\h\w\A5E609B0\w\A9D4091E\e\GC\API\WeakReference\IsAlive\\composite-r2r.dll.rsp"   --composite  --composite"
Emitting R2R PE file: C:\h\w\A5E609B0\w\A9D4091E\e\GC\API\WeakReference\IsAlive\\composite-r2r.dll
Emitting R2R PE file: C:\h\w\A5E609B0\w\A9D4091E\e\GC\API\WeakReference\IsAlive\IsAlive.dll
"Running R2RDump:  "dotnet" "C:\h\w\A5E609B0\p\r2rdump\r2rdump.dll" --header --sc --in C:\h\w\A5E609B0\w\A9D4091E\e\GC\API\WeakReference\IsAlive\\composite-r2r.dll --out C:\h\w\A5E609B0\w\A9D4091E\e\GC\API\WeakReference\IsAlive\\composite-r2r.dll.r2rdump --val"
 7:04:11.92
 "C:\h\w\A5E609B0\p\corerun.exe" -p "System.Reflection.Metadata.MetadataUpdater.IsSupported=false" -p "System.Runtime.Serialization.EnableUnsafeBinaryFormatterSerialization=true"  IsAlive.dll 
True
True
Test for WeakReference.IsAlive failed!
Xunit.Sdk.EqualException: Assert.Equal() Failure: Values differ
Expected: 100
Actual:   1
   at Xunit.Assert.Equal[T](T expected, T actual, IEqualityComparer`1 comparer) in /_/src/Microsoft.DotNet.XUnitAssert/src/EqualityAsserts.cs:line 174
   at __GeneratedMainWrapper.Main()
Expected: 100
Actual: 101
END EXECUTION - FAILED
FAILED

[kg]

This may be a test quality issue, based on examining the test. I don't think it necessarily indicates a bug in crossgen. There is a suspicious codepath in the test that intentionally returns 100 which, if I understand the GC correctly, should never be reachable:

runtime/src/tests/GC/API/WeakReference/IsAlive.cs

Lines 34 to 37 in 7721695

	if(ans1==false) { // GC.Collect() has already occurred..under GCStress
	Console.WriteLine("Test for WeakReference.IsAlive passed!");
	return 100;
	}

The test failure described in the original post indicates that the array pointed to by the weak reference is still alive after a single invocation of GC.Collect. My best guess for why this would happen is a dangling managed reference to the array at the point of the collection, but I'm not actually certain that the GC is obligated to get rid of the array under these circumstances even if it's unreachable?

When I run this test locally without crossgen involved at all, it fails:

PS Z:\runtime> Z:\runtime\artifacts\tests\coreclr\windows.x64.Debug\GC\API\WeakReference\IsAlive\IsAlive.cmd
BEGIN EXECUTION
 "Z:\runtime\artifacts\tests\coreclr\windows.x64.Debug\Tests\Core_Root\corerun.exe" -p "System.Runtime.Serialization.EnableUnsafeBinaryFormatterSerialization=true"  IsAlive.dll
True
True
Test for WeakReference.IsAlive failed!
Xunit.Sdk.EqualException: Assert.Equal() Failure: Values differ
Expected: 100
Actual:   1
   at Xunit.Assert.Equal[T](T expected, T actual, IEqualityComparer`1 comparer) in /_/src/arcade/src/Microsoft.DotNet.XUnitAssert/src/EqualityAsserts.cs:line 174
   at __GeneratedMainWrapper.Main()
Expected: 100
Actual: 101

[Maoni0]

if you induce a full blocking GC (which GC.Collect() does), GC is not in charge of determining what should be live at all. it just gets told by the rest of the runtime. so if the JIT tells the GC what that weakref points to is dead at the time of that GC, it's guaranteed that GC will null the target of the handle the weakref contains (which will cause IsAlive to return false); otherwise IsAlive will return true.