System.Diagnostics.Process.Kill(entireProcessTree: true) doesn't kill entire tree if an intermediate child process is in job object with JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE

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Description

Assume you have this process tree:

• Process 1
  • Process 2 (in job object)
    • Process 3

Process 1 creates process 2 and adds it to a new Windows job object with JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE such that process 2 is automatically killed when process 1 gets terminated.
Process 2 then creates process 3 which is not part of that job object.

In this scenario, calling Kill(entireProcessTree: true) on process 1 terminates process 1 and process 2 but NOT process 3.

This scenario occurs for example with the Windows Python Launcher py.exe.
py.exe starts python.exe in a job object, but subprocesses started by the Python script are not in that job object, by design:

the launcher will execute its command in a child process, remaining alive while the child process is executing

the Win32 Job API will be used to arrange so that the child process is automatically killed when the parent is terminated (although children of that child process will continue as is the case now.)

Reproduction Steps

1. Create DotnetKillTree.exe:

dotnet new console -n DotnetKillTree -o .
echo System.Diagnostics.Process.GetProcessById(int.Parse(args[0])).Kill(entireProcessTree: true); > Program.cs
dotnet publish -r win-x64 --sc -p:PublishSingleFile=true -p:PublishTrimmed=true -p:DebugType=None -p:DebugSymbols=false -o .

2. Download and install Python for Windows from https://www.python.org/downloads/
3. Download PsList from https://learn.microsoft.com/en-us/sysinternals/downloads/pslist
4. Start the process tree in a new console:

py.exe -c "import subprocess;subprocess.run(['ping.exe', '-t', 'localhost'])"

5. In another console show the process tree:

C:\Users\WDAGUtilityAccount\Desktop\Test>pslist -t

PsList v1.41 - Process information lister
Copyright (C) 2000-2023 Mark Russinovich
Sysinternals - www.sysinternals.com

Process information for A1ACB1A2-C520-4:

Name                             Pid Pri Thd  Hnd      VM      WS    Priv
Idle                               0   0  16    0       8       8      60
...
explorer                        4244   8  59 2497 4194303  164184   64388
  cmd                           1356   8   4   81 4194303    4800    5468
    pslist                      4596  13   4  223   63620    7752    2436
    conhost                     5788   8   7  201 4194303   18800    7236
  cmd                           4796   8   1   84 4194303    4860    2272
    conhost                      720   8   4  202 4194303   18880    7148
    py                          5416   8   4  144   67540    7012    1596
      python                    4724   8   4   90 4194303   10936    7216
        PING                    4268   8   6   94 4194303    4528     996

6. Kill the py process subtree:

C:\Users\WDAGUtilityAccount\Desktop\Test>DotnetKillTree.exe 5416

7. Check the process tree:

C:\Users\WDAGUtilityAccount\Desktop\Test>pslist -t

Name                             Pid Pri Thd  Hnd      VM      WS    Priv
Idle                               0   0  16    0       8       8      60
...
explorer                        4244   8  55 2483 4194303  164800   64732
  cmd                           1356   8   2   84 4194303    4896    3892
    pslist                      3608  13   4  223   63620    7748    2428
    conhost                     5788   8   8  204 4194303   18880    7276
  cmd                           4796   8   1   83 4194303    4860    2532
    conhost                      720   8   4  200 4194303   18880    7148
PING                            4268   8   6   94 4194303    4536    1004

Expected behavior

The PING process 4268 is also killed.

Actual behavior

The PING process 4268 remains running.

Regression?

I don't know. Probably not.

Known Workarounds

Avoid creating processes in job objects.
For example directly start python.exe instead of py.exe:

python.exe -c "import subprocess;subprocess.run(['ping.exe', '-t', 'localhost'])"

Configuration

.NET 8.0.202
Windows 10 Pro 22H2
x64
It's only an issue on Windows (because of job objects).
But it's probably not specific to this configuration.

Other information

Not sure, but the problem might be that the KillTree(SafeProcessHandle handle) method first kills a process (so that no further children can be created) and then lists its children and recursively kills them.

I suspect that in the above scenario killing process 1 closes the job object which already kills process 2 (but not process 3). And then process 2 is no longer a child of process 1 (and neither is process 3) and therefore the recursion ends and process 3 remains running.

To inspect the job object of each process (in a different test, hence the PIDs don't match) I used ProcessExplorer:

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