dotnet-watch run fails to reload on Debian machines that do not have the procps package installed

[MichaelSimons]

Describe the bug

dotnet-watch has some undocumented native dependencies. Specifically it depends on the pgrep command which is not installed on a clean Debian machine. dotnet-watch run silently fails to reload when the dependency is not present.

To Reproduce

Copied from dotnet/dotnet-docker#2396

Requirements: Linux Docker environment and the .NET 5 CLI

1. Create a new simple "Hello world" webapp

mkdir /tmp/test && cd /tmp/test
dotnet new web

2. Run it with dotnet watch run and the code mounted in a container

docker run -it --rm --workdir /app -v ${PWD}/:/app mcr.microsoft.com/dotnet/sdk:5.0 dotnet watch run

You'll notice that when opening, altering and saving the ./Startup.cs file, the service isn't reloaded.

Further technical details

1. The mcr.microsoft.com/dotnet/sdk:5.0 image in this scenario is based on Debian. Installing the procps package resolves the issue.
2. This is what I found with a quick investigation.

https://github.com/dotnet/aspnetcore/blob/master/src/Tools/dotnet-watch/src/Internal/ProcessRunner.cs#L164

_process.KillTree();

https://github.com/dotnet/aspnetcore/blob/master/src/Shared/Process/ProcessExtensions.cs#L20

public static void KillTree(this Process process, TimeSpan timeout)
{
    var pid = process.Id;
    if (_isWindows)
    {
        ...
    }
    else
    {
        var children = new HashSet<int>();
        GetAllChildIdsUnix(pid, children, timeout);
        foreach (var childId in children)
        {
            KillProcessUnix(childId, timeout);
        }
        KillProcessUnix(pid, timeout);
    }
}

private static void GetAllChildIdsUnix(int parentId, ISet<int> children, TimeSpan timeout)
{
    try
    {
        RunProcessAndWaitForExit(
            "pgrep",
            $"-P {parentId}",
            timeout,
            out var stdout);

3. In discussing this with @pranavkm, it seems plausible that the KillTree implementation could be replaced with System.Diagnostrics.Process.Kill(bool entireProcessTree). This would be desirable as it would remove this native dependency.
4. This was discovered while investigating a 5.0 regression with the .NET SDK Docker image. The procps package was removed in 5.0 as part of an effort to reduce the image size in order to provide a better UX. The dependency dotnet-watch has was unknown to all parties involved.

[ghost]

Thanks for contacting us.
We're moving this issue to the Next sprint planning milestone for future evaluation / consideration. We will evaluate the request when we are planning the work for the next milestone. To learn more about what to expect next and how this issue will be handled you can read more about our triage process here.

[MichaelSimons]

@mkArtakMSFT - Can this get scheduled for 6.0?

cc /@richlander

[pranavkm]

Resolved as part of dotnet/sdk#15821.