Process.StartTime on macOS returns different values

[mdolan92869]

When getting the StartTime for a process, each time it's called for a new instance of the same process, it has a different value. I believe that the issue is in StartTimeCore where the native "now" time is retrieved, a bunch of arithmetic is performed, then the C# "now" time is retrieved. Since operations were between the two "now" calls, the "now" values will obviously be different, but they're used as if they're the same. I've attached a sample program that will show the deltas between StartTime accesses.

System: macOS 10.14.5 Mojave

ProcessStartTime.zip

[mdolan92869]

In the attached code, the PID I was using was for Microsoft Outlook on my laptop. It was launched one time, but the StartTime drifted and no 2 consecutive calls got the same value.

[danmoseley]

https://github.com/dotnet/corefx/blob/ef25246c7ea01a96cda8a464b4b717213e8d9685/src/System.Diagnostics.Process/src/System/Diagnostics/Process.OSX.cs#L53-L60

@mdolan92869 perhaps you'd like to attempt a fix?

[jpfreyen]

@danmosemsft I'm reading this and I am concerned that there is a similar issue in Linux not being addressed by just fixing Process.OSX.cs

[danmoseley]

We likely won't have time to look at this for a a little while, so if either of you feel motivated to work on PR(s) that would be welcome.

[watfordgnf]

I've been away from OS X programming for some time (and my MBP is too old to run .NET Core), but would proc_bsdinfo.pbi_start_tvsec be a good start? Interop.libproc.GetProcessInfoById already has a call which retrieves it within proc_taskallinfo.pbsd. https://github.com/dotnet/corefx/blob/f7539b726c4bc2385b7f49e5751c1cff2f2c7368/src/Common/src/Interop/OSX/Interop.libproc.cs#L310-L323

Untested:

        /// <summary>Gets the time the associated process was started.</summary>
        private int StartTimeCore
        {
            get
            {
                EnsureState(State.HaveNonExitedId);
                Interop.libproc.proc_taskallinfo? info = Interop.libproc.GetProcessInfoById(Id);

                if (info == null)
                    throw new Win32Exception(SR.ProcessInformationUnavailable);

                // ignores microseconds
                return DateTime.UnixEpoch + TimeSpan.FromSeconds(info.pbsd.pbi_start_tvsec);
            }
        }

[watfordgnf]

@mdolan92869 I've put up a branch which attempts a fix if you can test (my MBP cannot run .NET Core).

@jpfreyen it looks like the Linux code falls back on UtcNow if it can't read the boot time from /proc/stat. It would be nice if /proc/{pid}/psinfo was available on *nix, as its start time is relative to the epoch rather than ticks since boot.

[jpfreyen]

@watfordgnf I would agree, pbi_start_tvsec/pbi_start_tvusec would be a good place to start for macOS.

And @watfordgnf do you mean /proc/{pid}/stat instead of /proc/stat? Or wait, if /proc/stat somehow can't be read then .NETCore doesn't bother with individual process stats at /proc/{pid}/stat, right?

[watfordgnf]

It pulls starttime from /proc/{pid}/stat, which is in ticks since boot. Boot time is then pulled from /proc/stat. It falls back to adding starttime to DateTime.UtcNow in the event /proc/stat parsing fails (for whatever reason).

[watfordgnf]

I've opened draft PR dotnet/corefx#39572 to iterate on a possible implementation.

[jpfreyen]

Thanks for the fix @watfordgnf; I can't help test, my work environment is totally not setup to build .NETCore code changes and try them.