Fix Windows Ppid Cache Race Condition

[AtakanColak]

As per #961, a RWMutex to Process struct and thread-safe ppid access functions are added which are now used for windows but virtually can be used for other OSes in the future.

Another solution would be to not cache at all, but this solution reduces overall system calls and in windows it makes quite the difference.

Please do review. @Lomanic @shirou

[AtakanColak]

go vet still fails, looking into it

[AtakanColak]

go vet check passes after changing the mutex to a pointer that is initialized with NewProcess call

[AtakanColak]

Squashed commits, waiting for review.

[Lomanic]

Thanks for PR, I didn't yet have a chance to test it. See my comment.

[shirou]

Sorry, #966 makes conflicts. But If you follow the comment and make the changes, this conflicts may resolved,

[AtakanColak]

@Lomanic thank you for the doc link, seeing a caching in windows ppid showed me that I was right to start this feature beforehand. Resolved it by carrying the methods to process_windows.go

@shirou resolved the conflicts.

Even though my tests passed, please do test before merge.

[shirou]

Thank you so much! I can reproduce race condition by using this test and this PR can fix.

% go test --race -v -run Test_Process_Ppid

func Test_Process_Ppid(t *testing.T) {
	wg := sync.WaitGroup{}
	testCount := 10
	p := testGetProcess()
	wg.Add(testCount)
	for i := 0; i < testCount; i++ {
		go func(j int) {
			ppid, err := p.Ppid()
			wg.Done()
			skipIfNotImplementedErr(t, err)
			if err != nil {
				t.Errorf("Ppid() failed, %v", err)
			}

			if j == 9 {
				t.Logf("Ppid(): %d", ppid)
			}
		}(i)
	}
	wg.Wait()
}

Could you replace the current Test_Process_Ppid() by this code, confirm, and commit again? Thanks!

[AtakanColak]

@shirou test passed on my windows. Added to the commit.

[Lomanic]

Problem with your test @shirou is that it doesn't test the same thing anymore (getting a non-zero value, and the same value as os.Getppid()). I would restore the previous one and put that one new test to a dedicated test like in https://github.com/shirou/gopsutil/blob/master/disk/disk_test.go#L70-L85.

And I would also suggest to be explicit that this test is made to detect race conditions (in its name). This test could even be put in a new process/process_race_test.go with // +build race flag on top so it only runs when the race detector is enabled.

[shirou]

Ah, yes, totally agree to @Lomanic .

I'm really sorry but could you revert that commit and create a new process/process_race_test.go file? > @AtakanColak .

[AtakanColak]

@Lomanic @shirou committed with a process_race_test.go instead.

[AtakanColak]

@Lomanic could you review and merge if deemed sufficient please.