Check if a process is running using it's name

[fuegas]

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

This PR relates to #5746. When you provide a pid_file that exists, has a (int32) number in it of a non-existing process, it will report the process as running right from the start of the agent:

> procstat_lookup,host=Kita.local,pid_finder=pgrep,pidfile=/tmp/process.pid,result=success pid_count=1i,result_code=0i,running=1i 1556141065000000000

A process is marked running if the procs map has at least one item:

"running":     len(procs),

This map is build/filled by updateProcesses:

info, ok := prevInfo[pid]
if ok {
  procs[pid] = info
}
...
proc, err := p.createProcess(pid)
if err != nil {
  // No problem; process may have ended after we found it
  continue
}
procs[pid] = proc

Because the PID is specified by a pid_file we have a valid PID (int32). This results in a valid Process from p.createProcess(pid), regardless whether the process exists or not. This Process is inserted into the procs map resulting in a map that has 1 or more entries. So if you provide a pid_file that has a number in it, procstat will always return running=1i.

In this PR I assume processes without name are rare and the chance that you want to monitor a process with no name is even rarer, a process without (or empty) name is seen as not running.

[sgtsquiggs]

Is this really necessary?

updateProcesses depends on gopsutil's NewProcess to check if the process exists, which achieves this by attempting to open /proc/<pid>.

[fuegas]

That is true for the Linux adapter. However, on Darwin, FreeBSD, OpenBSD and Windows the process is only created as a struct and no checks are done. So on those platforms the process from a pidfile always shows running=1i regardless of the actual state.

[sgtsquiggs]

That is unfortunate. For linux/bsd platforms we would confirm that the process exists via

err := process.Signal(syscall.Signal(0))

assuming process is type os.Process.

For windows, os.FindProcess actually checks to see if the process exists.