Hide the pid in global tracks if it is 0.

[fqueze]

This will make size profiles look nicer.

Example profile: https://share.firefox.dev/42MdeHQ

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.84%. Comparing base (7f30744) to head (10f6f0a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5361   +/-   ##
=======================================
  Coverage   85.84%   85.84%           
=======================================
  Files         312      312           
  Lines       29827    29829    +2     
  Branches     8213     8215    +2     
=======================================
+ Hits        25605    25607    +2     
  Misses       3627     3627           
  Partials      595      595           

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[mstange]

Here's a profile from samply import perf.data where perf.data was gathered with simpleperf in system-wide mode. It has an actual process with a zero PID. But it doesn't have any samples for that process. https://share.firefox.dev/415PzAR

But I think it's possible to have samples for PID 0. Here's a quote from a stackoverflow reply:

So nowadays process #0 on a Unix is the system process, which effectively holds a number of kernel threads doing a number of things, ranging from page-out operations, through filesystem cache flushes and buffer zeroing, to idling when there's nothing else to run.

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I'm not opposed to the suggested change in this PR, but we should be aware that sometimes, a pid of zero is legitimate.

[fqueze]

Should we instead change the size-profiler to make thread.pid an empty string, and hide the pid when the string is empty? (currently it's only hidden when the string is null, which isn't possible for actual thread, but happens for screenshot tracks)

[mstange]

Should we instead change the size-profiler to make thread.pid an empty string, and hide the pid when the string is empty? (currently it's only hidden when the string is null, which isn't possible for actual thread, but happens for screenshot tracks)

I think this might cause other problems because in the URL we encode some things keyed by pid, and I'm not sure if that would still work properly if the pid is empty. Let's go with what you have.

In the future we could change the URL encoding to use process indexes, and have a central list of processes so that the process start/stop/name information is deduplicated. Then we can afford empty pids.