Crash on initial pan left after start

[bryevdv]

If you start the profiler and hit the left arrow then the profiler crashes immediately. This is due to an overflow in compute_sample_utilization that was masked by:

       let start_time = interval.start.0 as u64;

which is not true when panning left from the initial starting range:

assertion failed: interval.start.0 >= 0:

So, one way to deal with this is to be more careful in compute_sample_utilization:

diff --git a/tools/legion_prof_rs/src/backend/data_source.rs b/tools/legion_prof_rs/src/backend/data_source.rs
index 12a6c5392..6d8036f2e 100644
--- a/tools/legion_prof_rs/src/backend/data_source.rs
+++ b/tools/legion_prof_rs/src/backend/data_source.rs
@@ -561,7 +561,16 @@ impl StateDataSource {
         interval: ts::Interval,
         samples: u64,
     ) -> Vec<UtilPoint> {
-        let start_time = interval.start.0 as u64;
+        if interval.start.0 + interval.duration_ns() <= 0 {
+            return [].to_vec();
+        }
+
+        let start_time = if interval.start.0 < 0 {
+            0
+        } else {
+            interval.start.0 as u64
+        };
+
         let duration = interval.duration_ns() as u64;

         let first_index = step_utilization

This seems to fix the issue, panning left before the initial start-up timestamp works as expected.

Questions for @elliottslaughter:

• This seems like reasonable defensive programming in any case, do you want a PR similar to the diff above made in the legion repo?
• If so, is zeroing the start but keeping the duration unchanged acceptable? An even more careful approach might special-case the first utilization point, but I am not sure that's worth it.
• Separately, should panning left beyond the initial start time be prohibited in the GUI? (This seems like it might be a bit clunky to accomplish)

[elliottslaughter]

No, I do not think we should prohibit the GUI from panning left from the origin.

I do think the GUI could be smarter about selecting tiles. That is, on this code path, intersect the request interval with the profile's actual interval:

https://github.com/StanfordLegion/prof-viewer/blob/99e0642d80c9b5af71dabb98d702fdbbdc095661/src/app.rs#L1331-L1333

At that point, it becomes invalid for the GUI to request bounds outside of the profile, and the backend can assert (if it wishes to do so) or ignore it. It's a matter of preference how defensive we want to be in this scenario.

[bryevdv]

Sure that also seems like a good option for the GUI https://github.com/StanfordLegion/prof-viewer/pull/40