fix duration in (fast)delta profiles

[pmbauer]

What does this PR do?

Fixes a bug in the duration computation for fast delta

The current version produces delta pprofs with a DurationNanos ~equal to the life of the process.

Note: this was a regression from a previously tested version of fastdelta. Unfortunately, there wasn't sufficient test coverage for this case. This pull addresses that too.

Describe how to test/QA your changes

follow test procedure executed for fast delta

• test in reliability env
• test on production shadow instance

Reviewer's Checklist

• If known, an appropriate milestone has been selected; otherwise the Triage milestone is set.
• Changed code has unit tests for its functionality.
• If this interacts with the agent in a new way, a system test has been added.

[nsrip-dd]

Thanks Paul!

The TestRepeatedHeapProfile test (which you have to enable with an env var since it's too expensive to run in our CI) also catches this bug if you add checking for the timestamps, which I neglected to do before.Since it's not run during our CI it's slightly less useful, though. Here's a patch if you think it's worth making that change as well:

diff --git a/profiler/internal/fastdelta/fd_test.go b/profiler/internal/fastdelta/fd_test.go
index 17404b80..1ac882dc 100644
--- a/profiler/internal/fastdelta/fd_test.go
+++ b/profiler/internal/fastdelta/fd_test.go
@@ -340,6 +340,9 @@ func makeGolden(t testing.TB, before, after []byte, fields ...pprofutils.ValueTy
        if err != nil {
                t.Fatal(err)
        }
+       c.TimeNanos = a.TimeNanos
+       // DurationNanos is the time period covered by the profile.
+       c.DurationNanos = a.TimeNanos - b.TimeNanos
        return c
 }
 
@@ -721,6 +724,10 @@ func TestRepeatedHeapProfile(t *testing.T) {
 
                golden := makeGolden(t, before, after, fields...)
 
+               if golden.DurationNanos != delta.DurationNanos {
+                       t.Fatalf("iter %d: want duration %v, got %v", i, golden.DurationNanos, delta.DurationNanos)
+               }
+
                golden.Scale(-1)
                diff, err := profile.Merge([]*profile.Profile{delta, golden})
                if err != nil {

[pmbauer]

Yep. I considered adding the time check to TestRepeatedHeapProfile, but as it doesn't run by default or in CI, I thought it worth breaking out. Too, that's a fairly heavyweight test.

[pr-commenter]

Benchmarks

Comparing candidate commit 477791a in PR branch pmbauer/fix-fastdelta-profile-duration with baseline commit ce33558 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 6 cases.