rbspy: bug: Early exits, duration parameter not respected in some cases

In some cases, we see that rbspy exits early, both with and without the --duration parameter.

Without the duration parameter, it can sometimes exit with an exit code of 0. The target process has not exited, and there is no apparent error in the output.

With the duration parameter, we can sometimes see it exit well before the duration has elapsed:

$ ./rbspy-x86_64-unknown-linux-gnu record -p 908389 -d 120

Time since start: 31s. Press Ctrl+C to stop.
Summary of profiling data so far:
% self  % total  name
 81.04    81.14  select [c function] - (unknown)
  9.38     9.38  pop [c function] - (unknown)
  2.42     2.42  sleep [c function] - (unknown)
  0.77     0.77  wait_readable [c function] - (unknown)
  0.77     0.77  initialize [c function] - (unknown)
  0.58     0.58  _query [c function] - (unknown)
  0.45     2.00  new [c function] - (unknown)
  0.19     0.52  initialize - /artifacts/bundle/ruby/2.7.0/gems/graphql-1.12.13/lib/grap
  0.19     0.19  tcp_listener_stats [c function] - (unknown)
  0.13     0.13  split [c function] - (unknown)
  0.10     0.10  match? [c function] - (unknown)
  0.06     5.58  public_send [c function] - (unknown)
  0.06     3.93  block in run - /app/app/models/engine.rb
  0.06     1.39  block in coerce_into_values - /artifacts/bundle/ruby/2.7.0/gems/graphql
  0.06     0.68  synchronize [c function] - (unknown)
  0.06     0.48  block in rules_from_db - /app/app/models/ordered_ruleset.rb
  0.06     0.06  legacy_connection_handling - /artifacts/bundle/ruby/2.7.0/gems/activere
  0.06     0.06  inheritance_column - /artifacts/bundle/ruby/2.7.0/gems/activerecord-6.1
  0.06     0.06  eval [c function] - (unknown)
  0.06     0.06  clock_gettime [c function] - (unknown)
1.8% (56/3109) of stack traces were sampled late because we couldn't sample at expected rate, results may be inaccurate. Current rate: 100. Try sampling at a lower rate with `--rate`.
Wrote raw data to /root/.cache/rbspy/records/rbspy-2021-07-22-1cem3j2wTP.raw.gz
Writing formatted output to /root/.cache/rbspy/records/rbspy-2021-07-22-yrFdFN7bCV.flamegraph.svg

In this example the target pid was still alive after rbspy exited.

I would expect that if no duration parameter is given, that rbspy waits until it is interrupted. If a duration parameter is given, it should wait until the duration has elapsed before exiting.

Any ideas @acj ?