Process tree retrieval

[charmitro]

The process tree retrieval failed when attempted after the monitored
process had terminated. Fix this by capturing the tree structure
during monitoring at peak memory usage.

Store the process tree snapshot when a new memory peak is detected and
return this stored tree instead of attempting post-termination
retrieval. This ensures child process information remains available
for display.

Verbose output for bash with children:

Command: cargo build
Started: 2025-05-28 00:38:31 UTC
Process ID: 1309040

Memory Usage:
  Peak RSS: 2.1 GiB (2243727360 bytes)
  Peak VSZ: 7.4 GiB (7979651072 bytes)

Process Tree: (18 processes monitored)
cargo (PID: 1309040) - Peak: 44.2 MiB
rustc (PID: 1309354) - Peak: 185.9 MiB
rustc (PID: 1309250) - Peak: 182.1 MiB
rustc (PID: 1309289) - Peak: 174.3 MiB
rustc (PID: 1309423) - Peak: 171.1 MiB
rustc (PID: 1309587) - Peak: 165.1 MiB
rustc (PID: 1309498) - Peak: 162.5 MiB
rustc (PID: 1309405) - Peak: 154.0 MiB
rustc (PID: 1309577) - Peak: 142.3 MiB
rustc (PID: 1309593) - Peak: 132.4 MiB
rustc (PID: 1309615) - Peak: 130.7 MiB
rustc (PID: 1309639) - Peak: 127.7 MiB
rustc (PID: 1309353) - Peak: 124.7 MiB
rustc (PID: 1309582) - Peak: 108.3 MiB
rustc (PID: 1309526) - Peak: 105.9 MiB
cc (PID: 1309630) - Peak: 940.0 KiB
collect2 (PID: 1309649) - Peak: 908.0 KiB
ld (PID: 1309650) - Peak: 26.7 MiB

Performance:
  Duration: 5.800s
  Samples collected: 59
  Sampling interval: 98ms

Exit Status: 0 (success)

[charmitro]

And a macOS one, FWIW:

Command: cargo b
Started: 2025-05-28 00:20:27 UTC
Process ID: 27137

Memory Usage:
  Peak RSS: 1.2 GiB (1262616576 bytes)
  Peak VSZ: 4.2 TiB (4630405857280 bytes)

Process Tree: (11 processes monitored)
cargo (PID: 27137) - Peak: 39.5 MiB
rustc (PID: 27164) - Peak: 177.6 MiB
rustc (PID: 27272) - Peak: 127.5 MiB
rustc (PID: 27274) - Peak: 125.9 MiB
rustc (PID: 27276) - Peak: 124.5 MiB
rustc (PID: 27275) - Peak: 118.1 MiB
rustc (PID: 27277) - Peak: 112.3 MiB
rustc (PID: 27280) - Peak: 98.2 MiB
rustc (PID: 27279) - Peak: 96.9 MiB
rustc (PID: 27278) - Peak: 93.3 MiB
rustc (PID: 27281) - Peak: 90.3 MiB

Performance:
  Duration: 3.501s
  Samples collected: 36
  Sampling interval: 97ms

Exit Status: 0 (success)

[charmitro]

Closed, now using srht https://lists.sr.ht/~charmitro/peak-mem-devel