Summary

The Linux branch of MEMORY_GRABBER (lib/sidekiq/launcher.rb) takes a pid argument but ignores it, reading the current process instead:

when /linux/
  ->(pid) {
    IO.readlines("/proc/#{$$}/status").each do |line|   # reads $$ , not pid
      ...

The darwin|bsd branch correctly uses pid (ps -o pid,rss -p #{pid}). The bug has been latent since #4717 because the only caller is memory_usage(::Process.pid), so $$ and pid are always equal in practice — but the lambda is parameterized by pid and should honor it (and stay consistent across platforms).

Fix: read /proc/#{pid}/status.

Testing

Adds a Linux-guarded regression test: calling the grabber with a non-existent pid must raise Errno::ENOENT (it now reads that pid's /proc entry). This fails against the old $$ behavior (which silently returns the current process's RSS) and passes after the fix. It skips on non-Linux platforms where this lambda isn't used.

bundle exec rake is green (standard + lint:herb + full suite; the new test skips locally on macOS and runs on Linux CI).

Note: I develop on macOS, so the Linux /proc path can't execute locally — this fix is by inspection/consistency with the darwin|bsd branch, and the regression test exercises it on Linux CI.