Add `PerfTools::FiberTrace.pretty_log_hierarchy`

[HertzDevil]

This:

Example code

spawn(name: "A1") do
  sleep
end

spawn(name: "B1") do
  spawn(name: "B2") do
    sleep
  end
  sleep
end

# C1 dies
# C2 is orphaned
spawn(name: "C1") do
  spawn(name: "C2") do
    sleep
  end
end

spawn(name: "D1") do
  spawn(name: "D2") do
    spawn(name: "D3") do
      sleep
    end
    sleep
  end
  sleep
end

# E1 dies
# E2 and E3 are orphaned
spawn(name: "E1") do
  spawn(name: "E2") do
    spawn(name: "E3") do
      sleep
    end
    sleep
  end
end

# F2 dies
# F3 is orphaned
spawn(name: "F1") do
  spawn(name: "F2") do
    spawn(name: "F3") do
      sleep
    end
  end
  sleep
end

# G1 and G2 die
# G3 is orphaned
spawn(name: "G1") do
  spawn(name: "G2") do
    spawn(name: "G3") do
      sleep
    end
  end
end

sleep 0.01
PerfTools::FiberTrace.pretty_log_hierarchy(STDOUT)

prints:

#<Fiber:0x1047c2e60: main>
  #<Fiber:0x1047c2f00: Fiber Clean Loop>
  #<Fiber:0x1047c2dc0: Signal Loop>
  #<Fiber:0x1047c2d20: A1>
  #<Fiber:0x1047c2c80: B1>
    #<Fiber:0x1047c28c0: B2>
  #<Fiber:0x1047c2b40: D1>
    #<Fiber:0x1047c2780: D2>
      #<Fiber:0x1047c2500: D3>
  #<Fiber:0x1047c2a00: F1>
(orphaned)
  #<Fiber:0x1047c2820: C2>
  #<Fiber:0x1047c26e0: E2>
    #<Fiber:0x1047c2460: E3>
  #<Fiber:0x1047c23c0: F3>
  #<Fiber:0x1047c2320: G3>

If -Dpreview_mt is active, the sibling order in the hierarchy might change slightly, and also the additional worker threads will have no child fibers at all, because all the spawning at the top level still takes place in the main thread, whereas spawning inside another fiber ultimately roots the fiber to the same main thread.

On the other hand, Thread.new starts a new tree with or without -Dpreview_mt, and if its body spawns a fiber, the hierarchy will be reflected too:

Thread.new do
  spawn(name: "H1") do
    sleep
  end
  sleep
end

sleep 0.1
PerfTools::FiberTrace.pretty_log_hierarchy(STDOUT)

#<Fiber:0x104a06e60: main>
  #<Fiber:0x104a06f00: Fiber Clean Loop>
  #<Fiber:0x104a06dc0: Signal Loop>
#<Fiber:0x104a06d20: main>
  #<Fiber:0x104a06c80: H1>

The line format is simply Fiber#inspect at the moment, without any unique counter nor associated thread. This can be changed easily.

No provisions for thread safety are done yet.