Detecting whether a debugger is attached to the current process

[HertzDevil]

Sometimes, it is desirable to perform extra actions in a process only when a debugger is already attached, e.g. log more information, trigger a debugger break, or implement #4263. Although the extra actions can be conditionally guarded using some compile-time flag, more convenient would be detecting the debugger's presence at execution time; this reduces recompilations, and allows the process to react to live debugger attachments and detachments.

On Windows, the implementation is very simple, and supports Visual Studio / LLDB / GDB:

lib LibC
  fun IsDebuggerPresent : BOOL
end

def debugger_present? : Bool
  LibC.IsDebuggerPresent != 0
end

On Linux, the conventional way seems to be reading from /proc/self/status. It is slower, but more expressive as it can obtain the debugger's process ID and handle other processes:

def debugger_present? : Bool
  File.each_line("/proc/self/status") do |line|
    if tracer_pid = line.lchop?("TracerPid:").try(&.to_i?)
      return true if tracer_pid != 0
    end
  end
  false
end

However, some BSDs, including macOS, do not have procfs. It seems sysctl would do the job.

On Solaris /proc/*/status is a binary blob.