Debug and getting a backtrace of stuck Ruby process with gdb

README.md

Steps

Get the PID ps -Af | grep ruby

Attach to the process sudo gdb -p PID

Enter in Python python

Run snippet gdb_ruby_backtrace.py

Execute python get_ruby_stacktrace() to get backtrace

Useful Links

Adventures In Production Rails Debugging

How to spy on a Ruby program

Using GDB to Inspect a Running Ruby Process

gdb_ruby_backtrace.py

# Updated for Ruby 2.3
string_t = None

def get_rstring(addr):
  s = addr.cast(string_t.pointer())
  if s['basic']['flags'] & (1 << 13):
    return s['as']['heap']['ptr'].string()
  else:
    return s['as']['ary'].string()

def get_lineno(iseq, pos):
  if pos != 0:
    pos -= 1
  t = iseq['line_info_table']
  t_size = iseq['line_info_size']

  if t_size == 0:
    return 0
  elif t_size == 1:
    return t[0]['line_no']

  for i in range(0, int(t_size)):
    if pos == t[i]['position']:
      return t[i]['line_no']
    elif t[i]['position'] > pos:
      return t[i-1]['line_no']

  return t[t_size-1]['line_no']

def get_ruby_stacktrace(th=None):
  global string_t

  try:
    control_frame_t = gdb.lookup_type('rb_control_frame_t')
    string_t = gdb.lookup_type('struct RString')
  except gdb.error:
    raise gdb.GdbError ("ruby extension requires symbols")

  if th == None:
    th = gdb.parse_and_eval('ruby_current_thread')
  else:
    th = gdb.parse_and_eval('(rb_thread_t *) %s' % th)

  last_cfp = th['cfp']
  start_cfp = (th['stack'] + th['stack_size']).cast(control_frame_t.pointer()) - 2
  size = start_cfp - last_cfp + 1
  cfp = start_cfp
  call_stack = []
  for i in range(0, int(size)):
    if cfp['iseq'].dereference().address != 0:
      if cfp['pc'].dereference().address != 0:
        s = "{}:{}:in `{}'".format(get_rstring(cfp['iseq']['body']['location']['path']),
          get_lineno(cfp['iseq']['body'], cfp['pc'] - cfp['iseq']['body']['iseq_encoded']),
          get_rstring(cfp['iseq']['body']['location']['label']))
        call_stack.append(s)

    cfp -= 1

  for i in reversed(call_stack):
    print(i)

end