Skip to content

Instantly share code, notes, and snippets.

Embed
What would you like to do?
debug SystemStackError in ruby 1.9

This is how I debug SystemStackError when there is no stack trace.

My first attempt was:

begin
  a_method_that_causes_infinite_recursion_in_a_not_obvious_way
rescue SystemStackError
  puts caller
end

But that was a total fail because the stack trace I got was this:

(irb):2:in `irb_binding'
/Users/jordan/.rvm/rubies/ruby-1.9.3-p286-perf/lib/ruby/1.9.1/irb/workspace.rb:80:in `eval'
/Users/jordan/.rvm/rubies/ruby-1.9.3-p286-perf/lib/ruby/1.9.1/irb/workspace.rb:80:in `evaluate'
/Users/jordan/.rvm/rubies/ruby-1.9.3-p286-perf/lib/ruby/1.9.1/irb/context.rb:254:in `evaluate'
/Users/jordan/.rvm/rubies/ruby-1.9.3-p286-perf/lib/ruby/1.9.1/irb.rb:159:in `block (2 levels) in eval_input'
/Users/jordan/.rvm/rubies/ruby-1.9.3-p286-perf/lib/ruby/1.9.1/irb.rb:273:in `signal_status'
/Users/jordan/.rvm/rubies/ruby-1.9.3-p286-perf/lib/ruby/1.9.1/irb.rb:156:in `block in eval_input'
/Users/jordan/.rvm/rubies/ruby-1.9.3-p286-perf/lib/ruby/1.9.1/irb/ruby-lex.rb:243:in `block (2 levels) in each_top_level_statement'
/Users/jordan/.rvm/rubies/ruby-1.9.3-p286-perf/lib/ruby/1.9.1/irb/ruby-lex.rb:229:in `loop'
/Users/jordan/.rvm/rubies/ruby-1.9.3-p286-perf/lib/ruby/1.9.1/irb/ruby-lex.rb:229:in `block in each_top_level_statement'
/Users/jordan/.rvm/rubies/ruby-1.9.3-p286-perf/lib/ruby/1.9.1/irb/ruby-lex.rb:228:in `catch'
/Users/jordan/.rvm/rubies/ruby-1.9.3-p286-perf/lib/ruby/1.9.1/irb/ruby-lex.rb:228:in `each_top_level_statement'
/Users/jordan/.rvm/rubies/ruby-1.9.3-p286-perf/lib/ruby/1.9.1/irb.rb:155:in `eval_input'
/Users/jordan/.rvm/rubies/ruby-1.9.3-p286-perf/lib/ruby/1.9.1/irb.rb:70:in `block in start'
/Users/jordan/.rvm/rubies/ruby-1.9.3-p286-perf/lib/ruby/1.9.1/irb.rb:69:in `catch'
/Users/jordan/.rvm/rubies/ruby-1.9.3-p286-perf/lib/ruby/1.9.1/irb.rb:69:in `start'
/Users/jordan/.rvm/gems/ruby-1.9.3-p286-perf/gems/railties-3.1.10/lib/rails/commands/console.rb:45:in `start'
/Users/jordan/.rvm/gems/ruby-1.9.3-p286-perf/gems/railties-3.1.10/lib/rails/commands/console.rb:8:in `start'
/Users/jordan/.rvm/gems/ruby-1.9.3-p286-perf/gems/railties-3.1.10/lib/rails/commands.rb:40:in `<top (required)>'
script/rails:6:in `require'
script/rails:6:in `<main>'

Yeah, so nothing useful. I expected to see thousands of method calls! Instead I see no methods beyond the IRB prompt.

No worries, ruby has tracing functionality built-in, so I used that instead. Here's the one I wrote. This will spit out a ton of information, so the challenge is filtering so that you only capture useful information. Oh, and, uh... you want to write this to a file because otherwise it will flood your irb prompt.

$enable_tracing = false
$trace_out = open('trace.txt', 'w')

set_trace_func proc { |event, file, line, id, binding, classname|
  if $enable_tracing && event == 'call'
    $trace_out.puts "#{file}:#{line} #{classname}##{id}"
  end
}

$enable_tracing = true
a_method_that_causes_infinite_recursion_in_a_not_obvious_way

Now let's check to see if I have some methods...

$ ls -lH | grep trace
-rw-r--r--   1 jordan  staff   5.1M Jan  9 09:03 trace.txt
$ wc -l trace.txt 
   37841 trace.txt

I sure do! 37841 method calls in fact. I'll leave it to an excercise for the reader to find out what's causing the stack overflow 😉

@jeandenis-k

This comment has been minimized.

Copy link

commented Oct 31, 2013

Thanks for the tip, it's a life saver! I just solved an infinite loop error with your tracing code. I grepped the name of the library I suspected on the trace file, and sure enough it was responsible for most of the method calls. I owe you one!

@steverice

This comment has been minimized.

Copy link

commented May 11, 2014

Hey this was great! Spent far too much time trying to run my Ruby 1.9 code against 1.8 in order to get a stack trace — this worked like a charm.
Big thanks!

@bendilley

This comment has been minimized.

Copy link

commented Feb 10, 2015

In case it helps anyone, I've gisted a tweak to this solution which attempts to raise an exception before the recursion gets too deep.

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
You can’t perform that action at this time.