Daniel Hoelbling-Inzko talks about programming

Debugging Go IOWait Hang: Sometimes it's really not your code

Posted by Daniel Hölbling on August 14, 2020

If something looks like a bug in the Language Runtime, Standard Library or the Operating System I tend to always approach it with caution: It's usually a bug in my code and I'm just not seeing it.

But sometimes it's not me - it's really the compiler and you spend a solid week debugging a Go program until you find out that cross-compiling from OSX to Linux leads to a stdlib Bug that manifests itself with the whole application just hanging in IOWait loops given enough concurrency.

Obviously the whole thing was really frustrating because:

  • The bug only happened on production servers (obviously - anything else would not be fun).
  • Could only be reproduced on a large dataset of 300 million items (so every test also takes quite a while)
  • I had to test if it works without concurrency (which took 2 days and yes it did)

But the important finding from this exercise was that you can print the full stacktrace of all running Goroutines as well as their status for a running/hanging program! You just have to send the kill -ABRT signal to a process! This is similar to what you see when a panic occurs and was massively helpful in hunting down this bug. Kudos to the Go team for that.

An example for this:

package main

func main() {
  for {}

The program will obviously hang and do a busy loop, but if you send the kill -ABRT signal you'll get something similar to this printed to stderr:

SIGABRT: abort
PC=0x1056d70 m=0 sigcode=0

goroutine 1 [running]:
        /Users/tigraine/projects/test/main.go:4 fp=0xc00003c788 sp=0xc00003c780 pc=0x1056d70
        /usr/local/Cellar/go/1.14.1/libexec/src/runtime/proc.go:203 +0x212 fp=0xc00003c7e0 sp=0xc00003c788 pc=0x102b3f2
        /usr/local/Cellar/go/1.14.1/libexec/src/runtime/asm_amd64.s:1373 +0x1 fp=0xc00003c7e8 sp=0xc00003c7e0 pc=0x10528f1
Filed under golang, go, debugging
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