Jesse Szwedko

j.szwedko@modcloth.com

@jesse_szwedko

This talk: github.com/jszwedko/gsp-go-debugging

Location: ModCloth, Inc. (Pittsburgh Office)

Date: 7/31/2014

Tracing

Interactive debugger

Post-mortem debugging

Remote debugger

Tracing

• log
• logrus

Interactive debugger

• gdb
• LiteIDE

Post-mortem debugging

• GOTRACEBACK
• SIGABRT

• Logs are streams
• stdlib log
• logrus
• GODEBUG
  • Can be used to output information about garbage collection and the scheduler

• Offers much of what you need from a logger
• Recommend (conditonally?) setting Flags to include source line
• Recommend allowing conditional enabling of logs (integration tests)

• Multiple output formats (text (l2met), json)
• Multiple levels (can configure output level)
• Hooks (e.g. sending to airbrake when an error is logged)
• Fields (can be used for metrics or context)

GDB

• The GNU Project Debugger

LiteIDE

• Cross-platform Go IDE

• Build application with go build -gcflags "-N -l" to disable optimizations
• For tests, run go test -gcflags "-N -l" -c to have Go compile the test, but not run it
• Run gdb -d $(go env GOROOT) <your binary>
• Go outputs DWARF debugging information with the binary
  • See Introduction to the DWARF Debugging Format for a good intro to DWARF

Gotchas:

• OSX ships with GDB 6.0 (you'll want > 7.0)
• Wasn't able to get Go GDB extensions working with versions built somewhere other than install location (ignores GOROOT)
• Hit or miss about which locals are available to be printed
• Unusable in Go 1.3 (for me at least)
  • gdb: Wrong values for local variables
  • gdb: breakpoints break things
  • gdb: nothing works (windows amd64)
• Debugging Go Code with GDB
• GDB user manual

• list Print next 10 source lines
• break
• break <file.go line> Insert breakpoint in file.go at line line
• break <p.Func> Insert breakpoint at function Func of package p
• bt Print backtrace
• continue Continue to next break point
• step Run until next source line
• next Run until next source line in the same stack frame

• info args Show function arguments
• info locals Show local variables
• p <variable> Print variable value
  • <slc>->array[<x>] print element x of slc
• whatis <variable> Print type of variable
• Leave empty to execute last command again

Go extensions

• Adds pretty printing of some non-standard types
• info goroutines Print goroutines
• goroutine <n> <cmd> Execute cmd in the context of goroutine n
• $len() Gets len
• $cap() Gets capacity
• $dtype() Get underlying type of interface value
• Walkthrough of installing gdb/LiteIDE on OSX

From http://golang.org/doc/gdb

"GDB does not understand Go programs well. The stack management, threading, and runtime contain aspects that differ enough from the execution model GDB expects that they can confuse the debugger, even when the program is compiled with gccgo. [...] Moreover, it is not a priority for the Go project to address these issues, which are difficult. [...]"

Graphical editor for Golang

Integrates with GDB

https://github.com/visualfc/liteide

Setup

• View -> Edit Environment
  • set GOPATH and GOROOT
• Click gear icon when file with main() is open
  • Set BUILDARGS to -gcflags "-N -l"
• Set breakpoints with the red orb
• Debug -> Start Debugging
• Walkthrough of installing gdb/LiteIDE on OSX

GOTRACEBACK: Dump stack traces when Go program encounters unrecovered panic or unexpected runtime condition

• 0: no stack traces
• 1: dump user goroutines backtrace (default)
• 2: dump run time and user goroutines backtrace
• crash: dump user and run time goroutines and crash in OS specific manner (core dump)
  • You can also get this manually by issuing a SIGABRT to a running process
• gdb -d $(go env GOROOT) -c core <your app>

Gotchas:

• Still want to compile with -gcflags "-N -l" to get debugging information
• Don't forget to set ulimit -c to non-zero value

Tracing

• log
• logrus

Interactive debugger

• gdb
• LiteIDE

Post-mortem debugging

• GOTRACEBACK
• SIGABRT

Jesse Szwedko

j.szwedko@modcloth.com

@jesse_szwedko

This talk: github.com/jszwedko/gsp-go-debugging