Go has a debugger—and it's awesome!

by Filippo Valsorda.

Something that often, uh... bugs1 Go developers is the lack of a proper debugger. Sure, builds are ridiculously fast and easy, and println(hex.Dump(b)) is your friend, but sometimes it would be nice to just set a breakpoint and step through that endless if chain or print a bunch of values without recompiling ten times.

CC BY 2.0 image by Carl Milner

You could try to use some dirty gdb hacks that will work if you built your binary with a certain linker and ran it on some architectures when the moon was in a waxing crescent phase, but let's be honest, it isn't an enjoyable experience.

Well, worry no more! godebug is here!

godebug is an awesome cross-platform debugger created by the Mailgun team. You can read their introduction for some under-the-hood details, but here's the cool bit: instead of wrestling with half a dozen different ptrace interfaces that would not be portable, godebug rewrites your source code and injects function calls like godebug.Line on every line, godebug.Declare at every variable declaration, and godebug.SetTrace for breakpoints (i.e. wherever you type _ = "breakpoint").

I find this solution brilliant. What you get out of it is a (possibly cross-compiled) debug-enabled binary that you can drop on a staging server just like you would with a regular binary. When a breakpoint is reached, the program will stop inline and wait for you on stdin. It's the single-binary, zero-dependencies philosophy of Go that we love applied to debugging. Builds everywhere, runs everywhere, with no need for tools or permissions on the server. It even compiles to JavaScript with gopherjs (check out the Mailgun post above—show-offs ;) ).

You might ask, "But does it get a decent runtime speed or work with big applications?" Well, the other day I was seeing RRDNS—our in-house Go DNS server—hit a weird branch, so I placed a breakpoint a couple lines above the if in question, recompiled the whole of RRDNS with godebug instrumentation, dropped the binary on a staging server, and replayed some DNS traffic.

[email protected]:~$ ./rrdns -config config.json  
-> _ = "breakpoint"
(godebug) l

    q := r.Query.Question[0]

--> _ = "breakpoint"

    if !isQtypeSupported(q.Qtype) {
(godebug) n
-> if !isQtypeSupported(q.Qtype) {
(godebug) q
dns.Question{Name:"filippo.io.", Qtype:0x1, Qclass:0x1}  
(godebug) c

Boom. The request and the debug log paused (make sure to kill any timeout you have in your tools), waiting for me to step through the code.

Sold yet? Here's how you use it: simply run godebug {build|run|test} instead of go {build|run|test}. We adapted godebug to resemble the go tool as much as possible. Remember to use -instrument if you want to be able to step into packages that are not main.

For example, here is part of the RRDNS Makefile:

ifdef GODEBUG  
    GOPATH="${PWD}" go install github.com/mailgun/godebug
    GOPATH="${PWD}" ./bin/godebug build -instrument "${GODEBUG}" -o bin/rrdns rrdns
    GOPATH="${PWD}" go install rrdns

ifdef GODEBUG  
    GOPATH="${PWD}" go install github.com/mailgun/godebug
    GOPATH="${PWD}" ./bin/godebug test -instrument "${GODEBUG}" rrdns/...
    GOPATH="${PWD}" go test rrdns/...

Debugging is just a make bin/rrdns GODEBUG=rrdns/... away.

This tool is still young, but in my experience, perfectly functional. The UX could use some love if you can spare some time (as you can see above it's pretty spartan), but it should be easy to build on what's there already.

About source rewriting

Before closing, I'd like to say a few words about the technique of source rewriting in general. It powers many different Go tools, like test coverage, fuzzing and, indeed, debugging. It's made possible primarily by Go’s blazing-fast compiles, and it enables amazing cross-platform tools to be built easily.

However, since it's such a handy and powerful pattern, I feel like there should be a standard way to apply it in the context of the build process. After all, all the source rewriting tools need to implement a subset of the following features:

  • Wrap the main function
  • Conditionally rewrite source files
  • Keep global state

Why should every tool have to reinvent all the boilerplate to copy the source files, rewrite the source, make sure stale objects are not used, build the right packages, run the right tests, and interpret the CLI..? Basically, all of godebug/cmd.go. And what about gb, for example?

I think we need a framework for Go source code rewriting tools. (Spoiler, spoiler, ...)

If you’re interested in working on Go servers at scale and developing tools to do it better, remember we’re hiring in London, San Francisco, and Singapore!

  1. I'm sorry.

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