How to properly debug a binary generated by `go test -c` using GDB?

The go test command has support for the -c flag, described as follows:

-c  Compile the test binary to pkg.test but do not run it.
    (Where pkg is the last element of the package's import path.)

As far as I understand, generating a binary like this is the way to run it interactively using GDB. However, since the test binary is created by combining the source and test files temporarily in some /tmp/ directory, this is what happens when I run list in gdb:

Loading Go Runtime support.
(gdb) list
42      github.com/<username>/<project>/_test/_testmain.go: No such file or directory.

This means I cannot happily inspect the Go source code in GDB like I'm used to. I know it is possible to force the temporary directory to stay by passing the -work flag to the go test command, but then it is still a huge hassle since the binary is not created in that directory and such. I was wondering if anyone found a clean solution to this problem.

Go 1.5 has been released, and there is still no officially sanctioned Go debugger. I haven't had much success using GDB for effectively debugging Go programs or test binaries. However, I have had success using Delve, a non-official debugger that is still undergoing development: https://github.com/derekparker/delve

To run your test code in the debugger, simply install delve:

go get -u github.com/derekparker/delve/cmd/dlv

... and then start the tests in the debugger from within your workspace:

dlv test

From the debugger prompt, you can single-step, set breakpoints, etc.

Give it a whirl!

Unfortunately, this appears to be a known issue that's not going to be fixed. See this discussion:

https://groups.google.com/forum/#!topic/golang-nuts/nIA09gp3eNU

> I've seen two solutions to this problem.
>
> 1) create a .gdbinit file with a set substitute-path command to
> redirect gdb to the actual location of the source. This file could be
> generated by the go tool but you'd risk overwriting someone's custom
> .gdbinit file and would tie the go tool to gdb which seems like a bad
> idea.
>
> 2) Replace the source file paths in the executable (which are pointing
> to /tmp/...) with the location they reside on disk. This is
> straightforward if the real path is shorter then the /tmp/... path.
> This would likely require additional support from the compiler /
> linker to make this solution more generic.

It spawned this issue on the Go Google Code issue tracker, to which the decision ended up being:

https://code.google.com/p/go/issues/detail?id=2881

> This is annoying, but it is the least of many annoying possibilities.
> As a rule, the go tool should not be scribbling in the source
> directories, which might not even be writable, and it shouldn't be
> leaving files elsewhere after it exits. There is next to nothing
> interesting in _testmain.go. People testing with gdb can break on
> testing.Main instead.
>
> Russ
> Status: Unfortunate

So, in short, it sucks, and while you can work around it and GDB a test executable, the development team is unlikely to make it as easy as it could be for you.

I'm still new to the golang game but for what it's worth basic debugging seems to work.

The list command you're trying to work can be used so long as you're already at a breakpoint somewhere in your code. For example:

(gdb) b aws.go:54
Breakpoint 1 at 0x61841: file /Users/mat/gocode/src/github.com/stellar/deliverator/aws/aws.go, line 54.
(gdb) r
Starting program: /Users/mat/gocode/src/github.com/stellar/deliverator/aws/aws.test
[snip: some arnings about BinaryCache]

Breakpoint 1, github.com/stellar/deliverator/aws.imageIsNewer (latest=0xc2081fe2d0, ami=0xc2081fe3c0, ~r2=false)
    at /Users/mat/gocode/src/github.com/stellar/deliverator/aws/aws.go:54
54	    layout := "2006-01-02T15:04:05.000Z"
(gdb) list
49	func imageIsNewer(latest *ec2.Image, ami *ec2.Image) bool {
50	    if latest == nil {
51	        return true
52	    }
53
54	    layout := "2006-01-02T15:04:05.000Z"
55
56	    amiCreationTime, amiErr := time.Parse(layout, *ami.CreationDate)
57	    if amiErr != nil {
58	        panic(amiErr)

This is just after running the following in the aws subdir of my project:

go test -c
gdb aws.test

As an additional caveat, it does seem very selective about where breakpoints can be placed. Seems like it has to be an expression but that conclusion is only via experimentation.

If you're willing to use tools besides GDB, check out godebug. To use it, first install with:

go get github.com/mailgun/godebug

Next, insert a breakpoint somewhere by adding the following statement to your code:

_ = "breakpoint"

Now run your tests with the godebug test command.

godebug test

It supports many of the parameters from the go test command.

> -test.bench string
> regular expression per path component to select benchmarks to run
> -test.benchmem
> print memory allocations for benchmarks
> -test.benchtime duration
> approximate run time for each benchmark (default 1s)
> -test.blockprofile string
> write a goroutine blocking profile to the named file after execution
> -test.blockprofilerate int
> if >= 0, calls runtime.SetBlockProfileRate() (default 1)
> -test.count n
> run tests and benchmarks n times (default 1)
> -test.coverprofile string
> write a coverage profile to the named file after execution
> -test.cpu string
> comma-separated list of number of CPUs to use for each test
> -test.cpuprofile string
> write a cpu profile to the named file during execution
> -test.memprofile string
> write a memory profile to the named file after execution
> -test.memprofilerate int
> if >=0, sets runtime.MemProfileRate
> -test.outputdir string
> directory in which to write profiles
> -test.parallel int
> maximum test parallelism (default 4)
> -test.run string
> regular expression to select tests and examples to run
> -test.short
> run smaller test suite to save time
> -test.timeout duration
> if positive, sets an aggregate time limit for all tests
> -test.trace string
> write an execution trace to the named file after execution
> -test.v
> verbose: print additional output