DEBUGGING.md

Debugging Tips

Golang debugging caveat

It is straightforward to run a Golang debugger on the lumogon program (e.g., by directing the debugger to start at the main function in lumogon.go and proceeding from there). However, once the lumogon program hands off execution to code running in a container in the Docker infrastructure under examination the debugger does not follow the execution inside the container.

Debugging with Visual Studio Code

If you use Visual Studio Code as your development environment, here is a sample launch.json file to use when running lumogon.go under the debugger:

{
    "version": "0.2.0",
    "configurations": [
        {
            "name": "Launch",
            "type": "go",
            "request": "launch",
            "mode": "debug",
            "remotePath": "",
            "port": 2345,
            "host": "127.0.0.1",
            "program": "${fileDirname}",
            "env": {},
            "args": ["container", "--all", "--debug", "--keep-harvesters"],
            "showLog": true
        }
    ]
}

Dumping Golang data structures

The go-spew Golang library implements a pretty-printer that can recursively and verbosely describe the contents of Go data structures. Here is an example of a helper method which wraps spew and provides tagged log output (including the file+line location of the calling function) for dumping an arbitrary data structure:

glide.yaml:

package: github.com/davecgh/go-spew/spew

debug.go:

import "github.com/davecgh/go-spew/spew"

// LogDump outputs a delimited log section with the message string and a structured dump of the provided data object
func LogDump(message string, data interface{}) {
    logging.Debug("\n\n====BEGIN======================================")
    _, file, line, _ := runtime.Caller(1)
    logging.Debug(fmt.Sprintf("Debug - %s @ %s:%d:\n", message, file, line), spew.Sdump(data))
    logging.Debug("====END=======================================\n\n")
}

(client code) dpkg.go:

import "github.com/puppetlabs/transparent-containers/cli/debug"

// ...

        for scanner.Scan() {
            txt := strings.Replace(scanner.Text(), "'", "", 2)
            if txt != "" {
                debug.LogDump("data", data)
                data = append(data, txt)
            }
        }

Sample output:

====BEGIN======================================
[lumogon] 2017/04/14 22:24:40.573682 Debug - data @ /Users/rick/go/src/github.com/puppetlabs/transparent-containers/cli/capability/dpkg.go:49:
%!(EXTRA string=([]string) (len=8 cap=8) {
 (string) (len=12) "acl,2.2.52-2",
 (string) (len=18) "adduser,3.113+nmu3",
 (string) (len=13) "apt,1.0.9.8.4",
 (string) (len=19) "base-files,8+deb8u7",
 (string) (len=18) "base-passwd,3.5.37",
 (string) (len=18) "bash,4.3-11+deb8u1",
 (string) (len=19) "bsdutils,1:2.25.2-6",
 (string) (len=16) "bzip2,1.0.6-7+b3"
}
)
[lumogon] 2017/04/14 22:24:40.573698 ====END=======================================