stacktrace

Stacktrace

Look at Palantir, such a Java shop. I can't believe they want stack traces in their Go code.

Why would anyone want stack traces in Go code?

This is difficult to debug:

Inverse tachyon pulse failed

This gives the full story and is easier to debug:

Failed to register for villain discovery
 --- at github.com/palantir/shield/agent/discovery.go:265 (ShieldAgent.reallyRegister) ---
 --- at github.com/palantir/shield/connector/impl.go:89 (Connector.Register) ---
Caused by: Failed to load S.H.I.E.L.D. config from /opt/shield/conf/shield.yaml
 --- at github.com/palantir/shield/connector/config.go:44 (withShieldConfig) ---
Caused by: There isn't enough time (4 picoseconds required)
 --- at github.com/palantir/shield/axiom/pseudo/resource.go:46 (PseudoResource.Adjust) ---
 --- at github.com/palantir/shield/axiom/pseudo/growth.go:110 (reciprocatingPseudo.growDown) ---
 --- at github.com/palantir/shield/axiom/pseudo/growth.go:121 (reciprocatingPseudo.verify) ---
Caused by: Inverse tachyon pulse failed
 --- at github.com/palantir/shield/metaphysic/tachyon.go:72 (TryPulse) ---

Note that stack traces are not designed to be user-visible. We have found them to be valuable in log files of server applications. Nobody wants to see these in CLI output or a web interface or a return value from library code.

Intent

The intent is not that we capture the exact state of the stack when an error happens, including every function call. For a library that does that, see github.com/go-errors/errors. The intent here is to attach relevant contextual information (messages, variables) at strategic places along the call stack, keeping stack traces compact and maximally useful.

Example Usage

func WriteAll(baseDir string, entities []Entity) error {
    err := os.MkdirAll(baseDir, 0755)
    if err != nil {
        return stacktrace.Propagate(err, "Failed to create base directory")
    }
    for _, ent := range entities {
        path := filepath.Join(baseDir, fileNameForEntity(ent))
        err = Write(path, ent)
        if err != nil {
            return stacktrace.Propagate(err, "Failed to write %v to %s", ent, path)
        }
    }
    return nil
}

Functions

stacktrace.Propagate(cause error, msg string, vals ...interface{}) error

Propagate wraps an error to include line number information. This is going to be your most common stacktrace call.

As in all of these functions, the msg and vals work like fmt.Errorf.

The message passed to Propagate should describe the action that failed, resulting in cause. The canonical call looks like this:

result, err := process(arg)
if err != nil {
    return nil, stacktrace.Propagate(err, "Failed to process %v", arg)
}

To write the message, ask yourself "what does this call do?" What does process(arg) do? It processes ${arg}, so the message is that we failed to process ${arg}.

Pay attention that the message is not redundant with the one in err. In the WriteAll example above, any error from os.MkdirAll will already contain the path it failed to create, so it would be redundant to include it again in our message. However, the error from os.MkdirAll will not identify that path as corresponding to the "base directory" so we propagate with that information.

If it is not possible to add any useful contextual information beyond what is already included in an error, msg can be an empty string:

func Something() error {
    mutex.Lock()
    defer mutex.Unlock()

    err := reallySomething()
    return stacktrace.Propagate(err, "")
}

The purpose of "" as opposed to a separate function is to make you feel a little guilty every time you do this.

This example also illustrates the behavior of Propagate when cause is nil – it returns nil as well. There is no need to check if err != nil.

stacktrace.NewError(msg string, vals ...interface{}) error

NewError is a drop-in replacement for fmt.Errorf that includes line number information. The canonical call looks like this:

if !IsOkay(arg) {
    return stacktrace.NewError("Expected %v to be okay", arg)
}

Error Codes

Occasionally it can be useful to propagate an error code while unwinding the stack. For example, a RESTful API may use the error code to set the HTTP status code.

The type stacktrace.ErrorCode is a typedef for uint32. You name the set of error codes relevant to your application.

const (
    EcodeManifestNotFound = stacktrace.ErrorCode(iota)
    EcodeBadInput
    EcodeTimeout
)

The special value stacktrace.NoCode is equal to math.MaxUint32, so avoid using that. NoCode is the error code of errors with no code explicitly attached.

An ordinary stacktrace.Propagate preserves the error code of an error.

stacktrace.PropagateWithCode(cause error, code ErrorCode, msg string, vals ...interface{}) error

stacktrace.NewErrorWithCode(code ErrorCode, msg string, vals ...interface{}) error

PropagateWithCode and NewErrorWithCode are analogous to Propagate and NewError but also attach an error code.

_, err := os.Stat(manifestPath)
if os.IsNotExist(err) {
    return stacktrace.PropagateWithCode(err, EcodeManifestNotFound, "")
}

stacktrace.NewMessageWithCode(code ErrorCode, msg string, vals ...interface{}) error

The error code mechanism can be useful by itself even where stack traces with line numbers are not required. NewMessageWithCode returns an error that prints just like fmt.Errorf with no line number, but including a code.

ttl := req.URL.Query().Get("ttl")
if ttl == "" {
    return 0, stacktrace.NewMessageWithCode(EcodeBadInput, "Missing ttl query parameter")
}

stacktrace.GetCode(err error) ErrorCode

GetCode extracts the error code from an error.

for i := 0; i < attempts; i++ {
    err := Do()
    if stacktrace.GetCode(err) != EcodeTimeout {
        return err
    }
    // try a few more times
}
return stacktrace.NewError("timed out after %d attempts", attempts)

GetCode returns the special value stacktrace.NoCode if err is nil or if there is no error code attached to err.

License

Stacktrace is released by Palantir Technologies, Inc. under the Apache 2.0 License. See the included LICENSE file for details.

Contributing

We welcome contributions of backward-compatible changes to this library.

• Write your code
• Add tests for new functionality
• Run go test and verify that the tests pass
• Fill out the Individual or Corporate Contributor License Agreement and send it to [email protected]
• Submit a pull request