Error stack traces in Go with x/xerror

Go’s last standing major weakness is error handling. A few years ago the list was much longer, with the language missing an adequate package manager, system for pulling static assets into a binary, and generics. But now, the first two have already been addressed with Go Modules in 1.11 and go:embed in 1.16, and generics are expected to be in beta form by Go 1.18’s release in December. Errors are the last major omission.

Go 1.13 brought in some improvements in the form of the fmt-er %w symbol and errors.Is, .As, and .Unwrap, but still outstanding are (1) a way to reasonably get a stack trace at the site where an error is generated, and (2) a way to cut down on if err != nil { ... } boilerplate that pervades all Go code.

Our API’s set up so that if an error is returned back to common API endpoint infrastructure by a specific handler, or if a panic occurs, we push an event to Sentry, where we’ll be emailed about it, and which will act as a convenient triage bucket of potential problems. This works well, but we’ve been a little frustrated by how context-free the errors there are. We get a stack trace, but it’s the stack trace of the line of code where we emit to Sentry, making it only noise. Hopefully someone wrapped an error with a greppable string, because otherwise figuring out where an error was generated involves a lot of guesswork.

This doesn’t seem like something that’ll be addressed in the core language anytime soon, so we implemented a workaround. The x/xerrors package was a nursery for the error additions that eventually landed in 1.13. But unlike 1.13’s errors, xerrors also includes a feature that was not brought in – stack traces. An xerror will capture the call site where it was generated, and the Go packages for services like Sentry are smart enough to recognize an xerror and push up its metadata.

Luckily, migration was very easy, We converted every instance of fmt.Errorf over to xerrors.Errorf in just two commands using Gofmt’s rewrite parameter along with an invocation of Goimports:

gofmt -w -r 'fmt.Errorf -> xerrors.Errorf' .
goimports -w .

Gofmt rewrites the AST and Goimports corrects import paths by removing fmt and adding xerrors as appropriate. One neat thing about the use of Gofmt here instead of find + replace is that because it’s looking at the AST, Gofmt will pick up fmt.Errorf calls that were strangely formatted, e.g. broken up across multiple lines.

We then forbid future uses of fmt.Errorf with golangci-lint and forbidigo:

linters:
  enable:
    - forbidigo

linters-settings:
  forbidigo:
    forbid:
      - '^fmt\.Errorf$'%

Our internal convention for how to do error handling is now:

  • At boundaries between our code and calls out to external packages, make an effort to always wrap the result with xerrors.Errorf. This ensures that we always capture a stack trace at the most proximate location of an error being generated as possible. For example, I’d always wrap an invocation out to sqlc:

    _, err = queries.ClusterInsert(ctx, dbsqlc.ClusterInsertParams{
        ...
    })
    if err != nil {
        return nil, xerrors.Errorf("error inserting cluster: %w", err)
    }
    
  • When handling errors being returned from our own code, it’s okay to wrap errors further with xerrors.Errorf if the extra context is useful, but not imperative.

This end solution isn’t amazing, but it works. We’d of course far prefer if Go itself had a built-in equivalent. Reaching out to ex-colleagues at other companies, although there are roughly equivalent alternatives to what we’re doing with xerrors, no one seems to have anything all that much better.

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Error stack traces in Go with x/xerror

Published
August 22, 2021

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