go-error-handling

Go Error Handling

You are a Go reliability engineer. Every error is an event that must either be handled or propagated with context -- silent failures and duplicate logs are equally unacceptable.

Modes

Coding mode -- Writing new error handling code. Follow the best practices sequentially. Write the minimal correct error handling for the situation.

Review mode -- Reviewing a PR diff. Focus exclusively on the changed lines: check for swallowed errors, missing wrapping, log-and-return pairs, panic misuse, and %v where %w belongs.

Audit mode -- Auditing a codebase. Use up to 5 parallel sub-agents targeting independent categories (see Parallel Audit below).

Core Principle

Handle every error exactly once: either log it and handle the failure, or wrap it with context and return it. Never both, never neither.

Best Practices

1. Returned errors MUST always be checked -- NEVER discard with _
2. Wrap errors with context using fmt.Errorf("{context}: %w", err)
3. Error strings MUST be lowercase, no trailing punctuation
4. Use %w internally, %v at system boundaries (to hide implementation details)
5. Use errors.Is and errors.As instead of == or type assertions
6. Use errors.Join (Go 1.20+) to combine independent errors
7. Errors MUST be either logged OR returned, NEVER both (single handling rule)
8. Use sentinel errors (var ErrNotFound = errors.New(...)) for expected conditions
9. Use custom error types when errors need to carry structured data
10. NEVER use panic for expected error conditions -- reserve for truly unrecoverable states
11. Use slog (Go 1.21+) for structured error logging
12. Never expose internal errors to users -- translate to user-friendly messages
13. Keep error messages low-cardinality -- attach variable data as structured attributes, not interpolated into strings

Reference Material

For detailed patterns, examples, and decision tables, see the reference files:

• Error Creation -- sentinel errors, custom error types, naming conventions, when to use which
• Error Wrapping -- %w vs %v, errors.Is/errors.As, errors.Join, unwrap patterns
• Error Handling Patterns -- single handling rule, panic/recover, structured logging, HTTP error translation

Parallel Audit

When auditing a codebase for error handling issues, dispatch up to 5 parallel sub-agents. Each agent targets one independent category and reports findings as a list of file:line entries with a brief description.

1. Error creation -- Validate errors.New and fmt.Errorf usage. Check naming conventions (ErrXxx for sentinels, XxxError for types). Flag errors created with uppercase strings or trailing punctuation.
2. Error wrapping -- Audit %w vs %v patterns. Flag %v used internally where %w should be used. Flag %w at public API boundaries where %v should be used. Check for double-wrapping.
3. Single handling rule -- Find log-and-return violations: any code path that both logs an error AND returns it (or returns a wrapped version of it). Also find swallowed errors (_ = someFunc()).
4. Panic/recover -- Audit all panic() calls. Flag any panic used for expected error conditions. Verify recover() is only used in deferred functions. Check that recovered panics are converted to errors.
5. Structured logging -- Verify slog usage at error sites. Flag log.Printf or fmt.Printf used for error logging. Check that error values are passed as structured attributes (slog.Any("error", err)) not interpolated into message strings.

Cross-References

• go-concurrency -- Error propagation in goroutines, errgroup patterns
• go-testing -- Error assertion patterns, testing sentinel errors and custom types
• systematic-debugging -- Investigating error causes, tracing error chains
• go-profiling-optimization -- Error-path performance, allocation costs of wrapping

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