37 Go interview questions on error handling & panics, with detailed answers and code.
In Go, error is an ordinary interface ( type error interface{ Error() string } ) returned as the last result. Failure paths are explicit in the signature and the control…
errors.New("x") == errors.New("x")a := errors.New("x") b := errors.New("x") fmt.Println(a == b) // false fmt.Println(a.Error() == b.Error()) // true type codeErr struct{ code int } func (e codeErr)…
%w and %v in fmt.Errorf?Both produce the same message text. %w also wraps : the returned error has an Unwrap() error method (or Unwrap() []error when there are several %w verbs, Go 1.20+), so…
errors.Is work internally, and how can a custom type change its matching?errors.Is(err, target) walks the error tree depth-first. At each node it returns true if the node == target (only when target's type is comparable) or if the node has an…
errors.As. Why must the target be a pointer, and what happens if you get it wrong?errors.As(err, target any) bool finds the first error in the tree that is assignable to the type *target points at, stores it in *target , and returns true. A node can…
errors.Join? How do multi-errors interact with errors.Is, errors.As and errors.Unwrap?errors.Join(errs ...error) (Go 1.20) returns an error that wraps every non-nil argument. It returns nil if all arguments are nil. Its message is the messages joined with…
Sentinel ( var ErrNotFound = errors.New(...) ): a fixed value checked with errors.Is . It is simple and cheap, but it can't carry context, and it becomes part of your…
Error() has a pointer or value receiver?A good custom error has structured fields, a message in lower case with no trailing punctuation, an Unwrap method if it wraps a cause, and usually a pointer receiver .…
type MyErr struct{} func (*MyErr) Error() string { return "my err" } func do(fail bool) error { var p *MyErr // nil pointer if fail { p = &MyErr{} } return p // converts…
err == ErrX wrong, and why can comparing errors with == panic?== matches only the outermost error. As soon as someone wraps it with %w , the comparison fails. Use errors.Is , which walks the chain. The exceptions are documented…
Wrap ( %w ) when callers need to inspect the cause and the cause is part of your stable contract. Don't wrap (use %v , or return your own error) when the cause is an…
panic instead of returning an error?Panic when there is a programmer error or an impossible state that the caller can't reasonably handle: Violated invariants, such as an unreachable default in an…
recover)func helper() { if r := recover(); r != nil { fmt.Println("recovered:", r) } } func a() { defer helper(); panic("A") } // helper IS the deferred func func b() { defer…
recover in main catch it? How do you protect background goroutines?No. Panics are per goroutine. A panic unwinds only its own goroutine's stack. If nothing in that goroutine recovers, the whole process crashes, however many recovers sit…
func main() { defer fmt.Println("1") defer func() { r := recover() fmt.Println("recovered", r) }() defer fmt.Println("3") fmt.Println(f()) panic("boom") } func f() (n…
Use a named result and assign it inside a deferred function that calls recover . This is the only way a deferred function can change what the caller receives. func…
panic(nil) and with re-panicking? How did Go 1.21 change this?Before Go 1.21, panic(nil) made recover() return nil , so code that tested if r := recover(); r != nil couldn't tell "no panic" from "panicked with nil" and swallowed…
recover?recover handles panics , both user panics and runtime-error panics (nil dereference, index out of range, nil map write, division by zero, failed type assertion, closing…
panic, os.Exit, log.Fatal and runtime.Goexit with respect to deferred functions.Call Runs defers? Effect panic(v) Yes (current goroutine) Unwinds. Crashes with exit code 2 unless recovered. os.Exit(n) No Exits immediately with code n. Buffers aren't…
errgroup.Goroutines can't return values, so errors have to be passed back explicitly. golang.org/x/sync/errgroup is the standard tool. g.Go runs functions that return error .…
Give each worker its own slot in a pre-sized slice. Each goroutine writes a different element, so there is no data race and no mutex. Then call errors.Join , which skips…
The standard library leaves stacks out of errors on purpose: capturing them costs time, and well-written wrap messages usually give enough context. When you do need one:…
Each error should be handled once. Handling means logging it, recovering from it (retry, fallback, default value), or turning it into a response. The alternative is to…
defer f.Close() potentially a bug when writing files? How do you handle the Close error?For files opened for writing , Close (or the final flush) can be where the error shows up: buffered data, NFS, a full disk. defer f.Close() throws that error away, so…
err)func parse(s string) error { var err error if s != "" { n, err := strconv.Atoi(s) // := declares a NEW err in this block if err == nil { fmt.Println("parsed", n) } }…
fmt.Errorf("op: %w", err) return when err is nil? Why does this matter in helper functions?It returns a non-nil error whose message is op: %!w(<nil>) . fmt.Errorf always returns a new error value, and it doesn't care whether the operand is nil. So an…
context.Canceled, DeadlineExceeded, and context.Cause.ctx.Err() returns nil while the context is active. After it ends, it returns context.Canceled (from an explicit cancel) or context.DeadlineExceeded (from a timeout).…
errors.Is(err, fs.ErrNotExist) preferred over os.IsNotExist(err)?os.IsNotExist , os.IsExist and os.IsPermission were written before error wrapping existed. They unwrap only a fixed set of types ( *PathError , *LinkError ,…
Instead of checking for a concrete type or sentinel, check whether the error implements a small interface that describes a behavior. The producer can then change its…
net/http handle a panic in a handler? What are the pitfalls?http.Server recovers panics in each connection's goroutine. It logs the panic with a stack trace to ErrorLog and closes the connection, so the client gets a broken…
A common senior-level design, inspired by Upspin and many internal Google and Uber codebases, is a single structured error type. It carries a machine-readable Kind (used…
io.Reader error contract, and why is if err != nil { return } wrong there?Read(p) can return n > 0 and a non-nil error in the same call, including io.EOF . The contract says callers should process the n bytes before looking at the error.…
errors.Is/As. What happens if you forget Unwrap?Any type with an Unwrap() error method is part of the chain. Without it, the wrapper is a dead end: errors.Is and errors.As stop at your type, and every caller checking…
Is method gets called)var ErrNotFound = errors.New("not found") type NotFoundError struct{ Name string } func (e *NotFoundError) Error() string { return e.Name + ": not found" } func (e…
func main() { defer func() { fmt.Println("recovered:", recover()) }() defer func() { panic("second") }() panic("first") } Output: recovered: second . The new panic…
sync.Once.Do panics? How do sync.OnceFunc and sync.OnceValue differ?With sync.Once , a panic in f still counts as "done". Do marks the Once as complete, the panic propagates to the first caller, and every later call returns immediately…
defer and panic/recover in hot paths?Sentinel errors are created once at package init. Returning one is just copying an interface value, with no allocation. fmt.Errorf formats a string and allocates the…