Reviewing Go

A reviewer's leverage is what the toolchain cannot say. Every check below describes a defect that survives the compiler, go vet, and a default golangci-lint run (errcheck, govet, ineffassign, staticcheck, unused).

The snippets are fragments cut down to the defect, not runnable programs.

Already covered — do not spend review on it

If the project has no linter configured, that is one review comment about CI — not thirty comments doing the linter's job by hand.

The table above dates; the tools do not. Verify a default set with mcp__documentation__fetch_docs on technology go-quality — topics go-vet, staticcheck-checks, golangci-lint-linters. Those entries point at the indexes the tool authors maintain, so they follow the version the project actually pins rather than the one this skill was written against.

The checks that earn their place

A typed nil in an interface is not a nil interface

type MyError struct{ msg string }

func (e *MyError) Error() string { return e.msg }

func find() *MyError { return nil }   // a typed nil

var err error = find()   // err != nil: the interface holds (*MyError, nil)
if err != nil { /* taken */ }

When you see it: a function returning a concrete pointer type whose result is assigned to error or any interface; an interface-typed struct field assigned from a typed nil.

Ask: does any path put a typed nil into an interface? Return the interface type, or return a literal nil on the success path.

Not reported by anything in the default set. (nilness catches a narrow subset, but it is an opt-in x/tools pass, not part of go vet.)

Errors compared instead of matched

if err == sql.ErrNoRows { }          // breaks as soon as anything wraps it
if errors.Is(err, sql.ErrNoRows) { } // survives %w

When you see it: == against a sentinel error, or a type assertion err.(*MyError) where errors.As belongs.

Ask: does any layer between the source and this comparison wrap with %w? When a wrap is added later, == silently stops matching: the branch quietly stops being taken and nothing fails loudly. (errorlint would catch this, but it is not in the default set.)

The iteration error is never checked

for rows.Next() {
    // scan
}
return items, nil   // a network error ended the loop exactly like a full result set

When you see it: for rows.Next(), for scanner.Scan() — any iterator with a separate Err() — and no rows.Err() / scanner.Err() after the loop.

Ask: how does this function distinguish "read everything" from "stopped early"? It cannot, and the caller gets a truncated result with a nil error.

No linter can report this: errcheck only inspects calls that are written, and here the call is missing.

A mutation applied to the range copy

for _, cfg := range configs {
    cfg.Enabled = true   // writes to a copy; discarded at the next iteration
}

When you see it: an assignment to a field of the value variable of a range over a slice or map of structs.

Ask: was this meant to mutate the collection? Then it needs configs[i] or a slice of pointers. Worth checking deliberately since Go 1.22: people now assume "the loop variable problem" is fixed, and this is the part that never changed.

A lock held across something that can block

mu.Lock()
if err := callService(ctx); err != nil {   // I/O inside the critical section
    return err                             // and an early return with no Unlock
}
mu.Unlock()

When you see it: Lock() without an immediate defer Unlock(), or a critical section containing I/O, a channel send, or a second lock.

Ask: does every path release it, and what is the worst case for a caller waiting on this mutex while that call hangs? copylocks covers copying a lock, never how long one is held.

A goroutine with no way to stop

go func() {
    for range ticker.C { poll() }   // lives until the process exits
}()

When you see it: go func containing a loop, with no ctx.Done(), no quit channel, and no way for the caller to observe completion.

Ask: who stops this, and how does the caller learn that it stopped? A leaked goroutine holding a connection or a lock is invisible until the process is.

err shadowed by an inner declaration

var err error
if err := do(); err != nil {   // inner err shadows the outer one
    log.Print(err)
}
return err                      // always nil

When you see it: := binding err inside an if, for or block where an outer err is the one returned.

Ask: which err does the return statement see? go vet's shadow analyzer finds these, but it is not in the default set and golangci-lint requires govet: enable: shadow, so in most repositories nothing reports it.

defer in a loop over an unbounded input

for _, path := range paths {
    f, err := os.Open(path)
    if err != nil { return err }
    defer f.Close()   // runs at function exit, not iteration exit
}

When you see it: a defer whose enclosing block is a loop body — over a slice, a map, or a paginated source. (SA9001 already covers the channel-range form, so that one is not yours.)

Ask: is the input bounded? Over a large or unbounded one this is a file descriptor leak. The fix is a closure per iteration, or an explicit Close() on both paths.

A sub-slice handed to a second owner

b := a[:1]
b = append(b, x)   // capacity permitting, this overwrites a[1]

When you see it: a slice expression (s[:n], s[i:j]) stored somewhere else or returned, and later appended to by either owner.

Ask: do two owners now share one backing array? The aliasing stays invisible until one of them appends within capacity and silently rewrites the other's data.

context.Background() below the entry point

When you see it: context.Background() or context.TODO() in a function that — or whose immediate caller — already has a ctx context.Context parameter.

Ask: why is the in-scope context not threaded through? Starting a fresh one mid-stack opts that call out of every deadline and cancellation the caller set up. (contextcheck would find it; not in the default set.)

Version-dependent — read go.mod before commenting

Loop variable capture. Before Go 1.22 the loop variable was shared across iterations — for range loops and three-clause for loops — so a closure capturing it saw the final value.

for _, v := range items {
    go func() { use(v) }()   // pre-1.22: every goroutine sees the last v
}

Since Go 1.22 each iteration gets its own variable and this is no longer a defect. Three things decide whether it applies:

• The module's go directive, not the installed toolchain, selects the semantics; a single file can override it with a //go:build go1.21 constraint.
• On pre-1.22 modules the common shape above is already reported by loopclosure, which is in go vet's default set — so the residual value here is narrow.
• It still bites on any version when the variable is declared outside the loop (v := …; for v = range …): that one is still shared, and it is the case worth looking for now.

Timers from time.After in a loop. On modules with go >= 1.23 an unreferenced timer becomes collectable immediately, so the old "leaks until it fires" framing is wrong. What remains is a timer allocated per iteration:

for {
    select {
    case v := <-ch:  handle(v)
    case <-time.After(time.Second):
    }
}

On a hot channel that is allocation and CPU per message, worth a comment as cost — and on a pre-1.23 module, still retention until each timer fires.

What to say

Anchor the comment to the line, name the condition that triggers the defect, and say what breaks: "this returns a typed nil into an error, so if err != nil is always true for callers" beats "avoid typed nils". If you cannot state the input that fails, it is a preference, not a defect.