valyala/fasthttp · error

prefork: command producer: %w

Error message

prefork: command producer: %w

What it means

Returned by Prefork.doCommand when the user-supplied CommandProducer function returns an error. The library delegates child-process construction to this callback and surfaces any failure from it wrapped with this prefix. It is a wrapper around user code, so the root cause is inside the custom CommandProducer.

Source

Thrown at prefork/prefork.go:353

func childEnv() []string {
	src := os.Environ()
	out := make([]string, 0, len(src)+1)
	prefix := preforkChildEnvVariable + "="
	for _, kv := range src {
		if len(kv) >= len(prefix) && kv[:len(prefix)] == prefix {
			continue
		}
		out = append(out, kv)
	}
	out = append(out, preforkChildEnvVariable+"="+preforkChildEnvValue)
	return out
}

func (p *Prefork) doCommand() (*exec.Cmd, error) {
	if p.CommandProducer != nil {
		cmd, err := p.CommandProducer(p.files)
		if err != nil {
			return nil, fmt.Errorf("prefork: command producer: %w", err)
		}
		if cmd == nil {
			return nil, ErrCommandProducerNilCmd
		}
		if cmd.Process == nil {
			return nil, ErrCommandProducerNotStarted
		}
		return cmd, nil
	}

	executable, err := os.Executable()
	if err != nil {
		return nil, fmt.Errorf("prefork: resolve executable: %w", err)
	}

	args := append([]string{executable}, os.Args[1:]...)

	cmd := &exec.Cmd{

View on GitHub (pinned to c96f600972)

Solutions

1. Inspect the wrapped (%w) cause to find the failure inside your CommandProducer.
2. Make CommandProducer defensive: validate inputs (files, env) and return clear errors or fall back to a default command.
3. If the producer cannot produce a command, consider removing it so doCommand uses the default executable path.
4. Add logging in the producer to capture its internal error before returning it.

Example fix

// before
p.CommandProducer = func(files []*os.File) (*exec.Cmd, error) { return buildCmd(missingCfg) }
// after
p.CommandProducer = func(files []*os.File) (*exec.Cmd, error) {
    if err := validateCfg(); err != nil { return nil, fmt.Errorf("cfg: %w", err) }
    return buildCmd()
}

// Validate the producer before assigning:
p.CommandProducer = func(files []*os.File) (*exec.Cmd, error) {
    if len(files) == 0 { return nil, errors.New("no listener files") }
    return buildCmd(files)
}

p.CommandProducer = func(files []*os.File) (*exec.Cmd, error) {
    cmd, err := buildCmd(files)
    if err != nil {
        log.Printf("command producer failed: %v", err)
        return nil, err // wrapped by prefork: command producer: %w
    }
    return cmd, nil
}

When it happens

Trigger: Prefork.CommandProducer is set and the callback returns a non-nil error when invoked with the listener files (e.g. it fails to build exec.Cmd, validate files, or resolve its own resources).

Common situations: Custom CommandProducer logic that reads config/env that is missing in child-spawn contexts; failing exec.LookPath inside the producer; transient resource errors while constructing the command.

Related errors

• prefork: listen tcp %s: %w
• prefork: dup listener fd: %w
• prefork: resolve executable: %w
• prefork: start child %q: %w
• must implement readat

AI-assisted analysis of valyala/fasthttp@c96f600972 (2026-08-31). Data as JSON: /api/errors/953bfde660263b89. Report an issue: GitHub.