valyala/fasthttp · error

prefork: resolve executable: %w

Error message

prefork: resolve executable: %w

What it means

Returned by Prefork.doCommand when os.Executable() fails to resolve the path of the current executable, which prefork needs to re-exec itself as the child process. This indicates the running binary's path could not be determined by the OS/runtime.

Source

Thrown at prefork/prefork.go:366

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{
		Path:       executable,
		Args:       args,
		Stdout:     os.Stdout,
		Stderr:     os.Stderr,
		Env:        childEnv(),
		ExtraFiles: p.files,
	}
	if err = cmd.Start(); err != nil {
		return nil, fmt.Errorf("prefork: start child %q: %w", executable, err)
	}
	return cmd, nil
}

View on GitHub (pinned to c96f600972)

Solutions

1. Ensure /proc is mounted in the container (mount -t proc proc /proc).
2. Do not delete/overwrite the running binary; deploy atomically (write new file, rename, restart).
3. Run prefork in a normal OS environment rather than a heavily restricted chroot.
4. If unavoidable, set a custom CommandProducer that supplies the executable path directly.

Example fix

// before
# Dockerfile FROM scratch without /proc handling, binary deleted mid-run
// after
# keep the binary on disk; use multi-stage copy and full restarts:
COPY app /app/app
ENTRYPOINT ["/app/app"]

// Pre-flight: ensure os.Executable works in this environment
if _, err := os.Executable(); err != nil {
    log.Fatalf("cannot resolve executable path: %v", err)
}

if err := p.Listen(addr); err != nil {
    if strings.Contains(err.Error(), "resolve executable") {
        log.Fatalf("environment cannot resolve /proc/self/exe: %v", err)
    }
    log.Fatal(err)
}

When it happens

Trigger: Calling prefork in an environment where /proc/self/exe is unavailable (Linux procfs not mounted), the binary was deleted after start, or the runtime cannot read /proc (e.g. minimal containers, some chroots).

Common situations: Distroless/minimal containers without /proc mounted properly; binary replaced/deleted while running (deleted-file /proc/self/exe pointing to '(deleted)'); running under exotic sandboxes (gVisor misconfigurations).

Related errors

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

AI-assisted analysis of valyala/fasthttp@c96f600972 (2026-08-31). Data as JSON: /api/errors/75c851d05cc60a73. Report an issue: GitHub.