golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898: Unwrap

Clean-room public code sample — generation instructions

Write a brand-new, minimal, self-contained code sample in this clean-room directory.
Do not copy, paraphrase, or reference any existing project source. Work only from this spec.

A csx.json manifest scaffold already exists. Do not recreate it from memory. Preserve its case.goal, packages and symbols; fill its empty case.contract with exact assertions and correct its environment, commands and verifierAdapter for the files you generate.

Goal: verify golang.org/x/xerrors.Unwrap in pkg:golang/golang.org/x/xerrors@v0.0.0-20191011141410-1b5146add898
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/golang.org/x/xerrors@v0.0.0-20191011141410-1b5146add898
Demonstrate these symbols/APIs:
  - golang.org/x/xerrors.Unwrap

Rules:
  - One focused purpose; the smallest project that proves the goal.
  - Include a contract test (test/contract.*) that runs OFFLINE and exits 0 exactly when the goal behavior works.
  - Pin every dependency with a lockfile so resolution is reproducible.
  - No secrets, credentials, or tokens. No real URLs (only example.com or localhost). No absolute paths.
  - No personal names, emails, company names, or project identifiers of any kind.
  - No binaries and no generated output (node_modules, dist, target, venv, .git, .env).
  - Keep it under 200 files and 256KB packed.

{"case":{"caseId":"case:sha256:e4f039f09b2490e9f2c009865b10c5b5c5879bf92ed14cc6a7f068212d983669","contract":["xerrors.Unwrap returns nil when called with nil or an error that does not implement Wrapper","xerrors.Unwrap returns the inner wrapped error when called on an error created with xerrors.Errorf using the ': %w' verb","xerrors.Unwrap returns the result of Unwrap() when called on a custom error type implementing Unwrap() error"],"goal":"verify golang.org/x/xerrors.Unwrap in pkg:golang/golang.org/x/xerrors@v0.0.0-20191011141410-1b5146add898","kind":"HOW","packages":["pkg:golang/golang.org/x/xerrors@v0.0.0-20191011141410-1b5146add898"],"schemaVersion":1,"symbols":["golang.org/x/xerrors.Unwrap"]},"contractCommand":["go","run","./test"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"golang","libc":"glibc","libcVersion":"2.39","os":"linux","osVersionBucket":"24","packageManager":"go","schemaVersion":1},"license":"MIT-0","packages":["pkg:golang/golang.org/x/xerrors@v0.0.0-20191011141410-1b5146add898"],"schemaVersion":1,"subject":"pkg:golang/golang.org/x/xerrors@v0.0.0-20191011141410-1b5146add898","symbols":["golang.org/x/xerrors.Unwrap"],"verifierAdapter":"golang@1"}

module sample

go 1.21

require golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898

golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898 h1:/atklqdjdhuosWIl6AIbOeHJjicWYPqR9bpxqxYG2pA=
golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=

{
  "schemaVersion": 1,
  "goal": "verify golang.org/x/xerrors.Unwrap in pkg:golang/golang.org/x/xerrors@v0.0.0-20191011141410-1b5146add898",
  "kind": "HOW",
  "packages": [
    "pkg:golang/golang.org/x/xerrors@v0.0.0-20191011141410-1b5146add898"
  ],
  "symbols": [
    "golang.org/x/xerrors.Unwrap"
  ]
}

package main

import (
	"fmt"
	"os"

	"golang.org/x/xerrors"
)

type customWrapper struct {
	msg string
	err error
}

func (c *customWrapper) Error() string {
	return c.msg
}

func (c *customWrapper) Unwrap() error {
	return c.err
}

func main() {
	base := xerrors.New("root failure")

	// Unwrap on a plain error without Wrapper returns nil.
	if got := xerrors.Unwrap(base); got != nil {
		fmt.Fprintf(os.Stderr, "expected nil for plain error, got %v\n", got)
		os.Exit(1)
	}

	// Unwrap on nil returns nil.
	if got := xerrors.Unwrap(nil); got != nil {
		fmt.Fprintf(os.Stderr, "expected nil for nil error, got %v\n", got)
		os.Exit(1)
	}

	// Unwrap on xerrors.Errorf with : %w returns the wrapped error.
	wrapped := xerrors.Errorf("outer context: %w", base)
	if got := xerrors.Unwrap(wrapped); got != base {
		fmt.Fprintf(os.Stderr, "expected base error from Errorf(: %%w), got %v\n", got)
		os.Exit(1)
	}

	// Unwrap on a custom type implementing Unwrap() error returns its inner error.
	custom := &customWrapper{msg: "custom outer", err: wrapped}
	if got := xerrors.Unwrap(custom); got != wrapped {
		fmt.Fprintf(os.Stderr, "expected wrapped error from customWrapper, got %v\n", got)
		os.Exit(1)
	}
	if got := xerrors.Unwrap(xerrors.Unwrap(custom)); got != base {
		fmt.Fprintf(os.Stderr, "expected base error after double Unwrap, got %v\n", got)
		os.Exit(1)
	}
}