gopkg.in/errgo.v2 v2.1.0: errors.Note

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 gopkg.in/errgo.v2/errors.Note in pkg:golang/gopkg.in/errgo.v2@v2.1.0
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/gopkg.in/errgo.v2@v2.1.0
Demonstrate these symbols/APIs:
  - gopkg.in/errgo.v2/errors.Note

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:d12e755679cf64821ea333e88f197a5dc0d16e5d489c5ca2a8dfe4d7de0793e3","contract":["Note returns nil when given a nil error","Note wraps an error with a prefix message","Note masks the underlying cause when shouldPreserveCause is nil or returns false","Note preserves the underlying cause when shouldPreserveCause returns true"],"goal":"verify gopkg.in/errgo.v2/errors.Note in pkg:golang/gopkg.in/errgo.v2@v2.1.0","kind":"HOW","packages":["pkg:golang/gopkg.in/errgo.v2@v2.1.0"],"schemaVersion":1,"symbols":["gopkg.in/errgo.v2/errors.Note"]},"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/gopkg.in/errgo.v2@v2.1.0"],"schemaVersion":1,"subject":"pkg:golang/gopkg.in/errgo.v2@v2.1.0","symbols":["gopkg.in/errgo.v2/errors.Note"],"verifierAdapter":"golang@1"}

module example.com/sample

go 1.22.0

require gopkg.in/errgo.v2 v2.1.0

github.com/kr/pretty v0.1.0 h1:L/CwN0zerZDmRFUapSPitk6f+Q3+0za1rQkzVuMiMFI=
github.com/kr/pretty v0.1.0/go.mod h1:dAy3ld7l9f0ibDNOQOHHMYYIIbhfbHSm3C4ZsoJORNo=
github.com/kr/pty v1.1.1/go.mod h1:pFQYn66WHrOpPYNljwOMqo10TkYh1fy3cYio2l3bCsQ=
github.com/kr/text v0.1.0 h1:45sCR5RtlFHMR4UwH9sdQ5TC8v0qDQCHnXt+kaKSTVE=
github.com/kr/text v0.1.0/go.mod h1:4Jbv+DJW3UT/LiOwJeYQe1efqtUx/iVham/4vfdArNI=
gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127 h1:qIbj1fsPNlZgppZ+VLlY7N33q108Sa+fhmuc+sWQYwY=
gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
gopkg.in/errgo.v2 v2.1.0 h1:0vLT13EuvQ0hNvakwLuFZ/jYrLp5F3kcWHXdRggjCE8=
gopkg.in/errgo.v2 v2.1.0/go.mod h1:hNsd1EY+bozCKY1Ytp96fpM3vjJbqLJn88ws8XvfDNI=

package sample

import (
	"gopkg.in/errgo.v2/errors"
)

// WrapWithNote wraps an error with an explanatory message, preserving the cause if predicate matches.
func WrapWithNote(err error, shouldPreserveCause func(error) bool, msg string) error {
	return errors.Note(err, shouldPreserveCause, msg)
}

// AnnotateError wraps an error with an additional message without preserving its underlying cause.
func AnnotateError(err error, msg string) error {
	return errors.Note(err, nil, msg)
}

// PreserveCause wraps an error with an additional message while preserving a specific target cause.
func PreserveCause(err, targetCause error, msg string) error {
	return errors.Note(err, errors.Is(targetCause), msg)
}

{
  "schemaVersion": 1,
  "goal": "verify gopkg.in/errgo.v2/errors.Note in pkg:golang/gopkg.in/errgo.v2@v2.1.0",
  "kind": "HOW",
  "packages": [
    "pkg:golang/gopkg.in/errgo.v2@v2.1.0"
  ],
  "symbols": [
    "gopkg.in/errgo.v2/errors.Note"
  ]
}

package main

import (
	"fmt"
	"os"

	"example.com/sample"
	"gopkg.in/errgo.v2/errors"
)

func main() {
	// 1. Note returns nil when given a nil error
	if err := errors.Note(nil, errors.Any, "some message"); err != nil {
		fmt.Fprintf(os.Stderr, "expected nil error for nil input, got: %v\n", err)
		os.Exit(1)
	}

	// 2. Note wraps an error with a prefix message
	baseErr := errors.New("base failure")
	wrapped := sample.AnnotateError(baseErr, "step failed")
	if wrapped == nil {
		fmt.Fprintf(os.Stderr, "expected non-nil wrapped error\n")
		os.Exit(1)
	}
	expectedMsg := "step failed: base failure"
	if wrapped.Error() != expectedMsg {
		fmt.Fprintf(os.Stderr, "expected message %q, got %q\n", expectedMsg, wrapped.Error())
		os.Exit(1)
	}

	// 3. Note masks the underlying cause when predicate is nil
	// When cause is masked, errors.Cause(wrapped) returns wrapped itself
	if cause := errors.Cause(wrapped); cause != wrapped {
		fmt.Fprintf(os.Stderr, "expected masked cause to equal wrapped error, got: %v\n", cause)
		os.Exit(1)
	}

	// 4. Note preserves the underlying cause when shouldPreserveCause returns true
	specificCause := errors.New("permission denied")
	annotated := sample.PreserveCause(specificCause, specificCause, "failed operation")
	if annotated == nil {
		fmt.Fprintf(os.Stderr, "expected non-nil annotated error\n")
		os.Exit(1)
	}
	if cause := errors.Cause(annotated); cause != specificCause {
		fmt.Fprintf(os.Stderr, "expected preserved cause %v, got %v\n", specificCause, cause)
		os.Exit(1)
	}

	// 5. Note masks cause when predicate returns false
	otherCause := errors.New("different error")
	notPreserved := sample.WrapWithNote(specificCause, errors.Is(otherCause), "not preserved")
	if cause := errors.Cause(notPreserved); cause == specificCause {
		fmt.Fprintf(os.Stderr, "expected cause NOT to be preserved, got %v\n", cause)
		os.Exit(1)
	}
}