github.com/go-kit/log v0.1.0: Prefix

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 github.com/go-kit/log.Prefix in pkg:golang/github.com/go-kit/log@v0.1.0
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/github.com/go-kit/log@v0.1.0
Demonstrate these symbols/APIs:
  - github.com/go-kit/log.Prefix

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:sha256:92ee25e5fa0271ac6bd1c91f5ee91faee64d4fcfbbf1823476dc5ed96f87d1ec","contract":["log.Prefix configures StdlibAdapter to strip prefix from log events when joinPrefixToMsg is false","log.Prefix configures StdlibAdapter to retain prefix in msg key when joinPrefixToMsg is true","log.Prefix extracts stdlib timestamps alongside the configured prefix","log.Prefix extracts caller file location and timestamps alongside prefix","log.Prefix combines with other StdlibAdapterOption configurations such as MessageKey and TimestampKey"],"goal":"verify github.com/go-kit/log.Prefix in pkg:golang/github.com/go-kit/log@v0.1.0","kind":"HOW","packages":["pkg:golang/github.com/go-kit/log@v0.1.0"],"schemaVersion":1,"symbols":["github.com/go-kit/log.Prefix"]},"contractCommand":["go","run","./test"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"golang","language":"go","libc":"glibc","libcVersion":"2.39","os":"linux","osVersionBucket":"24","packageManager":"go","schemaVersion":1},"license":"MIT-0","packages":["pkg:golang/github.com/go-kit/log@v0.1.0"],"schemaVersion":1,"subject":"pkg:golang/github.com/go-kit/log@v0.1.0","symbols":["github.com/go-kit/log.Prefix"],"verifierAdapter":"golang@1"}

module sample

go 1.26.6

require github.com/go-kit/log v0.1.0

require github.com/go-logfmt/logfmt v0.5.0 // indirect

github.com/go-kit/log v0.1.0 h1:DGJh0Sm43HbOeYDNnVZFl8BvcYVvjD5bqYJvp0REbwQ=
github.com/go-kit/log v0.1.0/go.mod h1:zbhenjAZHb184qTLMA9ZjW7ThYL0H2mk7Q6pNt4vbaY=
github.com/go-logfmt/logfmt v0.5.0 h1:TrB8swr/68K7m9CcGut2g3UOihhbcbiMAYiuTXdEih4=
github.com/go-logfmt/logfmt v0.5.0/go.mod h1:wCYkCAKZfumFQihp8CzCvQ3paCTfi41vtzG1KdI/P7A=
github.com/go-stack/stack v1.8.0/go.mod h1:v0f6uXyyMGvRgIKkXu+yp6POWl0qKG85gN/melR3HDY=

{
  "schemaVersion": 1,
  "goal": "verify github.com/go-kit/log.Prefix in pkg:golang/github.com/go-kit/log@v0.1.0",
  "kind": "HOW",
  "packages": [
    "pkg:golang/github.com/go-kit/log@v0.1.0"
  ],
  "symbols": [
    "github.com/go-kit/log.Prefix"
  ]
}

package main

import (
	"bytes"
	"fmt"
	stdlog "log"
	"os"
	"strings"

	"github.com/go-kit/log"
)

func main() {
	if err := runTests(); err != nil {
		fmt.Fprintf(os.Stderr, "Contract test failed: %v\n", err)
		os.Exit(1)
	}
	fmt.Println("All contract tests passed successfully.")
}

func runTests() error {
	// 1. log.Prefix configures StdlibAdapter to strip prefix from log events when joinPrefixToMsg is false
	var buf bytes.Buffer
	logger := log.NewLogfmtLogger(&buf)
	writer := log.NewStdlibAdapter(logger, log.Prefix("MYAPP: ", false))
	std := stdlog.New(writer, "MYAPP: ", 0)
	std.Print("service initialized")
	out := strings.TrimSpace(buf.String())
	if out != `msg="service initialized"` {
		return fmt.Errorf("contract 1 failed: expected %q, got %q", `msg="service initialized"`, out)
	}

	// 2. log.Prefix configures StdlibAdapter to retain prefix in msg key when joinPrefixToMsg is true
	buf.Reset()
	writer = log.NewStdlibAdapter(logger, log.Prefix("MYAPP: ", true))
	std = stdlog.New(writer, "MYAPP: ", 0)
	std.Print("service initialized")
	out = strings.TrimSpace(buf.String())
	if out != `msg="MYAPP: service initialized"` {
		return fmt.Errorf("contract 2 failed: expected %q, got %q", `msg="MYAPP: service initialized"`, out)
	}

	// 3. log.Prefix extracts stdlib timestamps alongside the configured prefix
	buf.Reset()
	writer = log.NewStdlibAdapter(logger, log.Prefix("WORKER: ", false))
	fmt.Fprint(writer, "WORKER: 2026/09/04 10:15:30 batch job completed\n")
	out = strings.TrimSpace(buf.String())
	if out != `ts="2026/09/04 10:15:30" msg="batch job completed"` {
		return fmt.Errorf("contract 3 failed: expected %q, got %q", `ts="2026/09/04 10:15:30" msg="batch job completed"`, out)
	}

	// 4. log.Prefix extracts caller file location and timestamps alongside prefix
	buf.Reset()
	writer = log.NewStdlibAdapter(logger, log.Prefix("API: ", false))
	fmt.Fprint(writer, "API: 2026/09/04 10:15:30 server.go:42: request received\n")
	out = strings.TrimSpace(buf.String())
	if out != `ts="2026/09/04 10:15:30" caller=server.go:42 msg="request received"` {
		return fmt.Errorf("contract 4 failed: expected %q, got %q", `ts="2026/09/04 10:15:30" caller=server.go:42 msg="request received"`, out)
	}

	// 5. log.Prefix combines with other StdlibAdapterOption configurations such as MessageKey and TimestampKey
	buf.Reset()
	writer = log.NewStdlibAdapter(logger,
		log.Prefix("DB: ", true),
		log.MessageKey("event"),
		log.TimestampKey("logged_at"),
	)
	fmt.Fprint(writer, "DB: 2026/09/04: connection established\n")
	out = strings.TrimSpace(buf.String())
	if out != `logged_at=2026/09/04 event="DB: connection established"` {
		return fmt.Errorf("contract 5 failed: expected %q, got %q", `logged_at=2026/09/04 event="DB: connection established"`, out)
	}

	return nil
}