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

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.NewLogfmtLogger 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.NewLogfmtLogger

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:f52090649cfd9dcdcedfa05c289dff4ca118d1cb389e9f0153e83fe2f70315c3","contract":["log.NewLogfmtLogger formats key-value pairs into logfmt formatted output lines","log.NewLogfmtLogger quotes and escapes values containing spaces, quotes, and newlines","log.NewLogfmtLogger formats an odd number of key-value arguments by pairing the orphan key with null","log.With wraps NewLogfmtLogger to inject contextual key-values across multiple calls","log.WithPrefix prepends key-values before log call key-values","log.Valuer dynamically evaluates values at log time","log.NewSyncLogger wraps NewLogfmtLogger for safe concurrent writes"],"goal":"verify github.com/go-kit/log.NewLogfmtLogger 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.NewLogfmtLogger"]},"contractCommand":["go","run","./test"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"golang","executionContext":"go","language":"go","libc":"glibc","libcVersion":"2.39","os":"linux","osVersionBucket":"24","packageManager":"go","runtime":"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.NewLogfmtLogger"],"verifierAdapter":"golang@1"}

module sample-logfmt-logger

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 h1:5SgMzNM5HxrEjV0ww2lTmX6E2Izsfxas4+YHWRs3Lsk=
github.com/go-stack/stack v1.8.0/go.mod h1:v0f6uXyyMGvRgIKkXu+yp6POWl0qKG85gN/melR3HDY=

{
  "schemaVersion": 1,
  "goal": "verify github.com/go-kit/log.NewLogfmtLogger 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.NewLogfmtLogger"
  ]
}

package main

import (
	"bytes"
	"errors"
	"fmt"
	"os"

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

func main() {
	// 1. log.NewLogfmtLogger formats key-value pairs into logfmt formatted output lines
	{
		var buf bytes.Buffer
		logger := log.NewLogfmtLogger(&buf)
		err := logger.Log("msg", "task started", "status", "ok", "count", 42)
		if err != nil {
			fmt.Fprintf(os.Stderr, "Contract 1 failed: unexpected error: %v\n", err)
			os.Exit(1)
		}
		expected := "msg=\"task started\" status=ok count=42\n"
		if buf.String() != expected {
			fmt.Fprintf(os.Stderr, "Contract 1 failed: expected %q, got %q\n", expected, buf.String())
			os.Exit(1)
		}
	}

	// 2. log.NewLogfmtLogger quotes and escapes values containing spaces, quotes, and newlines
	{
		var buf bytes.Buffer
		logger := log.NewLogfmtLogger(&buf)
		err := logger.Log("quote", `he said "hello"`, "path", "a b")
		if err != nil {
			fmt.Fprintf(os.Stderr, "Contract 2 failed: unexpected error: %v\n", err)
			os.Exit(1)
		}
		expected := "quote=\"he said \\\"hello\\\"\" path=\"a b\"\n"
		if buf.String() != expected {
			fmt.Fprintf(os.Stderr, "Contract 2 failed: expected %q, got %q\n", expected, buf.String())
			os.Exit(1)
		}
	}

	// 3. log.NewLogfmtLogger formats an odd number of key-value arguments by pairing the orphan key with null
	{
		var buf bytes.Buffer
		logger := log.NewLogfmtLogger(&buf)
		err := logger.Log("key1", "val1", "orphanKey")
		if err != nil {
			fmt.Fprintf(os.Stderr, "Contract 3 failed: unexpected error: %v\n", err)
			os.Exit(1)
		}
		expected := "key1=val1 orphanKey=null\n"
		if buf.String() != expected {
			fmt.Fprintf(os.Stderr, "Contract 3 failed: expected %q, got %q\n", expected, buf.String())
			os.Exit(1)
		}
	}

	// 4. log.With wraps NewLogfmtLogger to inject contextual key-values across multiple calls
	{
		var buf bytes.Buffer
		baseLogger := log.NewLogfmtLogger(&buf)
		contextLogger := log.With(baseLogger, "component", "worker", "attempt", 1)
		err := contextLogger.Log("event", "processing")
		if err != nil {
			fmt.Fprintf(os.Stderr, "Contract 4 failed: unexpected error: %v\n", err)
			os.Exit(1)
		}
		expected := "component=worker attempt=1 event=processing\n"
		if buf.String() != expected {
			fmt.Fprintf(os.Stderr, "Contract 4 failed: expected %q, got %q\n", expected, buf.String())
			os.Exit(1)
		}
	}

	// 5. log.WithPrefix prepends key-values before log call key-values
	{
		var buf bytes.Buffer
		baseLogger := log.NewLogfmtLogger(&buf)
		prefixLogger := log.WithPrefix(baseLogger, "level", "info")
		err := prefixLogger.Log("msg", "ready")
		if err != nil {
			fmt.Fprintf(os.Stderr, "Contract 5 failed: unexpected error: %v\n", err)
			os.Exit(1)
		}
		expected := "level=info msg=ready\n"
		if buf.String() != expected {
			fmt.Fprintf(os.Stderr, "Contract 5 failed: expected %q, got %q\n", expected, buf.String())
			os.Exit(1)
		}
	}

	// 6. log.Valuer dynamically evaluates values at log time
	{
		var buf bytes.Buffer
		counter := 0
		customValuer := log.Valuer(func() interface{} {
			counter++
			return counter
		})
		logger := log.With(log.NewLogfmtLogger(&buf), "call_id", customValuer)
		if err := logger.Log("action", "first"); err != nil {
			fmt.Fprintf(os.Stderr, "Contract 6 failed: %v\n", err)
			os.Exit(1)
		}
		if err := logger.Log("action", "second"); err != nil {
			fmt.Fprintf(os.Stderr, "Contract 6 failed: %v\n", err)
			os.Exit(1)
		}
		expected := "call_id=1 action=first\ncall_id=2 action=second\n"
		if buf.String() != expected {
			fmt.Fprintf(os.Stderr, "Contract 6 failed: expected %q, got %q\n", expected, buf.String())
			os.Exit(1)
		}
	}

	// 7. log.NewSyncLogger wraps NewLogfmtLogger for safe concurrent writes
	{
		var buf bytes.Buffer
		syncLogger := log.NewSyncLogger(log.NewLogfmtLogger(&buf))
		err := syncLogger.Log("msg", "safe concurrent write", "err", errors.New("sample error"))
		if err != nil {
			fmt.Fprintf(os.Stderr, "Contract 7 failed: %v\n", err)
			os.Exit(1)
		}
		expected := "msg=\"safe concurrent write\" err=\"sample error\"\n"
		if buf.String() != expected {
			fmt.Fprintf(os.Stderr, "Contract 7 failed: expected %q, got %q\n", expected, buf.String())
			os.Exit(1)
		}
	}

	fmt.Println("All contract assertions passed.")
}