github.com/go-logfmt/logfmt v0.6.0: Encoder

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-logfmt/logfmt.Encoder in pkg:golang/github.com/go-logfmt/logfmt@v0.6.0
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/github.com/go-logfmt/logfmt@v0.6.0
Demonstrate these symbols/APIs:
  - github.com/go-logfmt/logfmt.Encoder

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:sha256:8f52877177bf2e746aade9a388ca29344686cf13b944e57fc7fda1d677752d2e","contract":["logfmt.NewEncoder initializes an Encoder writing formatted logfmt records to an io.Writer","Encoder.EncodeKeyval writes formatted key=value pairs and EndRecord terminates the record with a newline","Encoder.EncodeKeyvals encodes alternating key-value pairs and automatically pairs odd keys with null","Encoder escapes quotes, newlines, and quotes strings containing whitespace in values","Encoder.EncodeKeyval rejects nil keys with ErrNilKey, empty or dropped-rune keys with ErrInvalidKey, and struct keys with ErrUnsupportedKeyType","Encoder.Reset resets encoder record separator state for a new record without writing a newline","Encoder formats time.Time in RFC3339 format and handles custom fmt.Stringer types"],"goal":"verify github.com/go-logfmt/logfmt.Encoder in pkg:golang/github.com/go-logfmt/logfmt@v0.6.0","kind":"HOW","packages":["pkg:golang/github.com/go-logfmt/logfmt@v0.6.0"],"schemaVersion":1,"symbols":["github.com/go-logfmt/logfmt.Encoder"]},"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/github.com/go-logfmt/logfmt@v0.6.0"],"schemaVersion":1,"subject":"pkg:golang/github.com/go-logfmt/logfmt@v0.6.0","symbols":["github.com/go-logfmt/logfmt.Encoder"],"verifierAdapter":"golang@1"}

module sample

go 1.22

require github.com/go-logfmt/logfmt v0.6.0

github.com/go-logfmt/logfmt v0.6.0 h1:wGYYu3uicYdqXVgoYbvnkrPVXkuLM1p1ifugDMEdRi4=
github.com/go-logfmt/logfmt v0.6.0/go.mod h1:WYhtIu8zTZfxdn5+rREduYbwxfcBr/Vr6KEVveWlfTs=

package sample

import (
	"bytes"
	"io"

	"github.com/go-logfmt/logfmt"
)

// StructuredLogger uses logfmt.Encoder to write structured log records.
type StructuredLogger struct {
	w   io.Writer
	enc *logfmt.Encoder
}

// NewStructuredLogger creates a new StructuredLogger wrapping logfmt.Encoder.
func NewStructuredLogger(w io.Writer) *StructuredLogger {
	return &StructuredLogger{
		w:   w,
		enc: logfmt.NewEncoder(w),
	}
}

// Log writes a complete log record from alternating key-value pairs.
func (l *StructuredLogger) Log(keyvals ...interface{}) error {
	if err := l.enc.EncodeKeyvals(keyvals...); err != nil {
		return err
	}
	return l.enc.EndRecord()
}

// LogField writes an individual key-value pair without ending the record.
func (l *StructuredLogger) LogField(key, val interface{}) error {
	return l.enc.EncodeKeyval(key, val)
}

// FlushRecord ends the current record with a newline delimiter.
func (l *StructuredLogger) FlushRecord() error {
	return l.enc.EndRecord()
}

// Reset resets the internal encoder state.
func (l *StructuredLogger) Reset() {
	l.enc.Reset()
}

// FormatRecord formats alternating key-value pairs into a logfmt string.
func FormatRecord(keyvals ...interface{}) (string, error) {
	var buf bytes.Buffer
	enc := logfmt.NewEncoder(&buf)
	if err := enc.EncodeKeyvals(keyvals...); err != nil {
		return "", err
	}
	if err := enc.EndRecord(); err != nil {
		return "", err
	}
	return buf.String(), nil
}

{
  "schemaVersion": 1,
  "goal": "verify github.com/go-logfmt/logfmt.Encoder in pkg:golang/github.com/go-logfmt/logfmt@v0.6.0",
  "kind": "HOW",
  "packages": [
    "pkg:golang/github.com/go-logfmt/logfmt@v0.6.0"
  ],
  "symbols": [
    "github.com/go-logfmt/logfmt.Encoder"
  ]
}

package main

import (
	"bytes"
	"errors"
	"fmt"
	"os"
	"strings"
	"time"

	"github.com/go-logfmt/logfmt"
	"sample"
)

type customStringer struct {
	text string
}

func (c customStringer) String() string {
	return c.text
}

type unhandledStruct struct {
	field int
}

func main() {
	var buf bytes.Buffer

	// Assertion 1: logfmt.NewEncoder initializes an Encoder writing formatted logfmt records to an io.Writer
	enc := logfmt.NewEncoder(&buf)
	if enc == nil {
		fmt.Fprintf(os.Stderr, "expected non-nil Encoder from logfmt.NewEncoder\n")
		os.Exit(1)
	}

	// Assertion 2: Encoder.EncodeKeyval writes formatted key=value pairs and EndRecord terminates the record with a newline
	if err := enc.EncodeKeyval("level", "info"); err != nil {
		fmt.Fprintf(os.Stderr, "EncodeKeyval level failed: %v\n", err)
		os.Exit(1)
	}
	if err := enc.EncodeKeyval("msg", "service started"); err != nil {
		fmt.Fprintf(os.Stderr, "EncodeKeyval msg failed: %v\n", err)
		os.Exit(1)
	}
	if err := enc.EncodeKeyval("port", 8080); err != nil {
		fmt.Fprintf(os.Stderr, "EncodeKeyval port failed: %v\n", err)
		os.Exit(1)
	}
	if err := enc.EndRecord(); err != nil {
		fmt.Fprintf(os.Stderr, "EndRecord failed: %v\n", err)
		os.Exit(1)
	}

	expected := "level=info msg=\"service started\" port=8080\n"
	if buf.String() != expected {
		fmt.Fprintf(os.Stderr, "expected %q, got %q\n", expected, buf.String())
		os.Exit(1)
	}

	// Assertion 3: Encoder.EncodeKeyvals encodes alternating key-value pairs and automatically pairs odd keys with null
	buf.Reset()
	if err := enc.EncodeKeyvals("tag", "prod", "solo_key"); err != nil {
		fmt.Fprintf(os.Stderr, "EncodeKeyvals with odd count failed: %v\n", err)
		os.Exit(1)
	}
	if err := enc.EndRecord(); err != nil {
		fmt.Fprintf(os.Stderr, "EndRecord failed: %v\n", err)
		os.Exit(1)
	}
	if buf.String() != "tag=prod solo_key=null\n" {
		fmt.Fprintf(os.Stderr, "expected odd keyval null output, got %q\n", buf.String())
		os.Exit(1)
	}

	// Assertion 4: Encoder escapes quotes, newlines, and quotes strings containing whitespace in values
	buf.Reset()
	if err := enc.EncodeKeyvals("quote", "quote \"inside\"", "multiline", "line1\nline2", "space", "has space"); err != nil {
		fmt.Fprintf(os.Stderr, "EncodeKeyvals with special chars failed: %v\n", err)
		os.Exit(1)
	}
	if err := enc.EndRecord(); err != nil {
		fmt.Fprintf(os.Stderr, "EndRecord failed: %v\n", err)
		os.Exit(1)
	}

	escapedOut := buf.String()
	if !strings.Contains(escapedOut, `quote="quote \"inside\""`) {
		fmt.Fprintf(os.Stderr, "expected escaped quotes in output, got %q\n", escapedOut)
		os.Exit(1)
	}
	if !strings.Contains(escapedOut, "multiline=\"line1\\nline2\"") {
		fmt.Fprintf(os.Stderr, "expected escaped newline in output, got %q\n", escapedOut)
		os.Exit(1)
	}
	if !strings.Contains(escapedOut, `space="has space"`) {
		fmt.Fprintf(os.Stderr, "expected quoted spaces in output, got %q\n", escapedOut)
		os.Exit(1)
	}

	// Assertion 5: Encoder.EncodeKeyval rejects nil keys with ErrNilKey, empty or dropped-rune keys with ErrInvalidKey, and struct keys with ErrUnsupportedKeyType
	buf.Reset()
	if err := enc.EncodeKeyval(nil, "value"); !errors.Is(err, logfmt.ErrNilKey) {
		fmt.Fprintf(os.Stderr, "expected logfmt.ErrNilKey for nil key, got %v\n", err)
		os.Exit(1)
	}
	if err := enc.EncodeKeyval("", "value"); !errors.Is(err, logfmt.ErrInvalidKey) {
		fmt.Fprintf(os.Stderr, "expected logfmt.ErrInvalidKey for empty key, got %v\n", err)
		os.Exit(1)
	}
	if err := enc.EncodeKeyval("   ", "value"); !errors.Is(err, logfmt.ErrInvalidKey) {
		fmt.Fprintf(os.Stderr, "expected logfmt.ErrInvalidKey for whitespace-only key, got %v\n", err)
		os.Exit(1)
	}
	if err := enc.EncodeKeyval(unhandledStruct{field: 1}, "value"); !errors.Is(err, logfmt.ErrUnsupportedKeyType) {
		fmt.Fprintf(os.Stderr, "expected logfmt.ErrUnsupportedKeyType for struct key, got %v\n", err)
		os.Exit(1)
	}

	// Assertion 6: Encoder.Reset resets encoder record separator state for a new record without writing a newline
	buf.Reset()
	if err := enc.EncodeKeyval("first", "val"); err != nil {
		fmt.Fprintf(os.Stderr, "EncodeKeyval failed: %v\n", err)
		os.Exit(1)
	}
	enc.Reset()
	buf.Reset()
	if err := enc.EncodeKeyval("status", "recovered"); err != nil {
		fmt.Fprintf(os.Stderr, "EncodeKeyval after reset failed: %v\n", err)
		os.Exit(1)
	}
	if err := enc.EndRecord(); err != nil {
		fmt.Fprintf(os.Stderr, "EndRecord after reset failed: %v\n", err)
		os.Exit(1)
	}
	if buf.String() != "status=recovered\n" {
		fmt.Fprintf(os.Stderr, "expected %q, got %q\n", "status=recovered\n", buf.String())
		os.Exit(1)
	}

	// Assertion 7: Encoder formats time.Time in RFC3339 format and handles custom fmt.Stringer types
	buf.Reset()
	fixedTime := time.Date(2026, 8, 27, 21, 50, 0, 0, time.UTC)
	if err := enc.EncodeKeyvals("ts", fixedTime, "custom", customStringer{text: "custom-value"}); err != nil {
		fmt.Fprintf(os.Stderr, "EncodeKeyvals with time and stringer failed: %v\n", err)
		os.Exit(1)
	}
	if err := enc.EndRecord(); err != nil {
		fmt.Fprintf(os.Stderr, "EndRecord failed: %v\n", err)
		os.Exit(1)
	}
	expectedFormatted := "ts=2026-08-27T21:50:00Z custom=custom-value\n"
	if buf.String() != expectedFormatted {
		fmt.Fprintf(os.Stderr, "expected %q, got %q\n", expectedFormatted, buf.String())
		os.Exit(1)
	}

	// Additional verification: StructuredLogger sample wrapper
	buf.Reset()
	logger := sample.NewStructuredLogger(&buf)
	if err := logger.Log("app", "sample", "status", "ok"); err != nil {
		fmt.Fprintf(os.Stderr, "logger.Log failed: %v\n", err)
		os.Exit(1)
	}
	if buf.String() != "app=sample status=ok\n" {
		fmt.Fprintf(os.Stderr, "logger.Log output mismatch: %q\n", buf.String())
		os.Exit(1)
	}

	formatted, err := sample.FormatRecord("event", "shutdown", "code", 0)
	if err != nil || formatted != "event=shutdown code=0\n" {
		fmt.Fprintf(os.Stderr, "sample.FormatRecord mismatch: %q, err=%v\n", formatted, err)
		os.Exit(1)
	}

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