github.com/go-logfmt/logfmt v0.5.0: Encoder.EncodeKeyval

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 pkg:golang/github.com/go-logfmt/logfmt@v0.5.0
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/github.com/go-logfmt/logfmt@v0.5.0
Demonstrate these symbols/APIs:
  - github.com/go-logfmt/logfmt.Encoder.EncodeKeyval

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:abbe8b8081550de2c538216831908649ccb2947490e0f1de52694c076fbd09bc","contract":["logfmt.Encoder.EncodeKeyval writes the logfmt encoding of a single key and value to the output stream","logfmt.Encoder.EncodeKeyval writes a single space separator before second and subsequent key-value pairs on the same record","logfmt.Encoder.EncodeKeyval formats nil values as null","logfmt.Encoder.EncodeKeyval returns ErrNilKey on nil key or nil pointer and writes nothing to the stream","logfmt.Encoder.EncodeKeyval returns ErrInvalidKey when key contains only invalid runes and writes nothing to the stream","logfmt.Encoder.EncodeKeyval returns ErrUnsupportedKeyType for array chan func map slice or struct key and writes nothing to the stream","logfmt.Encoder.EncodeKeyval returns ErrUnsupportedValueType for unsupported complex types and writes nothing to the stream"],"goal":"verify pkg:golang/github.com/go-logfmt/logfmt@v0.5.0","kind":"HOW","packages":["pkg:golang/github.com/go-logfmt/logfmt@v0.5.0"],"schemaVersion":1,"symbols":["github.com/go-logfmt/logfmt.Encoder.EncodeKeyval"]},"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@v0.5.0"],"schemaVersion":1,"subject":"pkg:golang/github.com/go-logfmt/logfmt@v0.5.0","symbols":["github.com/go-logfmt/logfmt.Encoder.EncodeKeyval"],"verifierAdapter":"golang@1"}

module sample

go 1.26.6

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

github.com/go-logfmt/logfmt v0.5.0 h1:TrB8swr/68K7m9CcGut2g3UOihhbcbiMAYiuTXdEih4=
github.com/go-logfmt/logfmt v0.5.0/go.mod h1:wCYkCAKZfumFQihp8CzCvQ3paCTfi41vtzG1KdI/P7A=

package sample

import (
	"bytes"
	"io"

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

// LogEntry writes key-value pairs one by one using EncodeKeyval and ends the record with a newline.
func LogEntry(w io.Writer, kvs [][2]interface{}) error {
	enc := logfmt.NewEncoder(w)
	for _, kv := range kvs {
		if err := enc.EncodeKeyval(kv[0], kv[1]); err != nil {
			return err
		}
	}
	return enc.EndRecord()
}

// FormatPair encodes a single key-value pair to a string using EncodeKeyval.
func FormatPair(key, value interface{}) (string, error) {
	var buf bytes.Buffer
	enc := logfmt.NewEncoder(&buf)
	if err := enc.EncodeKeyval(key, value); err != nil {
		return "", err
	}
	return buf.String(), nil
}

{
  "schemaVersion": 1,
  "goal": "verify pkg:golang/github.com/go-logfmt/logfmt@v0.5.0",
  "kind": "HOW",
  "packages": [
    "pkg:golang/github.com/go-logfmt/logfmt@v0.5.0"
  ],
  "symbols": [
    "github.com/go-logfmt/logfmt.Encoder.EncodeKeyval"
  ]
}

package main

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

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

func main() {
	// Assertion 1: logfmt.Encoder.EncodeKeyval writes the logfmt encoding of a single key and value to the output stream
	var buf bytes.Buffer
	enc := logfmt.NewEncoder(&buf)
	if err := enc.EncodeKeyval("msg", "hello world"); err != nil {
		fmt.Fprintf(os.Stderr, "Assertion 1 failed: %v\n", err)
		os.Exit(1)
	}
	if buf.String() != "msg=\"hello world\"" {
		fmt.Fprintf(os.Stderr, "Assertion 1 output mismatch: %q\n", buf.String())
		os.Exit(1)
	}

	// Assertion 2: logfmt.Encoder.EncodeKeyval writes a single space separator before second and subsequent key-value pairs on the same record
	if err := enc.EncodeKeyval("count", 42); err != nil {
		fmt.Fprintf(os.Stderr, "Assertion 2 failed: %v\n", err)
		os.Exit(1)
	}
	if buf.String() != "msg=\"hello world\" count=42" {
		fmt.Fprintf(os.Stderr, "Assertion 2 output mismatch: %q\n", buf.String())
		os.Exit(1)
	}
	if err := enc.EndRecord(); err != nil {
		fmt.Fprintf(os.Stderr, "EndRecord failed: %v\n", err)
		os.Exit(1)
	}
	if buf.String() != "msg=\"hello world\" count=42\n" {
		fmt.Fprintf(os.Stderr, "EndRecord output mismatch: %q\n", buf.String())
		os.Exit(1)
	}

	// Assertion 3: logfmt.Encoder.EncodeKeyval formats nil values as null
	buf.Reset()
	if err := enc.EncodeKeyval("data", nil); err != nil {
		fmt.Fprintf(os.Stderr, "Assertion 3 failed: %v\n", err)
		os.Exit(1)
	}
	if buf.String() != "data=null" {
		fmt.Fprintf(os.Stderr, "Assertion 3 output mismatch: %q\n", buf.String())
		os.Exit(1)
	}

	// Assertion 4: logfmt.Encoder.EncodeKeyval returns ErrNilKey on nil key or nil pointer and writes nothing to the stream
	buf.Reset()
	if err := enc.EncodeKeyval(nil, "val"); !errors.Is(err, logfmt.ErrNilKey) {
		fmt.Fprintf(os.Stderr, "Assertion 4 nil interface failed: expected ErrNilKey, got %v\n", err)
		os.Exit(1)
	}
	var nilPtr *string
	if err := enc.EncodeKeyval(nilPtr, "val"); !errors.Is(err, logfmt.ErrNilKey) {
		fmt.Fprintf(os.Stderr, "Assertion 4 nil pointer failed: expected ErrNilKey, got %v\n", err)
		os.Exit(1)
	}
	if buf.Len() != 0 {
		fmt.Fprintf(os.Stderr, "Assertion 4 stream mismatch: expected empty buffer, got %q\n", buf.String())
		os.Exit(1)
	}

	// Assertion 5: logfmt.Encoder.EncodeKeyval returns ErrInvalidKey when key contains only invalid runes and writes nothing to the stream
	buf.Reset()
	if err := enc.EncodeKeyval("   ===", "val"); !errors.Is(err, logfmt.ErrInvalidKey) {
		fmt.Fprintf(os.Stderr, "Assertion 5 failed: expected ErrInvalidKey, got %v\n", err)
		os.Exit(1)
	}
	if buf.Len() != 0 {
		fmt.Fprintf(os.Stderr, "Assertion 5 stream mismatch: expected empty buffer, got %q\n", buf.String())
		os.Exit(1)
	}

	// Assertion 6: logfmt.Encoder.EncodeKeyval returns ErrUnsupportedKeyType for array chan func map slice or struct key and writes nothing to the stream
	buf.Reset()
	type dummyKey struct{}
	if err := enc.EncodeKeyval(dummyKey{}, "val"); !errors.Is(err, logfmt.ErrUnsupportedKeyType) {
		fmt.Fprintf(os.Stderr, "Assertion 6 failed: expected ErrUnsupportedKeyType, got %v\n", err)
		os.Exit(1)
	}
	if buf.Len() != 0 {
		fmt.Fprintf(os.Stderr, "Assertion 6 stream mismatch: expected empty buffer, got %q\n", buf.String())
		os.Exit(1)
	}

	// Assertion 7: logfmt.Encoder.EncodeKeyval returns ErrUnsupportedValueType for unsupported complex types and writes nothing to the stream
	buf.Reset()
	type dummyVal struct{}
	if err := enc.EncodeKeyval("meta", dummyVal{}); !errors.Is(err, logfmt.ErrUnsupportedValueType) {
		fmt.Fprintf(os.Stderr, "Assertion 7 failed: expected ErrUnsupportedValueType, got %v\n", err)
		os.Exit(1)
	}
	if buf.Len() != 0 {
		fmt.Fprintf(os.Stderr, "Assertion 7 stream mismatch: expected empty buffer, got %q\n", buf.String())
		os.Exit(1)
	}

	// Helper verification: LogEntry and FormatPair
	pairStr, err := sample.FormatPair("status", "ok")
	if err != nil || pairStr != "status=ok" {
		fmt.Fprintf(os.Stderr, "FormatPair helper failed: %v, got %q\n", err, pairStr)
		os.Exit(1)
	}

	buf.Reset()
	err = sample.LogEntry(&buf, [][2]interface{}{
		{"module", "auth"},
		{"action", "login"},
	})
	if err != nil || buf.String() != "module=auth action=login\n" {
		fmt.Fprintf(os.Stderr, "LogEntry helper failed: %v, got %q\n", err, buf.String())
		os.Exit(1)
	}

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