github.com/go-kit/log v0.2.1: level.NewFilter

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/level.NewFilter in pkg:golang/github.com/go-kit/log@v0.2.1
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/github.com/go-kit/log@v0.2.1
Demonstrate these symbols/APIs:
  - github.com/go-kit/log/level.NewFilter

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:sha256:44d7122327032bb1bc86692af29a032ad0db96db6ce63a7ad37c5ae28b797514","contract":["level.NewFilter allows log events at or above configured AllowInfo level (info, warn, error) and squelches debug","level.NewFilter passes unleveled log events through unmodified by default","level.NewFilter squelches unleveled logs and returns custom ErrNoLevel when SquelchNoLevel(true) is configured","level.NewFilter returns custom ErrNotAllowed error when a disallowed log event is squelched","level.NewFilter with AllowNone squelches all leveled log events","level.NewFilter dynamically configures allowed levels using level.Parse and level.Allow"],"goal":"verify github.com/go-kit/log/level.NewFilter in pkg:golang/github.com/go-kit/log@v0.2.1","kind":"HOW","packages":["pkg:golang/github.com/go-kit/log@v0.2.1"],"schemaVersion":1,"symbols":["github.com/go-kit/log/level.NewFilter"]},"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@v0.2.1"],"schemaVersion":1,"subject":"pkg:golang/github.com/go-kit/log@v0.2.1","symbols":["github.com/go-kit/log/level.NewFilter"],"verifierAdapter":"golang@1"}

module sample-level-filter

go 1.26.6

require github.com/go-kit/log v0.2.1

require github.com/go-logfmt/logfmt v0.5.1 // indirect

github.com/go-kit/log v0.2.1 h1:MRVx0/zhvdseW+Gza6N9rVzU/IVzaeE1SFI4raAhmBU=
github.com/go-kit/log v0.2.1/go.mod h1:NwTd00d/i8cPZ3xOwwiv2PO5MOcx78fFErGNcVmBjv0=
github.com/go-logfmt/logfmt v0.5.1 h1:otpy5pqBCBZ1ng9RQ0dPu4PN7ba75Y/aA+UpowDyNVA=
github.com/go-logfmt/logfmt v0.5.1/go.mod h1:WYhtIu8zTZfxdn5+rREduYbwxfcBr/Vr6KEVveWlfTs=

{
  "schemaVersion": 1,
  "goal": "verify github.com/go-kit/log/level.NewFilter in pkg:golang/github.com/go-kit/log@v0.2.1",
  "kind": "HOW",
  "packages": [
    "pkg:golang/github.com/go-kit/log@v0.2.1"
  ],
  "symbols": [
    "github.com/go-kit/log/level.NewFilter"
  ]
}

package main

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

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

func main() {
	// 1. level.NewFilter allows log events at or above configured AllowInfo level (info, warn, error) and squelches debug
	{
		var buf bytes.Buffer
		baseLogger := log.NewLogfmtLogger(&buf)
		filterLogger := level.NewFilter(baseLogger, level.AllowInfo())

		if err := level.Debug(filterLogger).Log("msg", "debugging details"); err != nil {
			fmt.Fprintf(os.Stderr, "Contract 1 failed (debug log err): %v\n", err)
			os.Exit(1)
		}
		if buf.Len() != 0 {
			fmt.Fprintf(os.Stderr, "Contract 1 failed: expected debug to be filtered, got %q\n", buf.String())
			os.Exit(1)
		}

		if err := level.Info(filterLogger).Log("msg", "info message"); err != nil {
			fmt.Fprintf(os.Stderr, "Contract 1 failed (info log err): %v\n", err)
			os.Exit(1)
		}
		if err := level.Warn(filterLogger).Log("msg", "warning message"); err != nil {
			fmt.Fprintf(os.Stderr, "Contract 1 failed (warn log err): %v\n", err)
			os.Exit(1)
		}
		if err := level.Error(filterLogger).Log("msg", "error message"); err != nil {
			fmt.Fprintf(os.Stderr, "Contract 1 failed (error log err): %v\n", err)
			os.Exit(1)
		}

		expected := "level=info msg=\"info message\"\nlevel=warn msg=\"warning message\"\nlevel=error msg=\"error message\"\n"
		if buf.String() != expected {
			fmt.Fprintf(os.Stderr, "Contract 1 failed: expected %q, got %q\n", expected, buf.String())
			os.Exit(1)
		}
	}

	// 2. level.NewFilter passes unleveled log events through unmodified by default
	{
		var buf bytes.Buffer
		baseLogger := log.NewLogfmtLogger(&buf)
		filterLogger := level.NewFilter(baseLogger, level.AllowWarn())

		if err := filterLogger.Log("msg", "unleveled message", "status", "ok"); err != nil {
			fmt.Fprintf(os.Stderr, "Contract 2 failed: %v\n", err)
			os.Exit(1)
		}
		expected := "msg=\"unleveled message\" status=ok\n"
		if buf.String() != expected {
			fmt.Fprintf(os.Stderr, "Contract 2 failed: expected %q, got %q\n", expected, buf.String())
			os.Exit(1)
		}
	}

	// 3. level.NewFilter squelches unleveled logs and returns custom ErrNoLevel when SquelchNoLevel(true) is set
	{
		var buf bytes.Buffer
		errCustomNoLevel := errors.New("unleveled logs not permitted")
		baseLogger := log.NewLogfmtLogger(&buf)
		filterLogger := level.NewFilter(baseLogger,
			level.AllowAll(),
			level.SquelchNoLevel(true),
			level.ErrNoLevel(errCustomNoLevel),
		)

		err := filterLogger.Log("msg", "this should be dropped")
		if !errors.Is(err, errCustomNoLevel) {
			fmt.Fprintf(os.Stderr, "Contract 3 failed: expected %v, got %v\n", errCustomNoLevel, err)
			os.Exit(1)
		}
		if buf.Len() != 0 {
			fmt.Fprintf(os.Stderr, "Contract 3 failed: expected empty buffer, got %q\n", buf.String())
			os.Exit(1)
		}

		// Leveled log should still pass through
		if err := level.Info(filterLogger).Log("msg", "leveled ok"); err != nil {
			fmt.Fprintf(os.Stderr, "Contract 3 failed on leveled log: %v\n", err)
			os.Exit(1)
		}
		expected := "level=info msg=\"leveled ok\"\n"
		if buf.String() != expected {
			fmt.Fprintf(os.Stderr, "Contract 3 failed: expected %q, got %q\n", expected, buf.String())
			os.Exit(1)
		}
	}

	// 4. level.NewFilter returns custom ErrNotAllowed error when a disallowed log event is squelched
	{
		var buf bytes.Buffer
		errNotAllowed := errors.New("log level below threshold")
		baseLogger := log.NewLogfmtLogger(&buf)
		filterLogger := level.NewFilter(baseLogger,
			level.AllowError(),
			level.ErrNotAllowed(errNotAllowed),
		)

		err := level.Info(filterLogger).Log("msg", "should be squelched with error")
		if !errors.Is(err, errNotAllowed) {
			fmt.Fprintf(os.Stderr, "Contract 4 failed: expected %v, got %v\n", errNotAllowed, err)
			os.Exit(1)
		}
		if buf.Len() != 0 {
			fmt.Fprintf(os.Stderr, "Contract 4 failed: expected empty buffer, got %q\n", buf.String())
			os.Exit(1)
		}

		// Allowed level (Error) should succeed without error
		if err := level.Error(filterLogger).Log("msg", "critical failure"); err != nil {
			fmt.Fprintf(os.Stderr, "Contract 4 failed on allowed level: %v\n", err)
			os.Exit(1)
		}
		expected := "level=error msg=\"critical failure\"\n"
		if buf.String() != expected {
			fmt.Fprintf(os.Stderr, "Contract 4 failed: expected %q, got %q\n", expected, buf.String())
			os.Exit(1)
		}
	}

	// 5. level.NewFilter with AllowNone squelches all leveled log events
	{
		var buf bytes.Buffer
		baseLogger := log.NewLogfmtLogger(&buf)
		filterLogger := level.NewFilter(baseLogger, level.AllowNone())

		_ = level.Debug(filterLogger).Log("msg", "d")
		_ = level.Info(filterLogger).Log("msg", "i")
		_ = level.Warn(filterLogger).Log("msg", "w")
		_ = level.Error(filterLogger).Log("msg", "e")

		if buf.Len() != 0 {
			fmt.Fprintf(os.Stderr, "Contract 5 failed: expected no logs with AllowNone, got %q\n", buf.String())
			os.Exit(1)
		}
	}

	// 6. level.NewFilter dynamically configures allowed levels using Parse and Allow
	{
		var buf bytes.Buffer
		parsedLevel, err := level.Parse("warn")
		if err != nil {
			fmt.Fprintf(os.Stderr, "Contract 6 failed: parse error: %v\n", err)
			os.Exit(1)
		}

		baseLogger := log.NewLogfmtLogger(&buf)
		filterLogger := level.NewFilter(baseLogger, level.Allow(parsedLevel))

		_ = level.Info(filterLogger).Log("msg", "info skipped")
		_ = level.Warn(filterLogger).Log("msg", "warn logged")
		_ = level.Error(filterLogger).Log("msg", "error logged")

		expected := "level=warn msg=\"warn logged\"\nlevel=error msg=\"error logged\"\n"
		if buf.String() != expected {
			fmt.Fprintf(os.Stderr, "Contract 6 failed: expected %q, got %q\n", expected, buf.String())
			os.Exit(1)
		}
	}

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