github.com/quic-go/qpack v0.5.1: NewDecoder

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/quic-go/qpack.NewDecoder in pkg:golang/github.com/quic-go/qpack@v0.5.1
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/github.com/quic-go/qpack@v0.5.1
Demonstrate these symbols/APIs:
  - github.com/quic-go/qpack.NewDecoder

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:sha256:9b5736473076931b1ad889031d38bb918b7651f64c706c3efcf9e0515368e416","contract":["qpack.NewDecoder decodes full QPACK header blocks containing pseudo-headers and regular headers via DecodeFull","qpack.NewDecoder with emit callback processes header fields incrementally via Write","qpack.HeaderField.IsPseudo returns true for pseudo-headers starting with a colon and false for standard headers","qpack.NewDecoder.DecodeFull returns an error when decoding malformed or invalid header block data"],"goal":"verify github.com/quic-go/qpack.NewDecoder in pkg:golang/github.com/quic-go/qpack@v0.5.1","kind":"HOW","packages":["pkg:golang/github.com/quic-go/qpack@v0.5.1"],"schemaVersion":1,"symbols":["github.com/quic-go/qpack.NewDecoder"]},"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/quic-go/qpack@v0.5.1"],"schemaVersion":1,"subject":"pkg:golang/github.com/quic-go/qpack@v0.5.1","symbols":["github.com/quic-go/qpack.NewDecoder"],"verifierAdapter":"golang@1"}

module example.com/sample

go 1.24.0

require github.com/quic-go/qpack v0.5.1

require golang.org/x/net v0.28.0 // indirect

github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
github.com/quic-go/qpack v0.5.1 h1:giqksBPnT/HDtZ6VhtFKgoLOWmlyo9Ei6u9PqzIMbhI=
github.com/quic-go/qpack v0.5.1/go.mod h1:+PC4XFrEskIVkcLzpEkbLqq1uCoxPhQuvK5rH1ZgaEg=
github.com/stretchr/testify v1.9.0 h1:HtqpIVDClZ4nwg75+f6Lvsy/wHu+3BoSGCbBAcpTsTg=
github.com/stretchr/testify v1.9.0/go.mod h1:r2ic/lqez/lEtzL7wO/rwa5dbSLXVDPFyf8C91i36aY=
golang.org/x/net v0.28.0 h1:a9JDOJc5GMUJ0+UDqmLT86WiEy7iWyIhz8gz8E4e5hE=
golang.org/x/net v0.28.0/go.mod h1:yqtgsTWOOnlGLG9GFRrK3++bGOUEkNBoHZc8MEDWPNg=
gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

package main

import (
	"bytes"
	"fmt"
	"os"

	"github.com/quic-go/qpack"
)

// DecodeHeaders decodes QPACK encoded header block bytes into a list of HeaderFields.
func DecodeHeaders(data []byte) ([]qpack.HeaderField, error) {
	dec := qpack.NewDecoder(nil)
	return dec.DecodeFull(data)
}

// EncodeHeaders encodes a list of HeaderFields into QPACK byte slice.
func EncodeHeaders(fields []qpack.HeaderField) ([]byte, error) {
	var buf bytes.Buffer
	enc := qpack.NewEncoder(&buf)
	for _, f := range fields {
		if err := enc.WriteField(f); err != nil {
			return nil, fmt.Errorf("failed to encode field %s: %w", f.Name, err)
		}
	}
	if err := enc.Close(); err != nil {
		return nil, fmt.Errorf("failed to close encoder: %w", err)
	}
	return buf.Bytes(), nil
}

func main() {
	headers := []qpack.HeaderField{
		{Name: ":status", Value: "200"},
		{Name: ":path", Value: "/api/v1/resource"},
		{Name: "content-type", Value: "application/json"},
	}

	encoded, err := EncodeHeaders(headers)
	if err != nil {
		fmt.Fprintf(os.Stderr, "Error encoding headers: %v\n", err)
		os.Exit(1)
	}
	fmt.Printf("Encoded %d headers into %d bytes\n", len(headers), len(encoded))

	decoded, err := DecodeHeaders(encoded)
	if err != nil {
		fmt.Fprintf(os.Stderr, "Error decoding headers: %v\n", err)
		os.Exit(1)
	}

	for _, h := range decoded {
		pseudo := ""
		if h.IsPseudo() {
			pseudo = " (pseudo-header)"
		}
		fmt.Printf("Header: %s = %s%s\n", h.Name, h.Value, pseudo)
	}
}

{
  "schemaVersion": 1,
  "goal": "verify github.com/quic-go/qpack.NewDecoder in pkg:golang/github.com/quic-go/qpack@v0.5.1",
  "kind": "HOW",
  "packages": [
    "pkg:golang/github.com/quic-go/qpack@v0.5.1"
  ],
  "symbols": [
    "github.com/quic-go/qpack.NewDecoder"
  ]
}

package main

import (
	"bytes"
	"fmt"
	"os"

	"github.com/quic-go/qpack"
)

func main() {
	// Assertion 1: qpack.NewDecoder decodes full QPACK header blocks containing pseudo-headers and regular headers via DecodeFull
	var buf bytes.Buffer
	enc := qpack.NewEncoder(&buf)
	if err := enc.WriteField(qpack.HeaderField{Name: ":status", Value: "200"}); err != nil {
		fmt.Fprintf(os.Stderr, "FAIL: enc.WriteField :status: %v\n", err)
		os.Exit(1)
	}
	if err := enc.WriteField(qpack.HeaderField{Name: ":path", Value: "/index.html"}); err != nil {
		fmt.Fprintf(os.Stderr, "FAIL: enc.WriteField :path: %v\n", err)
		os.Exit(1)
	}
	if err := enc.WriteField(qpack.HeaderField{Name: "content-type", Value: "application/json"}); err != nil {
		fmt.Fprintf(os.Stderr, "FAIL: enc.WriteField content-type: %v\n", err)
		os.Exit(1)
	}
	if err := enc.Close(); err != nil {
		fmt.Fprintf(os.Stderr, "FAIL: enc.Close: %v\n", err)
		os.Exit(1)
	}

	dec := qpack.NewDecoder(nil)
	fields, err := dec.DecodeFull(buf.Bytes())
	if err != nil {
		fmt.Fprintf(os.Stderr, "FAIL: dec.DecodeFull: %v\n", err)
		os.Exit(1)
	}
	if len(fields) != 3 {
		fmt.Fprintf(os.Stderr, "FAIL: expected 3 decoded fields, got %d\n", len(fields))
		os.Exit(1)
	}
	if fields[0].Name != ":status" || fields[0].Value != "200" {
		fmt.Fprintf(os.Stderr, "FAIL: field 0 mismatch: %+v\n", fields[0])
		os.Exit(1)
	}
	if fields[1].Name != ":path" || fields[1].Value != "/index.html" {
		fmt.Fprintf(os.Stderr, "FAIL: field 1 mismatch: %+v\n", fields[1])
		os.Exit(1)
	}
	if fields[2].Name != "content-type" || fields[2].Value != "application/json" {
		fmt.Fprintf(os.Stderr, "FAIL: field 2 mismatch: %+v\n", fields[2])
		os.Exit(1)
	}

	// Assertion 2: qpack.NewDecoder with emit callback processes header fields incrementally via Write
	var emitted []qpack.HeaderField
	streamDec := qpack.NewDecoder(func(hf qpack.HeaderField) {
		emitted = append(emitted, hf)
	})
	n, err := streamDec.Write(buf.Bytes())
	if err != nil {
		fmt.Fprintf(os.Stderr, "FAIL: streamDec.Write: %v\n", err)
		os.Exit(1)
	}
	if n != buf.Len() {
		fmt.Fprintf(os.Stderr, "FAIL: streamDec.Write returned %d, expected %d\n", n, buf.Len())
		os.Exit(1)
	}
	if err := streamDec.Close(); err != nil {
		fmt.Fprintf(os.Stderr, "FAIL: streamDec.Close: %v\n", err)
		os.Exit(1)
	}
	if len(emitted) != 3 {
		fmt.Fprintf(os.Stderr, "FAIL: expected 3 emitted fields, got %d\n", len(emitted))
		os.Exit(1)
	}
	if emitted[0].Name != ":status" || emitted[1].Name != ":path" || emitted[2].Name != "content-type" {
		fmt.Fprintf(os.Stderr, "FAIL: emitted fields mismatch: %+v\n", emitted)
		os.Exit(1)
	}

	// Assertion 3: qpack.HeaderField.IsPseudo returns true for pseudo-headers starting with a colon and false for standard headers
	pseudoField := qpack.HeaderField{Name: ":method", Value: "GET"}
	if !pseudoField.IsPseudo() {
		fmt.Fprintf(os.Stderr, "FAIL: expected IsPseudo() to be true for :method\n")
		os.Exit(1)
	}
	regularField := qpack.HeaderField{Name: "user-agent", Value: "test-client"}
	if regularField.IsPseudo() {
		fmt.Fprintf(os.Stderr, "FAIL: expected IsPseudo() to be false for user-agent\n")
		os.Exit(1)
	}

	// Assertion 4: qpack.NewDecoder.DecodeFull returns an error when decoding malformed or invalid header block data
	errDec := qpack.NewDecoder(nil)
	_, err = errDec.DecodeFull([]byte{0xff, 0xff, 0xff})
	if err == nil {
		fmt.Fprintf(os.Stderr, "FAIL: expected error decoding malformed bytes, got nil\n")
		os.Exit(1)
	}

	fmt.Println("PASS: github.com/quic-go/qpack.NewDecoder contract verified")
}