examples: verified CAR file fetcher

examples/car-file-fetcher/.gitignore

@@ -0,0 +1,2 @@
+car-file-fetcher
+hello.txt

examples/car-file-fetcher/README.md

@@ -0,0 +1,27 @@
+# CAR File Fetcher
+
+This example shows how to download a UnixFS file or directory from a gateway that implements
+[application/vnd.ipld.car](https://www.iana.org/assignments/media-types/application/vnd.ipld.car)
+responses of the [Trustles Gateway](https://specs.ipfs.tech/http-gateways/trustless-gateway/)
+specification, in a trustless, verifiable manner.
+
+It relies on [IPIP-402](https://specs.ipfs.tech/ipips/ipip-0402/) to retrieve
+the file entity via a single CAR request with all blocks required for end-to-end
+verification.
+
+## Build
+
+```bash
+> go build -o fetcher
+```
+
+## Usage
+
+First, you need a gateway that complies with the Trustless Gateway specification.
+In our specific case, we need that the gateway supports CAR response type.
+
+As an example, you can verifiably fetch a `hello.txt` file from IPFS gateway at `https://trustless-gateway.link`:
+
+```
+./fetcher -g https://trustless-gateway.link -o hello.txt /ipfs/bafkreifzjut3te2nhyekklss27nh3k72ysco7y32koao5eei66wof36n5e
+```

examples/car-file-fetcher/main.go

@@ -0,0 +1,121 @@
+package main
+
+import (
+	"context"
+	"flag"
+	"fmt"
+	"io"
+	"log"
+	"net/http"
+	"os"
+
+	"github.com/ipfs/boxo/files"
+	"github.com/ipfs/boxo/gateway"
+	"github.com/ipfs/boxo/path"
+)
+
+func main() {
+	flag.Usage = func() {
+		fmt.Println("Usage: verified-fetch [flags] <path>")
+		flag.PrintDefaults()
+	}
+
+	gatewayUrlPtr := flag.String("g", "https://trustless-gateway.link", "trustless gateway to download the CAR file from")
+	userAgentPtr := flag.String("u", "", "user agent to use during the HTTP requests")
+	outputPtr := flag.String("o", "out", "output path to store the fetched path")
+	limitPtr := flag.Int64("l", 0, "file size limit for the gateway download")
+	flag.Parse()
+
+	ipfsPath := flag.Arg(0)
+	if len(ipfsPath) == 0 {
+		flag.Usage()
+		os.Exit(1)
+	}
+
+	if err := fetch(*gatewayUrlPtr, ipfsPath, *outputPtr, *userAgentPtr, *limitPtr); err != nil {
+		log.Fatal(err)
+	}
+}
+
+func fetch(gatewayURL, ipfsPath, outputPath, userAgent string, limit int64) error {
+	// Parse the given IPFS path to make sure it is valid.
+	p, err := path.NewPath(ipfsPath)
+	if err != nil {
+		return err
+	}
+
+	// Create a custom [http.Client] with the given user agent and the limit.
+	httpClient := &http.Client{
+		Timeout: gateway.DefaultGetBlockTimeout,
+		Transport: &limitedTransport{
+			RoundTripper: http.DefaultTransport,
+			limitBytes:   limit,
+			userAgent:    userAgent,
+		},
+	}
+
+	// Create the remote CAR gateway backend pointing to the given gateway URL and
+	// using our [http.Client]. A custom [http.Client] is not required and the called
+	// function would create a new one instead.
+	backend, err := gateway.NewRemoteCarBackend([]string{gatewayURL}, httpClient)
+	if err != nil {
+		return err
+	}
+
+	// Resolve the given IPFS path to ensure that it is not mutable. This will
+	// resolve both DNSLink and regular IPNS links. For the latter, it is
+	// necessary that the given gateway supports [IPNS Record] response format.
+	//
+	// [IPNS Record]: https://www.iana.org/assignments/media-types/application/vnd.ipfs.ipns-record
+	imPath, _, _, err := backend.ResolveMutable(context.Background(), p)
+	if err != nil {
+		return err
+	}
+
+	// Fetch the file or directory from the gateway. Since we're using a remote CAR
+	// backend gateway, this call will internally fetch a CAR file from the remote
+	// gateway and ensure that all blocks are present and verified.
+	_, file, err := backend.GetAll(context.Background(), imPath)
+	if err != nil {
+		return err
+	}
+	defer file.Close()
+
+	// Write the returned UnixFS file or directory to the file system.
+	return files.WriteTo(file, outputPath)
+}
+
+type limitedTransport struct {
+	http.RoundTripper
+	limitBytes int64
+	userAgent  string
+}
+
+func (r *limitedTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+	if r.userAgent != "" {
+		req.Header.Set("User-Agent", r.userAgent)
+	}
+	resp, err := r.RoundTripper.RoundTrip(req)
+	if resp != nil && resp.Body != nil && r.limitBytes > 0 {
+		resp.Body = &limitReadCloser{
+			limit:      r.limitBytes,
+			ReadCloser: resp.Body,
+		}
+	}
+	return resp, err
+}
+
+type limitReadCloser struct {
+	io.ReadCloser
+	limit     int64
+	bytesRead int64
+}
+
+func (l *limitReadCloser) Read(p []byte) (int, error) {
+	n, err := l.ReadCloser.Read(p)
+	l.bytesRead += int64(n)
+	if l.bytesRead > l.limit {
+		return 0, fmt.Errorf("reached read limit of %d bytes after reading %d bytes", l.limit, l.bytesRead)
+	}
+	return n, err
+}

examples/car-file-fetcher/main_test.go

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+package main
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+const (
+	HelloWorldCID = "bafkreifzjut3te2nhyekklss27nh3k72ysco7y32koao5eei66wof36n5e"
+)
+
+func TestErrorOnInvalidContent(t *testing.T) {
+	rs := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Write([]byte("wrong data"))
+	}))
+	t.Cleanup(rs.Close)
+
+	err := fetch(rs.URL, "/ipfs/"+HelloWorldCID, "hello.txt", "", 0)
+	require.Error(t, err)
+}
+
+func TestSuccessOnValidContent(t *testing.T) {
+	data, err := os.ReadFile("./hello.car")
+	require.NoError(t, err)
+
+	rs := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Write(data)
+	}))
+	t.Cleanup(rs.Close)
+
+	err = fetch(rs.URL, "/ipfs/"+HelloWorldCID, filepath.Join(t.TempDir(), "hello.txt"), "", 0)
+	require.NoError(t, err)
+}

examples/go.mod

@@ -3,7 +3,7 @@ module github.com/ipfs/boxo/examples
 go 1.21
 
 require (
-	github.com/ipfs/boxo v0.13.1
+	github.com/ipfs/boxo v0.18.0
 	github.com/ipfs/go-block-format v0.2.0
 	github.com/ipfs/go-cid v0.4.1
 	github.com/ipfs/go-datastore v0.6.0