feat: add carv2-messaging example for embedding messages in CARv2

Author: rvagg

examples/carv2-messaging.js

@@ -0,0 +1,90 @@
+#!/usr/bin/env node
+
+import fs from 'fs/promises'
+import { Readable } from 'stream'
+import { CarReader, CarWriter } from '@ipld/car'
+import * as raw from 'multiformats/codecs/raw'
+import * as dagCbor from '@ipld/dag-cbor'
+import { CID } from 'multiformats/cid'
+import { sha256 } from 'multiformats/hashes/sha2'
+import varint from 'varint'
+import bl from 'bl'
+
+async function example () {
+  /** WRITE **/
+  {
+    // create a CARv1 payload, just a single block
+    const bytes = new TextEncoder().encode('random meaningless bytes')
+    const hash = await sha256.digest(raw.encode(bytes))
+    const cid = CID.create(1, raw.code, hash)
+    const { writer, out } = await CarWriter.create([cid])
+    const v1Payload = bl()
+    Readable.from(out).pipe(v1Payload)
+    await writer.put({ cid, bytes })
+    await writer.close()
+
+    // _the_ CARv2 pragma, 11 bytes - a fake CARv1-like header that says {version:2}
+    const v2Pragma = new Uint8Array([10, 161, 103, 118, 101, 114, 115, 105, 111, 110, 2])
+    // sneaky message in CARv2 wrapper
+    const msg = dagCbor.encode({ sneaky: 'sending a message outside of CARv1 payload', expectedRoot: cid })
+    const msgLength = new Uint8Array(varint.encode(msg.byteLength))
+    /* v2 header is 40 bytes - characteristics: 16, v1 offset: 8, v1 size: 8, index offset: 8 */
+    const v2Data = new Uint8Array(v2Pragma.byteLength + 40 + msgLength.byteLength + msg.byteLength + v1Payload.length)
+    v2Data.set(v2Pragma, 0)
+    // characteristics are the first 16 bytes after the pragma, the leftmost bit means
+    // "is fully indexed", so we're using the 2nd to leftmost bit set to indicate
+    // "message after v2 header" (TODO: codify this)
+    let off = v2Pragma.byteLength
+    v2Data[off] = (1 << 6)
+    const dv = new DataView(v2Data.buffer, v2Data.byteOffset, v2Data.byteLength)
+    off += 16 // characteristics
+    // where is the v1 payload?
+    const dataOffset = v2Pragma.byteLength + 40 + msgLength.byteLength + msg.byteLength
+    dv.setBigUint64(off, BigInt(dataOffset), true) // position 16 is "data offset"
+    off += 8
+    dv.setBigUint64(off, BigInt(v1Payload.length), true) // position 16+8 is "data size"
+    off += 8
+    // position 16+8+8 is "index offset", leaving it as zero says "no index"
+    off += 8
+
+    v2Data.set(msgLength, off)
+    off += msgLength.byteLength
+    v2Data.set(msg, off)
+    off += msg.byteLength
+    v2Data.set(v1Payload.slice(), off)
+
+    await fs.writeFile('example-messaging.car', v2Data)
+  }
+
+  /** READ **/
+  {
+    const inputBytes = await fs.readFile('example-messaging.car')
+    // read and parse the entire stream in one go, this will cache the contents of
+    // the car in memory so is not suitable for large files.
+    const reader = await CarReader.fromBytes(inputBytes)
+
+    // read the CARv1 contents as normal
+    const roots = await reader.getRoots()
+    const got = await reader.get(roots[0])
+
+    console.log('Retrieved [%s] from example-messaging.car with CID [%s]',
+      new TextDecoder().decode(got.bytes),
+      roots[0].toString())
+
+    // BUT, our sneaky message is in here too
+    console.log('Is CARv2?', reader._header.version === 2)
+    console.log('Has message?', (reader._header.characteristics[0] & (1n << 6n)) !== 0)
+    let off = /* pragma */ 11 + /* header */ 40
+    const msgLength = varint.decode(inputBytes.slice(off))
+    console.log('Message length:', msgLength)
+    off += varint.decode.bytes
+    const msgBytes = inputBytes.slice(off, off + msgLength)
+    const msg = dagCbor.decode(msgBytes)
+    console.log('Message:', msg)
+  }
+}
+
+example().catch((err) => {
+  console.error(err)
+  process.exit(1)
+})