Serializer: name the packet in write errors from a compiled protocol

doc/api.md

@@ -56,7 +56,8 @@ Create a serializer of `mainType` defined in `proto`. This is a Transform stream
 
 ### Serializer.createPacketBuffer(packet)
 
-Returns a buffer of the `packet`.
+Returns a buffer of the `packet`. If serialization fails and `packet.name` is set, the error message is prefixed with
+`in packet <name>: ` unless the error's field path already contains that name.
 
 ## Parser(proto,mainType)
 

src/serializer.js

@@ -9,7 +9,16 @@ class Serializer extends Transform {
   }
 
   createPacketBuffer (packet) {
-    return this.proto.createPacketBuffer(this.mainType, packet)
+    try {
+      return this.proto.createPacketBuffer(this.mainType, packet)
+    } catch (e) {
+      // A compiled protocol reports no field path, so the packet name is the only way to say which packet failed
+      const name = packet?.name
+      if (name != null && !String(e.field ?? '').split('.').includes(String(name))) {
+        e.message = `in packet ${name}: ${e.message}`
+      }
+      throw e
+    }
   }
 
   _transform (chunk, enc, cb) {

test/misc.js

@@ -1,7 +1,7 @@
 /* eslint-env mocha */
 
 const assert = require('assert')
-const { ProtoDef } = require('../')
+const { ProtoDef, Serializer } = require('../')
 const { ProtoDefCompiler } = require('../').Compiler
 
 it('example works', () => {
@@ -25,3 +25,32 @@ describe('mapper', () => {
     })
   }
 })
+
+describe('Serializer', () => {
+  const types = {
+    packet_position: ['container', [{ name: 'x', type: 'f64' }, { name: 'face', type: ['mapper', { type: 'varint', mappings: { 0: 'down' } }] }]],
+    packet: ['container', [
+      { name: 'name', type: ['mapper', { type: 'varint', mappings: { 0: 'position' } }] },
+      { name: 'params', type: ['switch', { compareTo: 'name', fields: { position: 'packet_position' } }] }
+    ]]
+  }
+  const proto = new ProtoDef()
+  proto.addTypes(types)
+  const compiler = new ProtoDefCompiler()
+  compiler.addTypesToCompile(types)
+  const compiled = compiler.compileProtoDefSync()
+  const bad = { name: 'position', params: { x: 1, face: 'up' } }
+
+  it('names the packet when the error has no field path (compiled)', () => {
+    assert.throws(() => new Serializer(compiled, 'packet').createPacketBuffer(bad),
+      { message: 'in packet position: SizeOf error for undefined : up is not in the mappings value' })
+  })
+  it('does not repeat a packet name already in the field path (interpreted)', () => {
+    assert.throws(() => new Serializer(proto, 'packet').createPacketBuffer(bad),
+      { message: 'SizeOf error for params.position.face : up is not in the mappings value' })
+  })
+  it('leaves errors alone when the value has no name', () => {
+    assert.throws(() => new Serializer(compiled, 'packet_position').createPacketBuffer(bad.params),
+      { message: 'SizeOf error for undefined : up is not in the mappings value' })
+  })
+})