test_runner: use v8.serialize instead of TAP

doc/api/cli.md

@@ -2321,7 +2321,8 @@ on unsupported platforms will not be fixed.
 ### `NODE_TEST_CONTEXT=value`
 
 If `value` equals `'child'`, test reporter options will be overridden and test
-output will be sent to stdout in the TAP format.
+output will be sent to stdout in the TAP format. If any other value is provided,
+Node.js makes no guarantees about the reporter format used or its stability.
 
 ### `NODE_TLS_REJECT_UNAUTHORIZED=value`
 

lib/internal/test_runner/reporter/v8-serializer.js

@@ -0,0 +1,39 @@
+'use strict';
+
+const { DefaultSerializer } = require('v8');
+const { Buffer } = require('buffer');
+const { serializeError } = require('internal/error_serdes');
+
+
+module.exports = async function* v8Reporter(source) {
+  const serializer = new DefaultSerializer();
+
+  for await (const item of source) {
+    const originalError = item.data.details?.error;
+    if (originalError) {
+      // Error is overriden with a serialized version, so that it can be
+      // deserialized in the parent process.
+      // Error is restored after serialization.
+      item.data.details.error = serializeError(originalError);
+    }
+    // Add 4 bytes, to later populate with message length
+    serializer.writeRawBytes(Buffer.allocUnsafe(4));
+    serializer.writeHeader();
+    serializer.writeValue(item);
+
+    if (originalError) {
+      item.data.details.error = originalError;
+    }
+
+    const serializedMessage = serializer.releaseBuffer();
+    const serializedMessageLength = serializedMessage.length - 4;
+
+    serializedMessage.set([
+      serializedMessageLength >> 24 & 0xFF,
+      serializedMessageLength >> 16 & 0xFF,
+      serializedMessageLength >> 8 & 0xFF,
+      serializedMessageLength & 0xFF,
+    ], 0);
+    yield serializedMessage;
+  }
+};