child_process: fix parsing messages with splitted length field

lib/internal/child_process/serialization.js

@@ -61,7 +61,12 @@ const advanced = {
   *parseChannelMessages(channel, readData) {
     if (readData.length === 0) return;
 
-    ArrayPrototypePush(channel[kMessageBuffer], readData);
+    if (channel[kMessageBufferSize] && channel[kMessageBuffer][0].length < 4) {
+      // Message length split into two buffers, so let's concatenate it.
+      channel[kMessageBuffer][0] = Buffer.concat([channel[kMessageBuffer][0], readData]);
+    } else {
+      ArrayPrototypePush(channel[kMessageBuffer], readData);
+    }
     channel[kMessageBufferSize] += readData.length;
 
     // Index 0 should always be present because we just pushed data into it.

test/parallel/test-child-process-advanced-serialization-splitted-length-field.js

@@ -0,0 +1,24 @@
+'use strict';
+const common = require('../common');
+const child_process = require('child_process');
+
+// Regression test for https://github.com/nodejs/node/issues/55834
+const msgLen = 65521;
+let cnt = 10;
+
+if (process.argv[2] === 'child') {
+  const msg = Buffer.allocUnsafe(msgLen);
+  (function send() {
+    if (cnt--) {
+      process.send(msg, send);
+    } else {
+      process.disconnect();
+    }
+  })();
+} else {
+  const child = child_process.spawn(process.execPath, [__filename, 'child'], {
+    stdio: ['inherit', 'inherit', 'inherit', 'ipc'],
+    serialization: 'advanced'
+  });
+  child.on('message', common.mustCall(cnt));
+}