test: add buffer alignment regression tests

Buffers instances can have arbitrary alignment. `node-ffi` depends on
this. Add some regression tests to ensure we don't break this in the
future.

PR-URL: #5752

Author: addaleax

test/addons/buffer-free-callback/binding.cc

@@ -13,9 +13,16 @@ static void FreeCallback(char* data, void* hint) {
 void Alloc(const v8::FunctionCallbackInfo<v8::Value>& args) {
   v8::Isolate* isolate = args.GetIsolate();
   alive++;
+
+  uintptr_t alignment = args[1]->IntegerValue();
+  uintptr_t offset = args[2]->IntegerValue();
+
+  uintptr_t static_offset = reinterpret_cast<uintptr_t>(buf) % alignment;
+  char* aligned = buf + (alignment - static_offset) + offset;
+
   args.GetReturnValue().Set(node::Buffer::New(
         isolate,
-        buf,
+        aligned,
         args[0]->IntegerValue(),
         FreeCallback,
         nullptr).ToLocalChecked());

test/addons/buffer-free-callback/test.js

@@ -4,8 +4,8 @@
 require('../../common');
 var binding = require('./build/Release/binding');
 
-function check(size) {
-  var buf = binding.alloc(size);
+function check(size, alignment, offset) {
+  var buf = binding.alloc(size, alignment, offset);
   var slice = buf.slice(size >>> 1);
 
   buf = null;
@@ -16,7 +16,20 @@ function check(size) {
   gc();
 }
 
-check(64);
+check(64, 1, 0);
+
+// Buffers can have weird sizes.
+check(97, 1, 0);
+
+// Buffers can be unaligned
+check(64,  8, 0);
+check(64, 16, 0);
+check(64,  8, 1);
+check(64, 16, 1);
+check(97,  8, 1);
+check(97, 16, 1);
+check(97,  8, 3);
+check(97, 16, 3);
 
 // Empty ArrayBuffer does not allocate data, worth checking
-check(0);
+check(0, 1, 0);