[mcollina]

Building with --experimental-enable-pointer-compression --experimental-pointer-compression-shared-cage currently fails to compile. The shared cage turns on the V8 sandbox, and V8 only defines CppHeapPointerTag::kDefaultTag when the sandbox is off:

../src/cppgc_helpers-inl.h:18:43: error: 'kDefaultTag' is not a member of 'v8::CppHeapPointerTag'

V8 asks embedders that use the sandbox to give each type its own tag. Swapping in a different single tag would compile, but the tag would protect nothing: CppgcMixin::Unwrap() reads the raw pointer from the internal field and never checks it.

This PR:

• adds node::CppgcWrapperTag in src/node_v8_embedder.h, with one tag per cppgc wrapper type (ContextifyContext, ContextifyScript) and one for the deprecated SetCppgcReference()
• adds a SET_CPPGC_TAG() macro. CppgcMixin::Wrap<T>() uses T::kCppHeapPointerTag, so a wrapper without a tag fails to compile
• unwraps through v8::Object::Unwrap<T::kCppHeapPointerTag, T>(). With the sandbox on, unwrapping an object as the wrong type now returns nullptr. The isolate comes from v8::Isolate::GetCurrent(), so ASSIGN_OR_RETURN_UNWRAP_CPPGC and its callers stay the same
• updates the tests that used kDefaultTag directly

It also fixes three addon tests that crashed with the sandbox, because they created ArrayBuffers over memory outside it:

• cppgc-object now lets V8 allocate its counters
• null-buffer-neuter only checks for a nullptr data pointer without the sandbox, because empty buffers point into the sandbox
• worker-buffer-callback/test-free-called expects the main thread buffer to be freed too, because the sandbox copies the data and frees the original right away

Tested on main with a pointer-compression + shared-cage + sandbox build (default, addons, embedding, cctest) and with a default build (default, addons, embedding, cctest). All pass.

——-

AI generated, reviewed by me.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.68%. Comparing base (98dbf60) to head (76f24fb).
⚠️ Report is 49 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #67131      +/-   ##
==========================================
+ Coverage   90.65%   90.68%   +0.03%     
==========================================
  Files         797      797              
  Lines      278268   278805     +537     
  Branches    53517    53618     +101     
==========================================
+ Hits       252261   252838     +577     
+ Misses      16386    16334      -52     
- Partials     9621     9633      +12     

Files with missing lines	Coverage Δ
src/cppgc_helpers-inl.h	82.35% <100.00%> (-0.51%)	⬇️
src/cppgc_helpers.h	60.00% <ø> (ø)
src/env.cc	84.43% <100.00%> (+0.13%)	⬆️
src/node_contextify.h	72.22% <ø> (ø)

... and 51 files with indirect coverage changes

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