Add workaround for MSVC specific C++ issue

[samansmink]

Please describe why this is necessary.

To be able to compile the DuckDB Delta extension, a C++ based project linking against the delta kernel through the FFI, I currently need to manually patch the delta_kernel_ffi.hpp header with the following struct:

struct im_an_unused_struct_that_tricks_msvc_into_compilation {
    ExternResult<KernelBoolSlice> field;
    ExternResult<bool> field2;
    ExternResult<EngineBuilder*> field3;
    ExternResult<Handle<SharedExternEngine>> field4;
    ExternResult<Handle<SharedSnapshot>> field5;
    ExternResult<uintptr_t> field6;
    ExternResult<ArrowFFIData*> field7;
    ExternResult<Handle<SharedScanDataIterator>> field8;
    ExternResult<Handle<SharedScan>> field9;
    ExternResult<Handle<ExclusiveFileReadResultIterator>> field10;
    ExternResult<KernelRowIndexArray> field11;
};

I've taken this trickery from https://github.com/mozilla/cbindgen/issues/402#issuecomment-578680163

While this seems to work fine, this is a little impractical because it requires manually regenerating and patching the ffi header on every update. Furthermore, it makes it a little finnicky to set up a CI job in the kernel which tests kernel against the DuckDB Delta extension automatically.

First of all, this doesn't appear to be a compiler bug:

Linkage from C++ to objects defined in other languages and to objects defined in C++ from other languages is implementation-defined and language-dependent. Only where the object layout strategies of two language implementations are similar enough can such linkage be achieved (see https://stackoverflow.com/questions/8948443/interface-to-c-objects-through-extern-c-functions)

Note that while msvc throw an error, clang is also not super happy and throws a warning

There seem to be two solutions:

• Regular explicit instantiation
• Using the instantiated templates as struct field

I've chosen the latter because it seemed to cause less potential side effects. Explicit instantiation would be allowed to include the header once in the entire program.

Tbh I think explicit instantiation might work but is probably not really user-friendly compared to the struct approach

Describe the functionality you are proposing.

I think the best approach is to let the kernel generate this workaround automatically. It seems pretty harmless compared to explicit instantiation.

Additional context

No response

[scovich]

I notice the list includes only ExternResult -- does this issue only affect function return types?

[scovich]

I proposed a fix upstream with https://github.com/mozilla/cbindgen/pull/1024. Hopefully it gets accepted!

[scovich]

With a local checkout of the above fix and the following local changes:

diff --git a/ffi/Cargo.toml b/ffi/Cargo.toml
index 1bd69e7..f1a6634 100644
--- a/ffi/Cargo.toml
+++ b/ffi/Cargo.toml
@@ -30,7 +30,7 @@ arrow-data = { version = "53.0", default-features = false, features = [
 arrow-array = { version = "53.0", default-features = false, optional = true }

 [build-dependencies]
-cbindgen = "0.27.0"
+cbindgen = { path = "../../../cbindgen" }
 libc = "0.2.158"

 [dev-dependencies]
diff --git a/ffi/cbindgen.toml b/ffi/cbindgen.toml
index 491333a..b5951b4 100644
--- a/ffi/cbindgen.toml
+++ b/ffi/cbindgen.toml
@@ -13,6 +13,9 @@ namespace = "ffi"
 "feature = default-engine" = "DEFINE_DEFAULT_ENGINE"
 "feature = sync-engine" = "DEFINE_SYNC_ENGINE"

+[export]
+instantiate_return_value_monomorphs = true
+
 [export.mangle]
 remove_underscores = true

... the generated delta_kernel_ffi.hpp now includes this:

/// Dummy struct emitted by cbindgen to avoid compiler warnings/errors about
/// return type C linkage for template types returned by value from functions
struct __cbindgen_return_value_monomorphs {
  ExternResult<ArrowFFIData*> field0;
  ExternResult<EngineBuilder*> field1;
  ExternResult<Handle<ExclusiveFileReadResultIterator>> field2;
  ExternResult<Handle<SharedExternEngine>> field3;
  ExternResult<Handle<SharedScan>> field4;
  ExternResult<Handle<SharedScanDataIterator>> field5;
  ExternResult<Handle<SharedSnapshot>> field6;
  ExternResult<KernelBoolSlice> field7;
  ExternResult<KernelRowIndexArray> field8;
  ExternResult<bool> field9;
  ExternResult<uintptr_t> field10;
};

... and the overall header compiles cleanly if I copy it to tmp.cpp (other than lacking a main function):

$ clang++ -Wall -Werror -Wno-pragma-once-outside-header -std=gnu++17 -DDEFINE_DEFAULT_ENGINE tmp.cpp
ld: Undefined symbols:
  _main, referenced from:
      <initial-undefines>
clang: error: linker command failed with exit code 1 (use -v to see invocation)

[scovich]

Ah, it wasn't respecting conditional compilation before... now it does:

/// Dummy struct emitted by cbindgen to avoid compiler warnings/errors about
/// return type C linkage for template types returned by value from functions
struct __cbindgen_return_value_monomorphs {
#if defined(DEFINE_DEFAULT_ENGINE)
  ExternResult<ArrowFFIData*> field0
#endif
  ;
#if defined(DEFINE_DEFAULT_ENGINE)
  ExternResult<EngineBuilder*> field1
#endif
  ;
  ExternResult<Handle<ExclusiveFileReadResultIterator>> field2;
#if defined(DEFINE_DEFAULT_ENGINE)
  ExternResult<Handle<SharedExternEngine>> field3
#endif
  ;
#if defined(DEFINE_SYNC_ENGINE)
  ExternResult<Handle<SharedExternEngine>> field4
#endif
  ;
  ExternResult<Handle<SharedScan>> field5;
  ExternResult<Handle<SharedScanDataIterator>> field6;
  ExternResult<Handle<SharedSnapshot>> field7;
  ExternResult<KernelBoolSlice> field8;
  ExternResult<KernelRowIndexArray> field9;
  ExternResult<bool> field10;
  ExternResult<uintptr_t> field11;
};