C++: wrap generated DescriptorTable variable in extern "C" block

To implement extensions in Rust, we are going to need a symbol we can depend on
to force-link the C++ extension registration code. The `descriptor_table_xxxxx`
variable seems ideal for this: it already exists and is not public-facing, so
we can reuse it without increasing code size. This CL marks it `extern "C"` to
ensure that we can safely access it from Rust.

PiperOrigin-RevId: 696239471

src/google/protobuf/compiler/cpp/file.cc

@@ -1769,9 +1769,14 @@ void FileGenerator::GenerateGlobalStateFunctionDeclarations(io::Printer* p) {
   )cc");
 
   if (HasDescriptorMethods(file_, options_)) {
+    // The DescriptorTable needs to be extern "C" so that we can access it from
+    // Rust. We do not attempt to read the contents of the table in Rust, but
+    // just use the symbol to force-link the C++ generated code when necessary.
     p->Emit(R"cc(
+      extern "C" {
       $dllexport_decl $extern const ::$proto_ns$::internal::DescriptorTable
           $desc_table$;
+      }  // extern "C"
     )cc");
   }
 }

src/google/protobuf/descriptor_unittest.cc

@@ -44,6 +44,7 @@
 #include "absl/log/scoped_mock_log.h"
 #include "absl/status/status.h"
 #include "absl/status/statusor.h"
+#include "absl/strings/match.h"
 #include "absl/strings/numbers.h"
 #include "absl/strings/str_cat.h"
 #include "absl/strings/str_format.h"
@@ -14104,6 +14105,28 @@ TEST_F(LazilyBuildDependenciesTest, Dependency) {
 
 // ===================================================================
 
+// This is effectively a static_assert ensuring that the generated
+// descriptor_table variable is marked extern "C". The compiler will give us an
+// error if the generated declaration does not match this one. We need this
+// variable to be extern "C" so that we can refer to it from Rust.
+//
+// If this causes a linker error, it is likely because the name mangling
+// changed. That can be fixed by updating to the new name from the generated
+// code for unittest.proto.
+
+#define DESCRIPTOR_TABLE_NAME \
+  descriptor_table_google_2fprotobuf_2funittest_2eproto
+
+extern "C" {
+extern const ::google::protobuf::internal::DescriptorTable DESCRIPTOR_TABLE_NAME;
+}
+
+TEST(DescriptorTableExternLinkageTest, DescriptorTableExternLinkageTest) {
+  // The goal of this assertion is just to verify that the descriptor_table
+  // variable declaration above still refers to a real thing.
+  EXPECT_TRUE(absl::EndsWith(DESCRIPTOR_TABLE_NAME.filename, "unittest.proto"));
+}
+
 
 }  // namespace descriptor_unittest
 }  // namespace protobuf