Fix obscure bug where allocating an empty struct causes trouble. For reasons I…

Fix obscure bug where allocating an empty struct causes trouble.  For reasons I don't yet fully understand, this actually caused a crash on 32-bit OSX builds (but not 64-bit, nor 32-bit Linux).

c++/src/capnp/compiler/compiler.c++

@@ -521,8 +521,8 @@ const Compiler::Node::Content& Compiler::Node::getContent(Content::State minimum
       if (minimumState <= Content::BOOTSTRAP) break;
 
       // Create the final schema.
+      auto nodeSet = locked->translator->finish();
       KJ_IF_MAYBE(exception, kj::runCatchingExceptions([&](){
-        auto nodeSet = locked->translator->finish();
         locked->auxSchemas = KJ_MAP(auxNode, nodeSet.auxNodes) {
           return module->getCompiler().getFinalLoader().loadOnce(auxNode);
         };

c++/src/capnp/layout.c++

@@ -272,6 +272,20 @@ struct WireHelpers {
     //   segment belonging to the arena.  `ref` will be initialized as a non-far pointer, but its
     //   target offset will be set to zero.
 
+    if (amount == 0 * WORDS && kind == WirePointer::STRUCT) {
+      // Allocating a zero-sized struct.  If it happens to be allocated in the space immediately
+      // after the pointer, we'll have a problem:  the pointer will end up all-zero, so isNull()
+      // will be true.  This can lead to all kinds of weird behavior later on.  Since the target
+      // has zero-size anyway, we can really set the pointer to point anywhere, as long as it
+      // is in-bounds.  So, we can have the pointer point back at itself (an offset of -1).  This
+      // should be exceedingly rare in practice since empty structs are pretty useless.
+      //
+      // Note that the check for kind == WirePointer::STRUCT will hopefully cause this whole branch
+      // to be optimized away from all the call sites that are allocating non-structs.
+      ref->setKindAndTarget(WirePointer::STRUCT, reinterpret_cast<word*>(ref));
+      return reinterpret_cast<word*>(ref);
+    }
+
     if (orphanArena == nullptr) {
       if (!ref->isNull()) zeroObject(segment, ref);
 

c++/src/capnp/layout.h

@@ -816,7 +816,8 @@ private:
   // FAR pointer.
 
   word* location;
-  // Pointer to the object, or null if the tag is a FAR pointer.
+  // Pointer to the object.  Invalid if the tag is a FAR pointer (in which case you need to follow
+  // the FAR pointer instead).
 
   inline OrphanBuilder(const void* tagPtr, SegmentBuilder* segment, word* location)
       : segment(segment), location(location) {