base: one Option/Feature registry per process in shared-object builds

base/containers/init_chain.h

@@ -7,9 +7,24 @@
 #include <base/arch.h>
 #include <base/compiler.h>
 
+// The chain's root is a function-local static in a class template, so every
+// shared object that instantiates the template gets its own root when it is
+// compiled with -fvisibility=hidden (as a component build is): an Option
+// registered in one .so is then invisible to VisitAll in another. Default
+// visibility makes the root one symbol per process (ELF unique binding), so
+// all objects share one chain. An instantiation is only as visible as its
+// template argument, so every item type (OptionBase, Feature) carries the same
+// annotation. PE has no such binding; there base has to be linked as a DLL for
+// the same guarantee.
+#if defined(__GNUC__) || defined(__clang__)
+#define BASE_SHARED_REGISTRY __attribute__((visibility("default")))
+#else
+#define BASE_SHARED_REGISTRY
+#endif
+
 namespace base {
 template <typename TItem>
-class InitChain {
+class BASE_SHARED_REGISTRY InitChain {
  public:
   constexpr InitChain() noexcept = default;
 

base/feature.h

@@ -8,7 +8,7 @@
 #include <base/containers/init_chain.h>
 
 namespace base {
-struct BASE_EXPORT Feature : InitChain<Feature> {
+struct BASE_EXPORT BASE_SHARED_REGISTRY Feature : InitChain<Feature> {
   enum class Flags : arch_types::u32 {
     kNone = 1 << 0,
     kHidden = 1 << 1,  //< If this flag isn't set, assume public.

base/option.h

@@ -67,7 +67,7 @@ inline bool ParseOption(const char* text, T& out) {
 // Type-erased handle so every Option<T>, whatever T is, threads onto one
 // self-registering list (the same InitChain trick base::Feature uses). Walk
 // them with OptionBase::VisitAll, or let InitOptionsFromEnv() drive them.
-class BASE_EXPORT OptionBase : public InitChain<OptionBase> {
+class BASE_EXPORT BASE_SHARED_REGISTRY OptionBase : public InitChain<OptionBase> {
  public:
   OptionBase(const char* opt_name, const char* env_var, const char* description)
       : name_(opt_name), env_(env_var), desc_(description) {