From e5cf4883253476500a9b62f0935c87ce40579ec2 Mon Sep 17 00:00:00 2001
From: Johannes Stoelp <johannes.stoelp@gmail.com>
Date: Sun, 21 Apr 2024 21:57:03 +0200
Subject: fn: wrapper hook with deducing noexcept

---
 fn_hook.cc | 99 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 99 insertions(+)
 create mode 100644 fn_hook.cc

diff --git a/fn_hook.cc b/fn_hook.cc
new file mode 100644
index 0000000..1c70eab
--- /dev/null
+++ b/fn_hook.cc
@@ -0,0 +1,99 @@
+#if 0
+set -xe
+trap 'rm -f a.out' EXIT
+g++     -std=c++17 -rdynamic fn_hook.cc -Wno-ignored-attributes && ./a.out
+clang++ -std=c++17 -rdynamic fn_hook.cc && ./a.out
+g++     -std=c++20 -rdynamic fn_hook.cc -Wno-ignored-attributes && ./a.out
+clang++ -std=c++20 -rdynamic fn_hook.cc && ./a.out
+exit 0
+#endif
+
+#include <dlfcn.h>
+
+// Function wrapper hook with deducing noexcept.
+//
+// This is an experiment providing a function wrapper which automatically
+// deduces if the wrapped function is marked noexcept. The information is then
+// generated through the whole wrapper chain.
+//
+// To achieve this, a template specialization for function pointers is defined
+// over a non implemented primary template.
+
+// -- FN HOOK ------------------------------------------------------------------
+
+namespace detail {
+template <typename Derived, typename FnPtr>
+struct hook;
+
+template <typename Derived, typename Ret, typename... Args, bool IsNoexcept>
+struct hook<Derived, Ret (*)(Args...) noexcept(IsNoexcept)> {
+  // Function pointer type.
+  using fn_t = Ret (*)(Args...);
+
+  // Remember whether this hook was noexcept, to be used for the outer wrapper.
+  static constexpr bool is_noexcept = IsNoexcept;
+
+  constexpr hook() noexcept = default;
+
+  // Forwarding wrapper implementation. This also initializes the function
+  // pointer to the next function to call, on the first invocation (non-thread
+  // safe). Alternatively, the function pointer could be retrieved during
+  // construction, but then the construction and first invocation of the wrapped
+  // function must be coordinated.
+  Ret operator()(Args... args) noexcept(IsNoexcept) {
+    if (ptr == nullptr) {
+      init();
+    }
+    return ptr(args...);
+  }
+
+  // Initialize function pointer to next function to call.
+  void init() noexcept {
+    ptr = reinterpret_cast<fn_t>(::dlsym(RTLD_NEXT, Derived::sym));
+  }
+
+private:
+  fn_t ptr = nullptr;
+};
+}  // namespace detail
+
+// Used to ensure that global function hook objects are initialized during
+// compile time; constant initialization, which however allows mutation during
+// runtime, compared to constexpr object, which is non-mutable.
+#if defined(__cpp_constinit)
+#define CONSTINIT constinit
+#else
+#define CONSTINIT
+#endif
+
+// Helper macro to generate boilerplate code for function wrapper hook.
+#define WRAP(ret, fn, ...)                                               \
+  namespace detail {                                                     \
+  static CONSTINIT struct hook_##fn : hook<hook_##fn, decltype(&::fn)> { \
+    static constexpr const char* sym = #fn;                              \
+  } fn;                                                                  \
+  }                                                                      \
+                                                                         \
+  extern "C" ret fn(__VA_ARGS__) noexcept(detail::hook_##fn::is_noexcept)
+
+// -- EXAMPLE ------------------------------------------------------------------
+
+#include <stdio.h>
+#include <unistd.h>
+
+WRAP(ssize_t, write, int fd, const void* buf, size_t sz) {
+  puts("wrapper write(..)");
+  return detail::write(fd, buf, sz);
+}
+
+WRAP(pid_t, getpid) {
+  puts("wrapper getpid()");
+  return detail::getpid();
+}
+
+int main() {
+  const char msg[] = "hello moose\n";
+  write(STDOUT_FILENO, msg, sizeof(msg));
+
+  printf("pid = %d\n", getpid());
+}
-- 
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