[libc++abi] Add a rate limiter when logging dynamic_cast errors

[ldionne]

Summary

This upstreams a fix that Howard made a long time ago, where so many
errors would be logged that applications were becoming sluggish. With
this patch, the first three errors will be printed, and after that the
printing frequency decreases exponentially.

_LIBCXX_DYNAMIC_FALLBACK is only enabled on Apple platforms, so this
should be NFC for other platforms.

rdar://14996273

[ldionne]

Make the counter thread-safe per offline discussion with Michael

[Bigcheese]

LGTM

[jfb]

You probably want to assert that std::atomic<size_t>::is_always_lock_free so weird platforms don't suddenly require a lock, or the lock shard.

[ldionne]

In D78330#1995574, @jfb wrote:

You probably want to assert that std::atomic<size_t>::is_always_lock_free so weird platforms don't suddenly require a lock, or the lock shard.

I agree, but we build libcxxabi as C++11, not C++17 :-(. As soon as we upgrade our CMake, I'll change libcxx and libcxxabi to build as C++17 and I'll fix that.

[jfb]

In D78330#1996821, @ldionne wrote:

In D78330#1995574, @jfb wrote:

You probably want to assert that std::atomic<size_t>::is_always_lock_free so weird platforms don't suddenly require a lock, or the lock shard.

I agree, but we build libcxxabi as C++11, not C++17 :-(. As soon as we upgrade our CMake, I'll change libcxx and libcxxabi to build as C++17 and I'll fix that.

☹️

_Static_assert(__atomic_always_lock_free(sizeof(size_t), 0), "");

🤫

libcxxabi/src/private_typeinfo.cpp

 // Defining _LIBCXX_DYNAMIC_FALLBACK does not help since can_catch() calls
 // is_equal() with use_strcmp=false so the string names are not compared.
 
 #include <string.h>
 
 #ifdef _LIBCXX_DYNAMIC_FALLBACK
 #include "abort_message.h"
 #include <sys/syslog.h>
+#include <atomic>
 #endif
 
 static inline
 bool
 is_equal(const std::type_info* x, const std::type_info* y, bool use_strcmp)
 {
     // Use std::type_info's default comparison unless we've explicitly asked
     // for strcmp.
if (is_equal(dynamic_type, dst_type, false))
         dynamic_type->search_above_dst(&info, dynamic_ptr, dynamic_ptr, public_path, false);
 #ifdef _LIBCXX_DYNAMIC_FALLBACK
         // The following if should always be false because we should definitely
         //   find (static_ptr, static_type), either on a public or private path
         if (info.path_dst_ptr_to_static_ptr == unknown)
         {
             // We get here only if there is some kind of visibility problem
             //   in client code.
+            static std::atomic<size_t> error_count(0);
+            size_t error_count_snapshot = error_count.fetch_add(1, std::memory_order_relaxed);
+            if ((error_count_snapshot & (error_count_snapshot-1)) == 0)
                 syslog(LOG_ERR, "dynamic_cast error 1: Both of the following type_info's "
                         "should have public visibility. At least one of them is hidden. %s"
                         ", %s.\n", static_type->name(), dynamic_type->name());
             // Redo the search comparing type_info's using strcmp
             info = {dst_type, static_ptr, static_type, src2dst_offset, 0};
             info.number_of_dst_type = 1;
             dynamic_type->search_above_dst(&info, dynamic_ptr, dynamic_ptr, public_path, true);
         }
 #endif  // _LIBCXX_DYNAMIC_FALLBACK
         // Query the search.
         if (info.path_dst_ptr_to_static_ptr == public_path)
             dst_ptr = dynamic_ptr;
     }
     else
     {
         // Not using giant short cut.  Do the search
         dynamic_type->search_below_dst(&info, dynamic_ptr, public_path, false);
  #ifdef _LIBCXX_DYNAMIC_FALLBACK
         // The following if should always be false because we should definitely
         //   find (static_ptr, static_type), either on a public or private path
         if (info.path_dst_ptr_to_static_ptr == unknown &&
             info.path_dynamic_ptr_to_static_ptr == unknown)
         {
+            static std::atomic<size_t> error_count(0);
+            size_t error_count_snapshot = error_count.fetch_add(1, std::memory_order_relaxed);
+            if ((error_count_snapshot & (error_count_snapshot-1)) == 0)
                 syslog(LOG_ERR, "dynamic_cast error 2: One or more of the following type_info's "
                                 "has hidden visibility or is defined in more than one translation "
                                 "unit. They should all have public visibility. "
                                 "%s, %s, %s.\n", static_type->name(), dynamic_type->name(),
                         dst_type->name());
             // Redo the search comparing type_info's using strcmp
             info = {dst_type, static_ptr, static_type, src2dst_offset, 0};
             dynamic_type->search_below_dst(&info, dynamic_ptr, public_path, true);
         }
 #endif  // _LIBCXX_DYNAMIC_FALLBACK
         // Query the search.
         switch (info.number_to_static_ptr)
         {