From a156a06a5f0daac684e22febf9b80c2f583bc7ab Mon Sep 17 00:00:00 2001
From: Terence Parr <parrt@antlr.org>
Date: Sun, 19 Feb 2023 11:43:21 -0800
Subject: [PATCH] Revert "cpp runtime: Remove pthread dependency. (#4086)"

This reverts commit 207314792d851bcb11a409ce46ca34f37fd22fbd.
---
 runtime/Cpp/runtime/CMakeLists.txt            |  7 +++++++
 .../runtime/src/internal/Synchronization.h    | 20 +++++++++++--------
 2 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/runtime/Cpp/runtime/CMakeLists.txt b/runtime/Cpp/runtime/CMakeLists.txt
index 47497d566e..a4e4d1c6b0 100644
--- a/runtime/Cpp/runtime/CMakeLists.txt
+++ b/runtime/Cpp/runtime/CMakeLists.txt
@@ -28,6 +28,13 @@ file(GLOB libantlrcpp_SRC
 add_library(antlr4_shared SHARED ${libantlrcpp_SRC})
 add_library(antlr4_static STATIC ${libantlrcpp_SRC})
 
+# Make sure to link against threads (pthreads) library in order to be able to
+# make use of std::call_once in the code without producing runtime errors
+# (see also https://github.com/antlr/antlr4/issues/3708 and/or https://stackoverflow.com/q/51584960).
+find_package(Threads REQUIRED)
+target_link_libraries(antlr4_shared Threads::Threads)
+target_link_libraries(antlr4_static Threads::Threads)
+
 if (ANTLR_BUILD_CPP_TESTS)
   include(FetchContent)
 
diff --git a/runtime/Cpp/runtime/src/internal/Synchronization.h b/runtime/Cpp/runtime/src/internal/Synchronization.h
index 1c20338f09..4f969a8ab6 100644
--- a/runtime/Cpp/runtime/src/internal/Synchronization.h
+++ b/runtime/Cpp/runtime/src/internal/Synchronization.h
@@ -27,13 +27,12 @@
 
 #include "antlr4-common.h"
 
-#include <atomic>
-#include <functional>
 #include <mutex>
 #include <shared_mutex>
 #include <utility>
 
 #if ANTLR4CPP_USING_ABSEIL
+#include "absl/base/call_once.h"
 #include "absl/base/thread_annotations.h"
 #include "absl/synchronization/mutex.h"
 #define ANTLR4CPP_NO_THREAD_SAFTEY_ANALYSIS ABSL_NO_THREAD_SAFETY_ANALYSIS
@@ -136,15 +135,20 @@ namespace antlr4::internal {
     template <typename Callable, typename... Args>
     friend void call_once(OnceFlag &onceFlag, Callable &&callable, Args&&... args);
 
-    std::atomic_flag _flag = ATOMIC_FLAG_INIT;
+#if ANTLR4CPP_USING_ABSEIL
+    absl::once_flag _impl;
+#else
+    std::once_flag _impl;
+#endif
   };
 
   template <typename Callable, typename... Args>
-  void call_once(OnceFlag &onceFlag, Callable &&callable, Args &&...args) {
-    if (onceFlag._flag.test_and_set()) {
-      return;
-    }
-    std::invoke(std::forward<Callable>(callable), std::forward<Args>(args)...);
+  void call_once(OnceFlag &onceFlag, Callable &&callable, Args&&... args) {
+#if ANTLR4CPP_USING_ABSEIL
+    absl::call_once(onceFlag._impl, std::forward<Callable>(callable), std::forward<Args>(args)...);
+#else
+    std::call_once(onceFlag._impl, std::forward<Callable>(callable), std::forward<Args>(args)...);
+#endif
   }
 
 }  // namespace antlr4::internal