Call evthread_use_pthreads() on startup (#8258)

Summary:
libevent2 is not thread-safe without this.

- `[err] evmap.c:779: Assertion fdinfo->idxplus1 == i + 1 failed in
event_changelist_remove_all_`
- #8230
Pull Request resolved: #8258

Reviewed By: fugalh

Differential Revision: D8822279

Pulled By: fredemmott

fbshipit-source-id: 77365718fb4defee5c07eb9eb7a1da728cc01946

Author: fredemmott

CMake/FindLibEvent.cmake

@@ -12,10 +12,11 @@ endforeach()
 
 find_path(LIBEVENT_INCLUDE_DIR event.h PATHS ${LibEvent_INCLUDE_PATHS})
 find_library(LIBEVENT_LIB NAMES event PATHS ${LibEvent_LIB_PATHS})
+find_library(LIBEVENT_PTHREAD_LIB NAMES event_pthreads PATHS ${LibEvent_LIB_PATHS})
 
-if (LIBEVENT_LIB AND LIBEVENT_INCLUDE_DIR)
+if (LIBEVENT_LIB AND LIBEVENT_INCLUDE_DIR AND LIBEVENT_PTHREAD_LIB)
   set(LibEvent_FOUND TRUE)
-  set(LIBEVENT_LIB ${LIBEVENT_LIB})
+  set(LIBEVENT_LIB ${LIBEVENT_LIB} ${LIBEVENT_PTHREAD_LIB})
 else ()
   set(LibEvent_FOUND FALSE)
 endif ()
@@ -26,9 +27,9 @@ if (LibEvent_FOUND)
   endif ()
 else ()
   if (LibEvent_FIND_REQUIRED)
-    message(FATAL_ERROR "Could NOT find libevent.")
+    message(FATAL_ERROR "Could NOT find libevent and libevent_pthread.")
   endif ()
-  message(STATUS "libevent NOT found.")
+  message(STATUS "libevent and libevent_pthread NOT found.")
 endif ()
 
 mark_as_advanced(

hphp/runtime/base/program-functions.cpp

@@ -119,6 +119,11 @@
 #include <boost/program_options/variables_map.hpp>
 #include <boost/filesystem.hpp>
 
+#ifndef FACEBOOK
+// Needed on libevent2
+#include <event2/thread.h>
+#endif
+
 #include <oniguruma.h>
 // Onigurama defines UChar to unsigned char, but ICU4C defines it to signed
 // 16-bit int. This is supposed to be resolved by ONIG_ESCAPE_UCHAR_COLLISION,
@@ -1743,8 +1748,13 @@ static int execute_program_impl(int argc, char** argv) {
     RuntimeOption::VSDebuggerNoWait = po.vsDebugNoWait;
   }
 
-  // we need to initialize pcre cache table very early
+  // we need to to initialize these very early
   pcre_init();
+  // this is needed for libevent2 to be thread-safe, which backs Hack ASIO.
+  #ifndef FACEBOOK
+  // FB uses a custom libevent 1
+  evthread_use_pthreads();
+  #endif
 
   tl_heap.getCheck();
   if (RuntimeOption::ServerExecutionMode()) {