Use std::call_once for initialization

[davide]

Summary

Previously discussed with zturner on IRC.

[zturner]

source/Plugins/Process/FreeBSD/ProcessFreeBSD.cpp

59 ↗

(On Diff #23029)

Can you change all these to:

static llvm::ManagedStatic<std::once_flag> g_once_flag;

std::call_once(*g_once_flag, ...);

The reason for this is that Windows and MSVC don't yet have thread-safe initialization of function local statics that have a constructor. On most platforms, members of once_flag are linker initialized, but on Windows they have an actual constructor.

You can add a TODO line that indicates that once we require MSVC 2015 as the minimum version, we can remove these and use regular std::once_flags. Same comment applies to all the rest of the occurrences in the file as well.

[zturner]

Maybe a comment indicating why it's a ManagedStatic would also be helpful so people are aware of the history.

[davide]

In D8760#150578, @zturner wrote:

Maybe a comment indicating why it's a ManagedStatic would also be helpful so people are aware of the history.

Ugh, I'm sorry. For some reason I didn't receive your comments via mail and noticed them only after commit.
I'll change them tomorrow morning and submit a new review.

Thanks,

lldb/trunk/source/Plugins/Process/FreeBSD/ProcessFreeBSD.cpp

ProcessFreeBSD::CreateInstance(Target& target,
     if (crash_file_path == NULL)
         process_sp.reset(new ProcessFreeBSD (target, listener));
     return process_sp;
 }
 
 void
 ProcessFreeBSD::Initialize()
 {
-    static bool g_initialized = false;
+    static std::once_flag g_once_flag;
 
-    if (!g_initialized)
-    {
+    std::call_once(g_once_flag, []() {
         PluginManager::RegisterPlugin(GetPluginNameStatic(),
                                       GetPluginDescriptionStatic(),
                                       CreateInstance);
         ProcessPOSIXLog::Initialize(GetPluginNameStatic());
-        g_initialized = true;
-    }
+    });
 }
 
 lldb_private::ConstString
 ProcessFreeBSD::GetPluginNameStatic()
 {
     static ConstString g_name("freebsd");
     return g_name;
 }

lldb/trunk/source/Plugins/Process/Linux/ProcessLinux.cpp

 ProcessLinux::CreateInstance(Target &target, Listener &listener, const FileSpec *core_file)
 {
     return ProcessSP(new ProcessLinux(target, listener, (FileSpec *)core_file));
 }
 
 void
 ProcessLinux::Initialize()
 {
-    static bool g_initialized = false;
+    static std::once_flag g_once_flag;
 
-    if (!g_initialized)
-    {
-        g_initialized = true;
+    std::call_once(g_once_flag, []() {
         PluginManager::RegisterPlugin(GetPluginNameStatic(),
                                       GetPluginDescriptionStatic(),
                                       CreateInstance);
         ProcessPOSIXLog::Initialize(GetPluginNameStatic());
-    }
+    });
 }
 
 //------------------------------------------------------------------------------
 // Constructors and destructors.
 
 ProcessLinux::ProcessLinux(Target& target, Listener &listener, FileSpec *core_file)
     : ProcessPOSIX(target, listener, GetStaticLinuxSignalsSP ()), m_core_file(core_file), m_stopping_threads(false)
 {

lldb/trunk/source/Plugins/Process/MacOSX-Kernel/ProcessKDP.cpp

ProcessKDP::DoSignal (int signo)
     Error error;
     error.SetErrorString ("sending signals is not suppported in kdp remote debugging");
     return error;
 }
 
 void
 ProcessKDP::Initialize()
 {
-    static bool g_initialized = false;
-    
-    if (g_initialized == false)
+    static std::once_flag g_once_flag;
+    std::call_once(g_once_flag, []()
     {
-        g_initialized = true;
         PluginManager::RegisterPlugin (GetPluginNameStatic(),
                                        GetPluginDescriptionStatic(),
                                        CreateInstance,
-                                       DebuggerInitialize);
-        
+                                       DebuggerInitialize);a
         Log::Callbacks log_callbacks = {
             ProcessKDPLog::DisableLog,
             ProcessKDPLog::EnableLog,
             ProcessKDPLog::ListLogCategories
         };
-        
         Log::RegisterLogChannel (ProcessKDP::GetPluginNameStatic(), log_callbacks);
-    }
+    });
 }
 
 void
 ProcessKDP::DebuggerInitialize (lldb_private::Debugger &debugger)
 {
     if (!PluginManager::GetSettingForProcessPlugin(debugger, PluginProperties::GetSettingName()))
     {
         const bool is_global_setting = true;

lldb/trunk/source/Plugins/Process/Windows/ProcessWindows.cpp

 ProcessWindows::CreateInstance(Target &target, Listener &listener, const FileSpec *)
 {
     return ProcessSP(new ProcessWindows(target, listener));
 }
 
 void
 ProcessWindows::Initialize()
 {
-    static bool g_initialized = false;
+    static std::once_flag g_once_flag;
 
-    if (!g_initialized)
+    std::call_once(g_once_flag, []()
     {
-        g_initialized = true;
         PluginManager::RegisterPlugin(GetPluginNameStatic(),
                                       GetPluginDescriptionStatic(),
                                       CreateInstance);
     }
 }
 
 //------------------------------------------------------------------------------
 // Constructors and destructors.

lldb/trunk/source/Plugins/Process/elf-core/ProcessElfCore.cpp

ProcessElfCore::Clear()
 
     static UnixSignalsSP s_default_unix_signals_sp(new UnixSignals());
     SetUnixSignals(s_default_unix_signals_sp);
 }
 
 void
 ProcessElfCore::Initialize()
 {
-    static bool g_initialized = false;
+    static std::once_flag g_once_flag;
 
-    if (g_initialized == false)
+    std::call_once(g_once_flag, []()
     {
-        g_initialized = true;
-        PluginManager::RegisterPlugin (GetPluginNameStatic(), GetPluginDescriptionStatic(), CreateInstance);
-    }
+        PluginManager::RegisterPlugin (GetPluginNameStatic(),
+          GetPluginDescriptionStatic(), CreateInstance);
+    });
 }
 
 lldb::addr_t
 ProcessElfCore::GetImageInfoAddress()
 {
     Target *target = &GetTarget();
     ObjectFile *obj_file = target->GetExecutableModule()->GetObjectFile();
     Address addr = obj_file->GetImageInfoAddress(target);

lldb/trunk/source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp

if (m_debugserver_pid != LLDB_INVALID_PROCESS_ID)
         Host::Kill (m_debugserver_pid, SIGINT);
         m_debugserver_pid = LLDB_INVALID_PROCESS_ID;
     }
 }
 
 void
 ProcessGDBRemote::Initialize()
 {
-    static bool g_initialized = false;
+    static std::once_flag g_once_flag;
 
-    if (g_initialized == false)
+    std::call_once(g_once_flag, []()
     {
-        g_initialized = true;
         PluginManager::RegisterPlugin (GetPluginNameStatic(),
                                        GetPluginDescriptionStatic(),
                                        CreateInstance,
                                        DebuggerInitialize);
-    }
+    });
 }
 
 void
 ProcessGDBRemote::DebuggerInitialize (Debugger &debugger)
 {
     if (!PluginManager::GetSettingForProcessPlugin(debugger, PluginProperties::GetSettingName()))
     {
         const bool is_global_setting = true;

lldb/trunk/source/Plugins/Process/mach-core/ProcessMachCore.cpp

 ProcessMachCore::Clear()
 {
     m_thread_list.Clear();
 }
 
 void
 ProcessMachCore::Initialize()
 {
-    static bool g_initialized = false;
-    
-    if (g_initialized == false)
-    {
-        g_initialized = true;
+    static std::once_flag g_once_flag;
+    std::call_once(g_once_flag, []() {
         PluginManager::RegisterPlugin (GetPluginNameStatic(),
                                        GetPluginDescriptionStatic(),
-                                       CreateInstance);        
-    }
+                                       CreateInstance);
+    });
 }
 
 addr_t
 ProcessMachCore::GetImageInfoAddress()
 {
     // If we found both a user-process dyld and a kernel binary, we need to decide
     // which to prefer.
     if (GetCorefilePreference() == eKernelCorefile)