Implement globals shared across DLLs on Windows.

In the end was less painful than originally anticipated.

doc/corrade-changelog.dox

@@ -186,8 +186,6 @@ namespace Corrade {
 -   Global state used by @ref PluginManager::Manager, @ref Utility::Debug and
     @ref Utility::Resource internals is no longer duplicated when Corrade is
     built statically and linked to more than one dynamic library or executable.
-    Currently that holds only for Linux and macOS, Windows support will come
-    next.
 -   @ref PluginManager::Manager and @ref Utility::Debug now have the global
     state thread-local when the @ref CORRADE_BUILD_MULTITHREADED option is
     enabled (see also [mosra/corrade#65](https://github.com/mosra/corrade/issues/65))

src/Corrade/PluginManager/AbstractManager.cpp

@@ -64,6 +64,10 @@ using Corrade::Utility::Unicode::widen;
 #include "Corrade/PluginManager/configure.h"
 #endif
 
+#if defined(CORRADE_TARGET_WINDOWS) && defined(CORRADE_BUILD_STATIC) && !defined(CORRADE_TARGET_WINDOWS_RT)
+#include "Corrade/Utility/Implementation/WindowsWeakSymbol.h"
+#endif
+
 using namespace Corrade::Utility;
 
 namespace Corrade { namespace PluginManager {
@@ -131,17 +135,17 @@ struct AbstractManager::State {
 
 const int AbstractManager::Version = CORRADE_PLUGIN_VERSION;
 
-#ifndef CORRADE_BUILD_STATIC
-/* (Of course) can't be in an unnamed namespace in order to export it below */
+#if !defined(CORRADE_BUILD_STATIC) || defined(CORRADE_TARGET_WINDOWS)
+/* (Of course) can't be in an unnamed namespace in order to export it below
+   (except for Windows, where we do extern "C" so this doesn't matter) */
 namespace {
 #endif
 
-#if defined(CORRADE_BUILD_STATIC) && !defined(CORRADE_TARGET_WINDOWS)
+#if !defined(CORRADE_BUILD_STATIC) || (defined(CORRADE_BUILD_STATIC) && !defined(CORRADE_TARGET_WINDOWS)) || defined(CORRADE_TARGET_WINDOWS_RT)
+#ifdef CORRADE_BUILD_STATIC
 /* On static builds that get linked to multiple shared libraries and then used
    in a single app we want to ensure there's just one global symbol. On Linux
-   it's apparently enough to just export, macOS needs the weak attribute.
-   Windows not handled yet, as it needs a workaround using DllMain() and
-   GetProcAddress(). */
+   it's apparently enough to just export, macOS needs the weak attribute. */
 CORRADE_VISIBILITY_EXPORT
     #ifdef __GNUC__
     __attribute__((weak))
@@ -156,6 +160,11 @@ CORRADE_VISIBILITY_EXPORT
    static plugin initializers are executed, which means we don't hit any static
    initialization order fiasco. */
 Implementation::StaticPlugin* globalStaticPlugins = nullptr;
+#else
+/* On Windows the symbol is exported unmangled and then fetched via
+   GetProcAddress() to emulate weak linking. */
+extern "C" CORRADE_VISIBILITY_EXPORT Implementation::StaticPlugin* corradePluginManagerUniqueGlobalStaticPlugins = nullptr;
+#endif
 
 #ifdef CORRADE_BUILD_MULTITHREADED
 CORRADE_THREAD_LOCAL
@@ -165,7 +174,7 @@ CORRADE_THREAD_LOCAL
    static plugin registration *can't be* thread-local (it's written to from one
    thread but then only read from multiple) while the global plugin storage
    *has to be* thread-local as there's dependency management, refcounting etc.
-   going on. */
+   going on. Windows handled differently below. */
 CORRADE_VISIBILITY_EXPORT
     #ifdef __GNUC__
     __attribute__((weak))
@@ -187,10 +196,46 @@ CORRADE_VISIBILITY_EXPORT
    initialization order fiasco. */
 std::map<std::string, AbstractManager::Plugin*>* globalPlugins = nullptr;
 
-#ifndef CORRADE_BUILD_STATIC
+#if !defined(CORRADE_BUILD_STATIC) || defined(CORRADE_TARGET_WINDOWS)
 }
 #endif
 
+/* Windows can't have a symbol both thread-local and exported, moreover there
+   isn't any concept of weak symbols. Exporting thread-local symbols can be
+   worked around by exporting a function that then returns a reference to a
+   non-exported thread-local symbol; and finally GetProcAddress() on
+   GetModuleHandle(nullptr) "emulates" the weak linking as it's guaranteed to
+   pick up the same symbol of the final exe independently of the DLL it was
+   called from. To avoid #ifdef hell in code below, the globalStaticPlugins /
+   globalPlugins are redefined to return a value from this uniqueness-ensuring
+   function. */
+#if defined(CORRADE_TARGET_WINDOWS) && defined(CORRADE_BUILD_STATIC) && !defined(CORRADE_TARGET_WINDOWS_RT)
+extern "C" CORRADE_VISIBILITY_EXPORT std::map<std::string, AbstractManager::Plugin*>*& corradePluginManagerUniqueGlobalPlugins() {
+    return globalPlugins;
+}
+
+namespace {
+
+Implementation::StaticPlugin*& windowsGlobalStaticPlugins() {
+    /* A function-local static to ensure it's only initialized once without any
+       race conditions among threads */
+    static Implementation::StaticPlugin** const uniqueGlobals = reinterpret_cast<Implementation::StaticPlugin**>(Utility::Implementation::windowsWeakSymbol("corradePluginManagerUniqueGlobalStaticPlugins"));
+    return *uniqueGlobals;
+}
+
+std::map<std::string, AbstractManager::Plugin*>*& windowsGlobalPlugins() {
+    /* A function-local static to ensure it's only initialized once without any
+       race conditions among threads */
+    static std::map<std::string, AbstractManager::Plugin*>*&(*const uniqueGlobals)() = reinterpret_cast<std::map<std::string, AbstractManager::Plugin*>*&(*)()>(Utility::Implementation::windowsWeakSymbol("corradePluginManagerUniqueGlobalPlugins"));
+    return uniqueGlobals();
+}
+
+}
+
+#define globalStaticPlugins windowsGlobalStaticPlugins()
+#define globalPlugins windowsGlobalPlugins()
+#endif
+
 static_assert(std::is_pod<Implementation::StaticPlugin>::value,
     "static plugins shouldn't cause any global initialization / finalization to happen on their own");
 

src/Corrade/PluginManager/Test/GlobalStateAcrossLibrariesTest.cpp

@@ -51,12 +51,7 @@ void GlobalStateAcrossLibrariesTest::test() {
 
     /* Avoid accidentally loading the dynamic plugins as well */
     PluginManager::Manager<AbstractAnimal> manager{"nonexistent"};
-    {
-        #ifdef CORRADE_TARGET_WINDOWS
-        CORRADE_EXPECT_FAIL("Deduplication of global data across shared libraries isn't implemented on Windows yet.");
-        #endif
-        CORRADE_COMPARE(manager.pluginList(), std::vector<std::string>{"Canary"});
-    }
+    CORRADE_COMPARE(manager.pluginList(), std::vector<std::string>{"Canary"});
 }
 
 }}}}

src/Corrade/PluginManager/Test/ManagerInitFiniTest.cpp

@@ -85,23 +85,13 @@ void ManagerInitFiniTest::dynamicPlugin() {
            initialization is not called again. */
         out.str({});
         CORRADE_COMPARE(manager.load("InitFiniDynamic"), LoadState::Loaded);
-        {
-            #if defined(CORRADE_TARGET_WINDOWS) && defined(CORRADE_BUILD_STATIC)
-            CORRADE_EXPECT_FAIL("Deduplication of global data (Debug output redirection) across shared libraries isn't implemented on Windows yet.");
-            #endif
-            CORRADE_COMPARE(out.str(), "Dynamic plugin initialized\n");
-        }
+        CORRADE_COMPARE(out.str(), "Dynamic plugin initialized\n");
 
         /* Finalization is right before manager unloads them. Base finalization
            is not called yet. */
         out.str({});
         CORRADE_COMPARE(manager.unload("InitFiniDynamic"), LoadState::NotLoaded);
-        {
-            #if defined(CORRADE_TARGET_WINDOWS) && defined(CORRADE_BUILD_STATIC)
-            CORRADE_EXPECT_FAIL("Deduplication of global data (Debug output redirection) across shared libraries isn't implemented on Windows yet.");
-            #endif
-            CORRADE_COMPARE(out.str(), "Dynamic plugin finalized\n");
-        }
+        CORRADE_COMPARE(out.str(), "Dynamic plugin finalized\n");
 
         out.str({});
     }

src/Corrade/Utility/CMakeLists.txt

@@ -99,6 +99,12 @@ if(WITH_UTILITY)
         endif()
     endif()
 
+    # Functionality specific to static Windows builds
+    if(CORRADE_TARGET_WINDOWS AND NOT CORRADE_TARGET_WINDOWS_RT AND CORRADE_BUILD_STATIC)
+        list(APPEND CorradeUtility_SRCS Implementation/WindowsWeakSymbol.cpp)
+        list(APPEND CorradeUtility_PRIVATE_HEADERS Implementation/WindowsWeakSymbol.h)
+    endif()
+
     # Objects shared between main and test library
     add_library(CorradeUtilityObjects OBJECT
         ${CorradeUtility_SRCS}
@@ -189,6 +195,13 @@ if(NOT CMAKE_CROSSCOMPILING)
     if(CORRADE_TARGET_WINDOWS)
         # Needed for dealing with the design failure that's called "Unicode WINAPI"
         list(APPEND CorradeUtilityRc_SRCS Unicode.cpp)
+
+        # Functionality specific to static Windows builds. Not really needed
+        # for corrade-rc as it doesn't load any plugins yet but easier than
+        # having a special handling.
+        if(NOT CORRADE_TARGET_WINDOWS_RT)
+            list(APPEND CorradeUtilityRc_SRCS Implementation/WindowsWeakSymbol.cpp)
+        endif()
     endif()
 
     # Having a tool used during a build depend on a shared library brings a lot

src/Corrade/Utility/Debug.cpp

@@ -56,6 +56,10 @@
 #include "Corrade/Containers/EnumSet.hpp"
 #include "Corrade/Utility/DebugStl.h"
 
+#if defined(CORRADE_TARGET_WINDOWS) && defined(CORRADE_BUILD_STATIC) && !defined(CORRADE_TARGET_WINDOWS_RT)
+#include "Corrade/Utility/Implementation/WindowsWeakSymbol.h"
+#endif
+
 namespace Corrade { namespace Utility {
 
 namespace {
@@ -78,8 +82,10 @@ HANDLE streamOutputHandle(const std::ostream* s) {
 
 }
 
-#ifndef CORRADE_BUILD_STATIC
-/* (Of course) can't be in an unnamed namespace in order to export it below */
+#if !defined(CORRADE_BUILD_STATIC) || defined(CORRADE_TARGET_WINDOWS)
+/* (Of course) can't be in an unnamed namespace in order to export it below
+   (except for Windows, where we do extern "C" so this doesn't matter, but we
+   don't want to expose the DebugGlobals symbols if not needed) */
 namespace {
 #endif
 
@@ -98,8 +104,7 @@ CORRADE_THREAD_LOCAL
 /* On static builds that get linked to multiple shared libraries and then used
    in a single app we want to ensure there's just one global symbol. On Linux
    it's apparently enough to just export, macOS needs the weak attribute.
-   Windows not handled yet, as it needs a workaround using DllMain() and
-   GetProcAddress(). */
+   Windows handled differently below. */
 CORRADE_VISIBILITY_EXPORT
     #ifdef __GNUC__
     __attribute__((weak))
@@ -114,8 +119,35 @@ DebugGlobals debugGlobals{
     #endif
 };
 
-#ifndef CORRADE_BUILD_STATIC
+#if !defined(CORRADE_BUILD_STATIC) || defined(CORRADE_TARGET_WINDOWS)
+}
+#endif
+
+/* Windows can't have a symbol both thread-local and exported, moreover there
+   isn't any concept of weak symbols. Exporting thread-local symbols can be
+   worked around by exporting a function that then returns a reference to a
+   non-exported thread-local symbol; and finally GetProcAddress() on
+   GetModuleHandle(nullptr) "emulates" the weak linking as it's guaranteed to
+   pick up the same symbol of the final exe independently of the DLL it was
+   called from. To avoid #ifdef hell in code below, the debugGlobals are
+   redefined to return a value from this uniqueness-ensuring function. */
+#if defined(CORRADE_TARGET_WINDOWS) && defined(CORRADE_BUILD_STATIC) && !defined(CORRADE_TARGET_WINDOWS_RT)
+extern "C" CORRADE_VISIBILITY_EXPORT DebugGlobals& corradeUtilityUniqueDebugGlobals() {
+    return debugGlobals;
+}
+
+namespace {
+
+DebugGlobals& windowsDebugGlobals() {
+    /* A function-local static to ensure it's only initialized once without any
+       race conditions among threads */
+    static DebugGlobals&(*const uniqueGlobals)() = reinterpret_cast<DebugGlobals&(*)()>(Implementation::windowsWeakSymbol("corradeUtilityUniqueDebugGlobals"));
+    return uniqueGlobals();
 }
+
+}
+
+#define debugGlobals windowsDebugGlobals()
 #endif
 
 template<Debug::Color c, bool bold> Debug::Modifier Debug::colorInternal() {

src/Corrade/Utility/Implementation/WindowsWeakSymbol.cpp

@@ -0,0 +1,43 @@
+/*
+    This file is part of Corrade.
+
+    Copyright © 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016,
+                2017, 2018, 2019 Vladimír Vondruš <mosra@centrum.cz>
+
+    Permission is hereby granted, free of charge, to any person obtaining a
+    copy of this software and associated documentation files (the "Software"),
+    to deal in the Software without restriction, including without limitation
+    the rights to use, copy, modify, merge, publish, distribute, sublicense,
+    and/or sell copies of the Software, and to permit persons to whom the
+    Software is furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included
+    in all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+    THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+    FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+    DEALINGS IN THE SOFTWARE.
+*/
+
+#include "WindowsWeakSymbol.h"
+
+#include <Corrade/Utility/Assert.h>
+
+#define WIN32_LEAN_AND_MEAN 1
+#define VC_EXTRALEAN
+#include <windows.h>
+
+namespace Corrade { namespace Utility { namespace Implementation {
+
+void* windowsWeakSymbol(const char* name) {
+    /* FARPROC?! I want either a function pointer or a variable pointer */
+    void* address = reinterpret_cast<void*>(GetProcAddress(GetModuleHandleA(nullptr), name));
+    CORRADE_INTERNAL_ASSERT(address);
+    return address;
+}
+
+}}}

src/Corrade/Utility/Implementation/WindowsWeakSymbol.h

@@ -0,0 +1,40 @@
+#ifndef Corrade_Utility_WindowsWeakSymbol_h
+#define Corrade_Utility_WindowsWeakSymbol_h
+/*
+    This file is part of Corrade.
+
+    Copyright © 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016,
+                2017, 2018, 2019 Vladimír Vondruš <mosra@centrum.cz>
+
+    Permission is hereby granted, free of charge, to any person obtaining a
+    copy of this software and associated documentation files (the "Software"),
+    to deal in the Software without restriction, including without limitation
+    the rights to use, copy, modify, merge, publish, distribute, sublicense,
+    and/or sell copies of the Software, and to permit persons to whom the
+    Software is furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included
+    in all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+    THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+    FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+    DEALINGS IN THE SOFTWARE.
+*/
+
+#include <Corrade/configure.h>
+
+#if !defined(CORRADE_TARGET_WINDOWS) || !defined(CORRADE_BUILD_STATIC) || defined(CORRADE_TARGET_WINDOWS_RT)
+#error this file is only meant to be used in non-RT Windows static builds
+#endif
+
+namespace Corrade { namespace Utility { namespace Implementation {
+
+void* windowsWeakSymbol(const char* name);
+
+}}}
+
+#endif