applied patch #1912699 - "Pinned" / always loaded plugins support

this patch also adds plugin_api_version to exec_flags so your plug-in can now tell if features it needs are available
more plug-ins that need this will be converted once the patch to make both the stubs and the plug-ins use the same header file is in place

git-svn-id: https://svn.code.sf.net/p/nsis/code/NSIS/trunk@5809 212acab6-be3b-0410-9dea-997c60f758d6

Source/exehead/plugin.c

@@ -0,0 +1,81 @@
+#include "plugin.h"
+
+#ifdef NSIS_CONFIG_PLUGIN_SUPPORT
+
+typedef struct _loaded_plugin
+{
+	struct _loaded_plugin* next;
+	NSISPLUGINCALLBACK proc;
+	HMODULE dll;
+}
+loaded_plugin;
+
+static loaded_plugin* g_plugins = 0; // not thread safe!
+
+void NSISCALL Plugins_SendMsgToAllPlugins(int msg)
+{
+  loaded_plugin* p;
+
+	for (p = g_plugins; p; p = p->next)
+  {
+		p->proc(msg);
+  }
+}
+
+void NSISCALL Plugins_UnloadAll() 
+{
+	loaded_plugin* p = g_plugins;
+
+	Plugins_SendMsgToAllPlugins(NSPIM_UNLOAD);
+
+	while (p)
+	{
+		loaded_plugin* oldp = p;
+    p = oldp->next;
+		FreeLibrary(oldp->dll);
+		GlobalFree(oldp);
+	}
+
+	g_plugins = NULL;
+}
+
+BOOL NSISCALL Plugins_CanUnload(HANDLE pluginHandle)
+{
+  loaded_plugin* p;
+
+	for (p = g_plugins; p; p = p->next)
+  {
+    if (p->dll == pluginHandle)
+    {
+      return FALSE;
+    }
+  }
+	return TRUE;
+}
+
+BOOL NSISCALL RegisterPluginCallback(HMODULE pluginHandle, NSISPLUGINCALLBACK proc)
+{
+	loaded_plugin* p;
+
+	if (!Plugins_CanUnload(pluginHandle))
+  {
+    // already registered
+    return FALSE;
+  }
+	
+	p = (loaded_plugin*) GlobalAlloc(LPTR, sizeof(loaded_plugin));
+	if (p)
+	{
+		p->proc   = proc;
+		p->dll    = pluginHandle;
+		p->next   = g_plugins;
+
+		g_plugins = p;
+
+		return TRUE;
+	}
+
+	return FALSE;
+}
+
+#endif /* #ifdef NSIS_CONFIG_PLUGIN_SUPPORT */