[sim] Add API for extensions to discover each other (#2681)

hal/src/main/native/include/hal/Extensions.h

@@ -1,5 +1,5 @@
 /*----------------------------------------------------------------------------*/
-/* Copyright (c) 2017-2019 FIRST. All Rights Reserved.                        */
+/* Copyright (c) 2017-2020 FIRST. All Rights Reserved.                        */
 /* Open Source Software - may be modified and shared by FRC teams. The code   */
 /* must be accompanied by the FIRST BSD license file in the root directory of */
 /* the project.                                                               */
@@ -44,6 +44,31 @@ int HAL_LoadOneExtension(const char* library);
  */
 int HAL_LoadExtensions(void);
 
+/**
+ * Registers an extension such that other extensions can discover it.
+ *
+ * The passed data pointer is retained and the extension must keep this
+ * pointer valid.
+ *
+ * @param name extension name (may embed version number)
+ * @param data data pointer
+ */
+void HAL_RegisterExtension(const char* name, void* data);
+
+/**
+ * Registers an extension registration listener function. The function will
+ * be called immediately with any currently registered extensions, and will
+ * be called later when any additional extensions are registered.
+ *
+ * @param param parameter data to pass to callback function
+ * @param func callback function to be called for each registered extension;
+ *             parameters are the parameter data, extension name, and extension
+ *             data pointer passed to HAL_RegisterExtension()
+ */
+void HAL_RegisterExtensionListener(void* param,
+                                   void (*func)(void*, const char* name,
+                                                void* data));
+
 /**
  * Enables or disables the message saying no HAL extensions were found.
  *

hal/src/main/native/sim/Extensions.cpp

@@ -7,6 +7,8 @@
 
 #include "hal/Extensions.h"
 
+#include <vector>
+
 #include <wpi/Path.h>
 #include <wpi/SmallString.h>
 #include <wpi/StringRef.h>
@@ -37,6 +39,10 @@
 #define DLERROR dlerror()
 #endif
 
+static std::vector<std::pair<const char*, void*>> gExtensionRegistry;
+static std::vector<std::pair<void*, void (*)(void*, const char*, void*)>>
+    gExtensionListeners;
+
 namespace hal {
 namespace init {
 void InitializeExtensions() {}
@@ -116,6 +122,20 @@ int HAL_LoadExtensions(void) {
   return rc;
 }
 
+void HAL_RegisterExtension(const char* name, void* data) {
+  gExtensionRegistry.emplace_back(name, data);
+  for (auto&& listener : gExtensionListeners)
+    listener.second(listener.first, name, data);
+}
+
+void HAL_RegisterExtensionListener(void* param,
+                                   void (*func)(void*, const char* name,
+                                                void* data)) {
+  gExtensionListeners.emplace_back(param, func);
+  for (auto&& extension : gExtensionRegistry)
+    func(param, extension.first, extension.second);
+}
+
 void HAL_SetShowExtensionsNotFoundMessages(HAL_Bool showMessage) {
   GetShowNotFoundMessage() = showMessage;
 }