include: add internal fields struct to uv_loop_t

Add struct `uv__loop_internal_fields_t` as a location for future
additions to `uv_loop_t` while also maintaining v1.x compatibility.

Currently `uv__loop_internal_fields_t` only contains the `flags` field.
The reason for adding the `flags` field is because the same field was
never added to `UV_LOOP_PRIVATE_FIELDS` in Windows as it was in Unix.

The idea for creating a struct and attaching it to `uv_loop_t` for
future API enhancements was taken from a comment by bnoordhuis in:
https://github.com/libuv/libuv/issues/2506#issuecomment-540050665

Also add `internal_fields` to `uv_loop_t` to store the pointer to
`uv__loop_internal_fields_t`. This naming makes more sense than just
using `active_reqs.unused[1]`. To maintain ABI compatibility, shrink the
`unused` array.

PR-URL: https://github.com/libuv/libuv/pull/2725
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Jameson Nash <vtjnash@gmail.com>

include/uv.h

@@ -1775,9 +1775,11 @@ struct uv_loop_s {
   unsigned int active_handles;
   void* handle_queue[2];
   union {
-    void* unused[2];
+    void* unused;
     unsigned int count;
   } active_reqs;
+  /* Internal storage for future extensions. */
+  void* internal_fields;
   /* Internal flag to signal loop stop. */
   unsigned int stop_flag;
   UV_LOOP_PRIVATE_FIELDS

src/unix/loop.c

@@ -28,6 +28,7 @@
 #include <unistd.h>
 
 int uv_loop_init(uv_loop_t* loop) {
+  uv__loop_internal_fields_t* lfields;
   void* saved_data;
   int err;
 
@@ -36,6 +37,11 @@ int uv_loop_init(uv_loop_t* loop) {
   memset(loop, 0, sizeof(*loop));
   loop->data = saved_data;
 
+  lfields = (uv__loop_internal_fields_t*) uv__calloc(1, sizeof(*lfields));
+  if (lfields == NULL)
+    return UV_ENOMEM;
+  loop->internal_fields = lfields;
+
   heap_init((struct heap*) &loop->timer_heap);
   QUEUE_INIT(&loop->wq);
   QUEUE_INIT(&loop->idle_handles);
@@ -105,6 +111,8 @@ fail_rwlock_init:
 
 fail_signal_init:
   uv__platform_loop_delete(loop);
+  uv__free(lfields);
+  loop->internal_fields = NULL;
 
   uv__free(loop->watchers);
   loop->nwatchers = 0;
@@ -146,6 +154,8 @@ int uv_loop_fork(uv_loop_t* loop) {
 
 
 void uv__loop_close(uv_loop_t* loop) {
+  uv__loop_internal_fields_t* lfields;
+
   uv__signal_loop_cleanup(loop);
   uv__platform_loop_delete(loop);
   uv__async_stop(loop);
@@ -181,6 +191,10 @@ void uv__loop_close(uv_loop_t* loop) {
   uv__free(loop->watchers);
   loop->watchers = NULL;
   loop->nwatchers = 0;
+
+  lfields = uv__get_internal_fields(loop);
+  uv__free(lfields);
+  loop->internal_fields = NULL;
 }
 
 

src/uv-common.h

@@ -333,6 +333,9 @@ void uv__threadpool_cleanup(void);
   }                                                                           \
   while (0)
 
+#define uv__get_internal_fields(loop)                                         \
+  ((uv__loop_internal_fields_t*) loop->internal_fields)
+
 /* Allocator prototypes */
 void *uv__calloc(size_t count, size_t size);
 char *uv__strdup(const char* s);
@@ -342,4 +345,10 @@ void uv__free(void* ptr);
 void* uv__realloc(void* ptr, size_t size);
 void* uv__reallocf(void* ptr, size_t size);
 
+typedef struct uv__loop_internal_fields_s uv__loop_internal_fields_t;
+
+struct uv__loop_internal_fields_s {
+  unsigned int flags;
+};
+
 #endif /* UV_COMMON_H_ */

src/win/core.c

@@ -233,6 +233,11 @@ int uv_loop_init(uv_loop_t* loop) {
   if (loop->iocp == NULL)
     return uv_translate_sys_error(GetLastError());
 
+  lfields = (uv__loop_internal_fields_t*) uv__calloc(1, sizeof(*lfields));
+  if (lfields == NULL)
+    return UV_ENOMEM;
+  loop->internal_fields = lfields;
+
   /* To prevent uninitialized memory access, loop->time must be initialized
    * to zero before calling uv_update_time for the first time.
    */
@@ -297,6 +302,8 @@ fail_mutex_init:
   loop->timer_heap = NULL;
 
 fail_timers_alloc:
+  uv__free(lfields);
+  loop->internal_fields = NULL;
   CloseHandle(loop->iocp);
   loop->iocp = INVALID_HANDLE_VALUE;
 
@@ -347,6 +354,10 @@ void uv__loop_close(uv_loop_t* loop) {
   uv__free(loop->timer_heap);
   loop->timer_heap = NULL;
 
+  lfields = uv__get_internal_fields(loop);
+  uv__free(lfields);
+  loop->internal_fields = NULL;
+
   CloseHandle(loop->iocp);
 }