heap: Support adding new heap regions at runtime

To facilitate this, the list of registered heap regions is now a linked list
(allowing entries to be appended at runtime.)

components/heap/heap_private.h

@@ -18,23 +18,36 @@
 #include <freertos/FreeRTOS.h>
 #include <soc/soc_memory_layout.h>
 #include "multi_heap.h"
+#include "rom/queue.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
 
 /* Some common heap registration data structures used
    for heap_caps_init.c to share heap information with heap_caps.c
 */
 
 /* Type for describing each registered heap */
-typedef struct {
-    size_t type;
+typedef struct heap_t_ {
     uint32_t caps[SOC_MEMORY_TYPE_NO_PRIOS]; ///< Capabilities for the type of memory in this heap (as a prioritised set). Copied from soc_memory_types so it's in RAM not flash.
     intptr_t start;
     intptr_t end;
     portMUX_TYPE heap_mux;
     multi_heap_handle_t heap;
+    SLIST_ENTRY(heap_t_) next;
 } heap_t;
 
-extern heap_t *registered_heaps;
-extern size_t num_registered_heaps;
+/* All registered heaps.
+
+   Forms a single linked list, even though most entries are contiguous.
+   This means at the expense of 4 bytes per heap, new heaps can be
+   added at runtime in a fast & thread-safe way.
+*/
+extern SLIST_HEAD(registered_heap_ll, heap_t_) registered_heaps;
 
 bool heap_caps_match(const heap_t *heap, uint32_t caps);
 
+#ifdef __cplusplus
+}
+#endif