Add support for atomvm_minimum_free_size key to system_info/1 on STM32 platform

Adds support for getting the current free heap size on STM32 platform using
`erlang:system_info(atomvm_minimum_free_size)`.

Signed-off-by: Winford <winford@object.stream>

Author: UncleGrumpy

CHANGELOG.md

@@ -49,6 +49,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Added Function.ex and Protocol.ex improving Elixir 1.18 support
 - Added WiFi support for ESP32P4 via esp-wifi-external for build with ESP-IDF v5.4 and later
 - Added Process.link/1 and unlink/1 to Elixir Process.ex
+- Added `erlang:system_info/1` keys `atomvm_free_heap_size` and `atomvm_minimum_free_size` on STM32 platform
 
 ### Changed
 

src/platforms/stm32/src/lib/sys.c

@@ -51,6 +51,7 @@ extern uint8_t _ebss, _stack;
 
 static uint8_t *_cur_brk = NULL;
 static uint8_t *_heap_end = NULL;
+static uint8_t *_heap_high_mark = NULL;
 
 /*
  * If not overridden, this puts the heap into the left
@@ -63,6 +64,7 @@ static void __local_ram(uint8_t **start, uint8_t **end)
 {
     *start = &_ebss;
     *end = (uint8_t *) (((uintptr_t) &_stack - RESERVE_STACK_SIZE));
+    _heap_high_mark = 0;
 }
 
 void *_sbrk_r(struct _reent *reent, ptrdiff_t diff)
@@ -79,6 +81,10 @@ void *_sbrk_r(struct _reent *reent, ptrdiff_t diff)
         return (void *) -1;
     }
     _cur_brk += diff;
+    if (_cur_brk > _heap_high_mark) {
+        _heap_high_mark = _cur_brk;
+    }
+
     return _old_brk;
 }
 
@@ -87,6 +93,11 @@ static int sys_get_free_heap()
     return (int) ((uint8_t *) (((uintptr_t) &_stack - RESERVE_STACK_SIZE))) - (_cur_brk == NULL ? (int) &_ebss : (int) _cur_brk);
 }
 
+static int sys_least_free_heap()
+{
+    return (int) ((uint8_t *) (((uintptr_t) &_stack - RESERVE_STACK_SIZE))) - (int) _heap_high_mark;
+}
+
 // Monotonically increasing number of milliseconds from reset
 static volatile uint64_t system_millis;
 
@@ -272,10 +283,12 @@ term sys_get_info(Context *ctx, term key)
     if (key == globalcontext_existing_term_from_atom_string(glb, ATOM_STR("\x15", "atomvm_free_heap_size"))) {
         return term_from_int32(sys_get_free_heap());
     }
+    if (key == globalcontext_existing_term_from_atom_string(glb, ATOM_STR("\x18", "atomvm_minimum_free_size"))) {
+        return term_from_int32(sys_least_free_heap());
+    }
     return UNDEFINED_ATOM;
 }
 
-
 void sys_enable_flash_cache()
 {
     flash_unlock_option_bytes();