[BUGFIX] LWIP system mailbox overflow fix

Author: pstolarz

features/lwipstack/lwip-sys/arch/lwip_sys_arch.c

@@ -116,6 +116,9 @@ u32_t sys_now(void) {
 /* CMSIS-RTOS implementation of the lwip operating system abstraction */
 #include "arch/sys_arch.h"
 
+/* modulus subtract: (a - b) mod m, where a, b belongs to mod m ring */
+#define SUB_MOD(a, b, m) ((a) >= (b) ? (a) - (b) : (m) - (b) + (a))
+
 /*---------------------------------------------------------------------------*
  * Routine:  sys_mbox_new
  *---------------------------------------------------------------------------*
@@ -174,11 +177,11 @@ void sys_mbox_post(sys_mbox_t *mbox, void *msg) {
 
     int state = osKernelLock();
 
-    mbox->queue[mbox->post_idx % MB_SIZE] = msg;
-    mbox->post_idx += 1;
+    mbox->queue[mbox->post_idx] = msg;
+    mbox->post_idx = (mbox->post_idx + 1) % MB_SIZE;
 
     osEventFlagsSet(mbox->id, SYS_MBOX_FETCH_EVENT);
-    if (mbox->post_idx - mbox->fetch_idx == MB_SIZE-1)
+    if (SUB_MOD(mbox->post_idx, mbox->fetch_idx, MB_SIZE) >= MB_SIZE-1)
         osEventFlagsClear(mbox->id, SYS_MBOX_POST_EVENT);
 
     osKernelRestoreLock(state);
@@ -207,11 +210,11 @@ err_t sys_mbox_trypost(sys_mbox_t *mbox, void *msg) {
 
     int state = osKernelLock();
 
-    mbox->queue[mbox->post_idx % MB_SIZE] = msg;
-    mbox->post_idx += 1;
+    mbox->queue[mbox->post_idx] = msg;
+    mbox->post_idx = (mbox->post_idx + 1) % MB_SIZE;
 
     osEventFlagsSet(mbox->id, SYS_MBOX_FETCH_EVENT);
-    if (mbox->post_idx - mbox->fetch_idx == MB_SIZE-1)
+    if (SUB_MOD(mbox->post_idx, mbox->fetch_idx, MB_SIZE) >= MB_SIZE-1)
         osEventFlagsClear(mbox->id, SYS_MBOX_POST_EVENT);
 
     osKernelRestoreLock(state);
@@ -261,8 +264,8 @@ u32_t sys_arch_mbox_fetch(sys_mbox_t *mbox, void **msg, u32_t timeout) {
     int state = osKernelLock();
 
     if (msg)
-        *msg = mbox->queue[mbox->fetch_idx % MB_SIZE];
-    mbox->fetch_idx += 1;
+        *msg = mbox->queue[mbox->fetch_idx];
+    mbox->fetch_idx = (mbox->fetch_idx + 1) % MB_SIZE;
 
     osEventFlagsSet(mbox->id, SYS_MBOX_POST_EVENT);
     if (mbox->post_idx == mbox->fetch_idx)
@@ -297,8 +300,8 @@ u32_t sys_arch_mbox_tryfetch(sys_mbox_t *mbox, void **msg) {
     int state = osKernelLock();
 
     if (msg)
-        *msg = mbox->queue[mbox->fetch_idx % MB_SIZE];
-    mbox->fetch_idx += 1;
+        *msg = mbox->queue[mbox->fetch_idx];
+    mbox->fetch_idx = (mbox->fetch_idx + 1) % MB_SIZE;
 
     osEventFlagsSet(mbox->id, SYS_MBOX_POST_EVENT);
     if (mbox->post_idx == mbox->fetch_idx)