fix places-GC trigger when a large message is pending

A large message that hasn't been delivered can trigger a inter-place
GC. The intent is to force a GC to avoid messages piling up that can
never be delivered, but the GC didn't adjust to a state where messages
stay both undelivered and uncollected, and it would continuosly
trigger GCs. Trigger a GC only if the pending-message size has grown
relative to the previous GC.

Author: mflatt

racket/src/racket/gc2/newgc.c

@@ -932,7 +932,7 @@ void GC_check_master_gc_request() {
 
   if (mgc) {
     /* check for GC needed due to GC_report_unsent_message_delta(): */
-    if ((mgc->gen0.current_size + mgc->pending_msg_size) >= mgc->gen0.max_size) {
+    if ((mgc->gen0.current_size + mgc->pending_msg_size) >= (mgc->gen0.max_size + mgc->prev_pending_msg_size)) {
       NewGC *gc = GC_get_GC();
 
       if (!postmaster_and_master_gc(gc))
@@ -2749,6 +2749,8 @@ static void collect_master(Log_Master_Info *lmi) {
     GCVERBOSEprintf(gc, "END MASTER COLLECTION\n");
 #endif
 
+    MASTERGC->prev_pending_msg_size = MASTERGC->pending_msg_size;
+
     {
       int i = 0;
       int alive = MASTERGCINFO->alive;

racket/src/racket/gc2/newgc.h

@@ -153,6 +153,7 @@ typedef struct NewGC {
   void (*unsafe_allocation_abort)(struct NewGC *);
   uintptr_t memory_in_use; /* the amount of memory in use */
   uintptr_t pending_msg_size; /* set in master, only */
+  uintptr_t prev_pending_msg_size; /* set in master, only */
 
   /* blame the child thread infos */
   GC_Thread_Info *thread_infos;