From 1537d6680bb66dc2484e11815bc2cd0e31ca39cc Mon Sep 17 00:00:00 2001
From: Roberto Ierusalimschy <roberto@inf.puc-rio.br>
Date: Fri, 26 Feb 2021 11:41:02 -0300
Subject: New control for reentrancy of emergency collections

Instead of assuming that shrinking a block may be an emergency
collection, use an explicit field ('gcstopem') to stop emergency
collections while GC is working.
---
 lmem.c | 25 ++++++++++++-------------
 1 file changed, 12 insertions(+), 13 deletions(-)

(limited to 'lmem.c')

diff --git a/lmem.c b/lmem.c
index e90f991a..9029d588 100644
--- a/lmem.c
+++ b/lmem.c
@@ -24,12 +24,12 @@
 
 #if defined(EMERGENCYGCTESTS)
 /*
-** First allocation will fail whenever not building initial state
-** and not shrinking a block. (This fail will trigger 'tryagain' and
-** a full GC cycle at every allocation.)
+** First allocation will fail whenever not building initial state.
+** (This fail will trigger 'tryagain' and a full GC cycle at every
+** allocation.)
 */
 static void *firsttry (global_State *g, void *block, size_t os, size_t ns) {
-  if (completestate(g) && ns > os)
+  if (completestate(g) && ns > 0)  /* frees never fail */
     return NULL;  /* fail */
   else  /* normal allocation */
     return (*g->frealloc)(g->ud, block, os, ns);
@@ -138,15 +138,17 @@ void luaM_free_ (lua_State *L, void *block, size_t osize) {
 
 
 /*
-** In case of allocation fail, this function will call the GC to try
-** to free some memory and then try the allocation again.
-** (It should not be called when shrinking a block, because then the
-** interpreter may be in the middle of a collection step.)
+** In case of allocation fail, this function will do an emergency
+** collection to free some memory and then try the allocation again.
+** The GC should not be called while state is not fully built, as the
+** collector is not yet fully initialized. Also, it should not be called
+** when 'gcstopem' is true, because then the interpreter is in the
+** middle of a collection step.
 */
 static void *tryagain (lua_State *L, void *block,
                        size_t osize, size_t nsize) {
   global_State *g = G(L);
-  if (completestate(g)) {  /* is state fully build? */
+  if (completestate(g) && !g->gcstopem) {
     luaC_fullgc(L, 1);  /* try to free some memory... */
     return (*g->frealloc)(g->ud, block, osize, nsize);  /* try again */
   }
@@ -156,8 +158,6 @@ static void *tryagain (lua_State *L, void *block,
 
 /*
 ** Generic allocation routine.
-** If allocation fails while shrinking a block, do not try again; the
-** GC shrinks some blocks and it is not reentrant.
 */
 void *luaM_realloc_ (lua_State *L, void *block, size_t osize, size_t nsize) {
   void *newblock;
@@ -165,8 +165,7 @@ void *luaM_realloc_ (lua_State *L, void *block, size_t osize, size_t nsize) {
   lua_assert((osize == 0) == (block == NULL));
   newblock = firsttry(g, block, osize, nsize);
   if (l_unlikely(newblock == NULL && nsize > 0)) {
-    if (nsize > osize)  /* not shrinking a block? */
-      newblock = tryagain(L, block, osize, nsize);
+    newblock = tryagain(L, block, osize, nsize);
     if (newblock == NULL)  /* still no memory? */
       return NULL;  /* do not update 'GCdebt' */
   }
-- 
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