core: fix cdata decrementing

When cdata has custom finalizer (and so LJ_GC_CDATA_FIN flag) it is not
collected immediately, when lj_cdata_free() is called. Instead, it is
resurrected and marked finalized, so it is collected at the next GC
cycle. The reason of the bug is that gc_cdatanum is decremented when
cdata is resurrected too (i.e. twice).

This patch excludes cdata decrementing from resurrection branch and
adds corresponding tests.

Follows up tarantool/tarantool#5187
Closes tarantool/tarantool#5820

Author: Buristan

src/lj_cdata.c

@@ -80,10 +80,11 @@ void LJ_FASTCALL lj_cdata_free(global_State *g, GCcdata *cd)
     lua_assert(ctype_hassize(ct->info) || ctype_isfunc(ct->info) ||
 	       ctype_isextern(ct->info));
     lj_mem_free(g, cd, sizeof(GCcdata) + sz);
+    g->gc.cdatanum--;
   } else {
     lj_mem_free(g, memcdatav(cd), sizecdatav(cd));
+    g->gc.cdatanum--;
   }
-  g->gc.cdatanum--;
 }
 
 void lj_cdata_setfin(lua_State *L, GCcdata *cd, GCobj *obj, uint32_t it)

test/misclib-getmetrics-capi.test.lua

@@ -14,7 +14,7 @@ local jit_opt_default = {
 local tap = require('tap')
 
 local test = tap.test("clib-misc-getmetrics")
-test:plan(10)
+test:plan(11)
 
 local testgetmetrics = require("testgetmetrics")
 
@@ -62,6 +62,19 @@ test:ok(testgetmetrics.objcount(function(iterations)
     jit.opt.start(unpack(jit_opt_default))
 end))
 
+test:ok(testgetmetrics.objcount_cdata_decrement(function()
+    -- cdata decrement test.
+    -- See https://github.com/tarantool/tarantool/issues/5820.
+    local ffi = require("ffi")
+    local function nop() end
+    ffi.gc(ffi.cast("void *", 0), nop)
+    -- Does not collect cdata, but removes LJ_GC_CDATA_FIN flag
+    -- and resurrects object.
+    collectgarbage()
+    -- Collects cdata.
+    collectgarbage()
+end))
+
 -- Compiled loop with a direct exit to the interpreter.
 test:ok(testgetmetrics.snap_restores(function()
     jit.opt.start(0, "hotloop=1")

test/misclib-getmetrics-capi/testgetmetrics.c

@@ -155,6 +155,33 @@ static int objcount(lua_State *L)
 	return 1;
 }
 
+static int objcount_cdata_decrement(lua_State *L)
+{
+	/*
+	 * cdata decrement test.
+	 * See https://github.com/tarantool/tarantool/issues/5820.
+	 */
+	struct luam_Metrics oldm, newm;
+	int n = lua_gettop(L);
+	if (n != 1 || !lua_isfunction(L, 1))
+		luaL_error(L, "incorrect argument: 1 function is required");
+
+	/* Force up garbage collect all dead objects. */
+	lua_gc(L, LUA_GCCOLLECT, 0);
+
+	luaM_metrics(L, &oldm);
+	/*
+	 * The function generates and collects cdata with
+	 * LJ_GC_CDATA_FIN flag.
+	 */
+	lua_call(L, 0, 0);
+	luaM_metrics(L, &newm);
+	assert(newm.gc_cdatanum - oldm.gc_cdatanum == 0);
+
+	lua_pushboolean(L, 1);
+	return 1;
+}
+
 static int snap_restores(lua_State *L)
 {
 	struct luam_Metrics oldm, newm;
@@ -229,6 +256,7 @@ static const struct luaL_Reg testgetmetrics[] = {
 	{"gc_allocated_freed", gc_allocated_freed},
 	{"gc_steps", gc_steps},
 	{"objcount", objcount},
+	{"objcount_cdata_decrement", objcount_cdata_decrement},
 	{"snap_restores", snap_restores},
 	{"strhash", strhash},
 	{"tracenum_base", tracenum_base},

test/misclib-getmetrics-lapi.test.lua

@@ -172,7 +172,7 @@ test:test("gc-steps", function(subtest)
 end)
 
 test:test("objcount", function(subtest)
-    subtest:plan(4)
+    subtest:plan(5)
     local ffi = require("ffi")
 
     jit.opt.start(0)
@@ -231,6 +231,19 @@ test:test("objcount", function(subtest)
     subtest:is(new_metrics.gc_cdatanum, old_metrics.gc_cdatanum,
                "cdatanum don't change")
 
+    -- cdata decrement test.
+    -- See https://github.com/tarantool/tarantool/issues/5820.
+    local function nop() end
+    local cdatanum_old = misc.getmetrics().gc_cdatanum
+    ffi.gc(ffi.cast("void *", 0), nop)
+    -- Does not collect cdata, but removes LJ_GC_CDATA_FIN flag
+    -- and resurrects object.
+    collectgarbage()
+    -- Collects cdata.
+    collectgarbage()
+    subtest:is(misc.getmetrics().gc_cdatanum, cdatanum_old,
+               "cdatanum is decremented correctly")
+
     -- Restore default jit settings.
     jit.opt.start(unpack(jit_opt_default))
 end)