CRE: avoid assert failure in sub-processes (#1766)

Will allow frontend to disable CRE finalizers when
running code in a subprocess and explicitely exiting
quickly without any proper cleanup.

cre.cpp

@@ -4108,6 +4108,13 @@ static int freeImage(lua_State *L) {
     return 0;
 }
 
+static bool skip_teardown = false;
+
+static int setSkipTearDown(lua_State *L) {
+    skip_teardown = lua_toboolean(L, 1);
+    return 0;
+}
+
 static const struct luaL_Reg cre_func[] = {
     {"initCache", initCache},
     {"initHyphDict", initHyphDict},
@@ -4133,6 +4140,7 @@ static const struct luaL_Reg cre_func[] = {
     {"renderImageData", renderImageData},
     {"smoothScaleBlitBuffer", smoothScaleBlitBuffer},
     {"setImageReplacementChar", setImageReplacementChar},
+    {"setSkipTearDown", setSkipTearDown},
     {NULL, NULL}
 };
 
@@ -4302,6 +4310,8 @@ int luaopen_cre(lua_State *L) {
 
 // Library finalizer (c.f., dlopen(3)). This serves no real purpose except making Valgrind's output slightly more useful.
 __attribute__((destructor)) static void cre_teardown(void) {
+    if (skip_teardown)
+        return;
     if (cre_callback_forwarder) {
         delete cre_callback_forwarder;
         cre_callback_forwarder = NULL;

ffi/util.lua

@@ -299,6 +299,16 @@ function util.execute(...)
     end
 end
 
+-- Frontend can register functions to be run in all subprocesses
+-- just after the fork, ie. for some cleanup work.
+local _run_in_subprocess_after_fork_funcs = {}
+function util.addRunInSubProcessAfterForkFunc(id, func)
+    _run_in_subprocess_after_fork_funcs[id] = func
+end
+function util.removeRunInSubProcessAfterForkFunc(id)
+    _run_in_subprocess_after_fork_funcs[id] = nil
+end
+
 --- Run lua code (func) in a forked subprocess
 --
 -- With with_pipe=true, sets up a pipe for communication
@@ -329,6 +339,9 @@ function util.runInSubProcess(func, with_pipe, double_fork)
     end
     local pid = C.fork()
     if pid == 0 then -- child process
+        for _, f in pairs(_run_in_subprocess_after_fork_funcs) do
+            f()
+        end
         if double_fork then
             pid = C.fork()
             if pid ~= 0 then