[WEKA] Calling fork() and D runtime

[Jonathan Shamir via Digitalmars-d]

Hey,

This is my first time writing in the D forums!

I have an application written in D that runs as a linux daemon 
(some python service script is in charge of running and 
daemonizing it).
This "agent" works similar to docker - a service that accepts 
commands and runs in the background, and starts our application 
container. The container is itself a daemon (ppid = 1) with 
unshared namespaces etc.

So, normally, implementing such an application would look 
something like:
1. Main "agent" process runs fork() to create a child process 
(child_1). All memory is copy-on-write.
2. Child_1 malloc()s a stack, and calls clone() to create yet 
another child (child_2), which will eventually become the 
container pid 1.
3. Child_2 initializes the container (mounts, unshare, chroot, 
etc) then eventually exec()s into the container init process.
4. child_1 exit()s, which causes child_2 to become a daemon.
5. The agent main process should wait() on the forked pid since 
it's impolite to leave zombies (I do this in a thread).

The problem I encounter is with the forked process (child_1).

Here is the code I wrote handling the fork() (Note: this 
functionality should really be provided by core.threads or 
something, for unix environments).

```private void deferToForkProcess(int delegate() entryPoint, 
Timeout timeout = Timeout.infinite) {
     import core.runtime : Runtime;
     import core.sys.posix.unistd : fork, pid_t;
     import core.sys.posix.sys.wait;
     import core.stdc.stdlib : exit;

     int rc = theReactor.deferToThread({
         pid_t pid = fork();
         errnoEnforce(pid >= 0, "Fork failed");

         // child process
         if (pid == 0) {
             try {
                 int rc = entryPoint();
                 exit(rc);
             } catch (Throwable ex) {
                 try {
                     LOG_ERROR(ex.toString);
                 } catch (Throwable) {}
                 exit(1);
             }
             //assert(false);
         }

         // parent - wait for child to exit

         int status = 0;
         do {
             errnoEnforce(waitpid(pid, &status, 0) != -1, "Waitpid 
failed");
         } while (!WIFEXITED(status));

         int rc = WEXITSTATUS(status);
         return rc;
     }, timeout);

     enforce(rc == 0, "Child process failed (rc = %d)".format(rc));
}
```

entryPoint() returns 0, but the exit(0) raises an 
OutOfMemoryError:
```0x4e6472
exit
??:0
0x4e6428
__run_exit_handlers
??:0
0x4df976
__libc_csu_fini
??:0
0x40327e
ldc.register_dso
crtstuff.c:0
0x4caee4
_d_dso_registry
??:0
0x4ccdba
_D2rt4util9container6common8xreallocFNbNiPvmZPv
??:0
0x4b873d
onOutOfMemoryError
??:0```

I tried to call Runtime.initialize() and Runtime.terminate() 
surrounding the call to entryPoint(), but this didn't help. I 
suspect calling initialize() was a no-op since the forked process 
shares (copy-on-write) the VM space with it's parent, that 
already initialized the runtime. (Note: confirmed, initialize() 
returns true indicating it was already inited).

What is the correct way to handle fork() with the D runtime?