process.c: fix ETXTBUSY from MJIT compiler process

This affects test/ruby/test_process.rb (test_execopt_env_path).

Since MJIT uses vfork+execve in a separate thread, there can be
small window in-between vfork and execve where tmp_script.cmd is
held open by the vforked child.  vfork only pauses the MJIT
thread, not any Ruby Threads, so our call to Process.spawn will
hit ETXTBUSY in that window unless we fork.

main thread                       | MJIT thread
----------------------------------------------------
fd = open(tmp)                    |                |
                                  | vfork for CC   |   CC running
write                             |                | ---------------
fchmod                            |                |  sees "fd" here
close(fd)                         |                |
    Process.spawn called          |                |
vfork  (spawn)|    (new process)  |                |
              | execve => TXTBUSY |                |
              |                   |                | execve (FD_CLOEXEC on fd)
              |                   | vfork returns  |

Holding the waitpid_lock whenever we intend to spawn a process
prevents the MJIT thread from spawning a process while we are
spawning in Ruby-land.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66171 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

process.c

@@ -916,6 +916,8 @@ do_waitpid(rb_pid_t pid, int *st, int flags)
 #endif
 }
 
+#define WAITPID_LOCK_ONLY ((struct waitpid_state *)-1)
+
 struct waitpid_state {
     struct list_node wnode;
     rb_execution_context_t *ec;
@@ -3923,12 +3925,13 @@ retry_fork_async_signal_safe(int *status, int *ep,
     volatile int try_gc = 1;
     struct child_handler_disabler_state old;
     int err;
+    rb_vm_t *vm = w && WAITPID_USE_SIGCHLD ? GET_VM() : 0;
 
     while (1) {
         prefork();
         disable_child_handler_before_fork(&old);
-        if (w && WAITPID_USE_SIGCHLD) {
-            rb_native_mutex_lock(&rb_ec_vm_ptr(w->ec)->waitpid_lock);
+        if (vm) {
+            rb_native_mutex_lock(&vm->waitpid_lock);
         }
 #ifdef HAVE_WORKING_VFORK
         if (!has_privilege())
@@ -3954,12 +3957,12 @@ retry_fork_async_signal_safe(int *status, int *ep,
 #endif
         }
 	err = errno;
-        if (w && WAITPID_USE_SIGCHLD) {
-            if (pid > 0) {
+        if (vm) {
+            if (pid > 0 && w != WAITPID_LOCK_ONLY) {
                 w->pid = pid;
-                list_add(&rb_ec_vm_ptr(w->ec)->waiting_pids, &w->wnode);
+                list_add(&vm->waiting_pids, &w->wnode);
             }
-            rb_native_mutex_unlock(&rb_ec_vm_ptr(w->ec)->waitpid_lock);
+            rb_native_mutex_unlock(&vm->waitpid_lock);
         }
 	disable_child_handler_fork_parent(&old);
         if (0 < pid) /* fork succeed, parent process */
@@ -3995,7 +3998,8 @@ fork_check_err(int *status, int (*chfunc)(void*, char *, size_t), void *charg,
     error_occurred = recv_child_error(ep[0], &err, errmsg, errmsg_buflen);
     if (error_occurred) {
         if (status) {
-            VM_ASSERT(w == 0 && "only used by extensions");
+            VM_ASSERT((w == 0 || w == WAITPID_LOCK_ONLY) &&
+                      "only used by extensions");
             rb_protect(proc_syswait, (VALUE)pid, status);
         }
         else if (!w) {
@@ -4340,7 +4344,7 @@ rb_spawn_process(struct rb_execarg *eargp, char *errmsg, size_t errmsg_buflen)
     rb_last_status_set((status & 0xff) << 8, 0);
     pid = 1;			/* dummy */
 # endif
-    if (eargp->waitpid_state) {
+    if (eargp->waitpid_state && eargp->waitpid_state != WAITPID_LOCK_ONLY) {
         eargp->waitpid_state->pid = pid;
     }
     rb_execarg_run_options(&sarg, NULL, errmsg, errmsg_buflen);
@@ -4369,6 +4373,15 @@ static rb_pid_t
 rb_execarg_spawn(VALUE execarg_obj, char *errmsg, size_t errmsg_buflen)
 {
     struct spawn_args args;
+    struct rb_execarg *eargp = rb_execarg_get(execarg_obj);
+
+    /*
+     * Prevent a race with MJIT where the compiler process where
+     * can hold an FD of ours in between vfork + execve
+     */
+    if (!eargp->waitpid_state && mjit_enabled) {
+        eargp->waitpid_state = WAITPID_LOCK_ONLY;
+    }
 
     args.execarg = execarg_obj;
     args.errmsg.ptr = errmsg;

test/ruby/test_process.rb

@@ -343,11 +343,6 @@ class TestProcess < Test::Unit::TestCase
   end
 
   def test_execopt_env_path
-    # http://ci.rvm.jp/results/trunk-mjit@silicon-docker/1455223
-    # http://ci.rvm.jp/results/trunk-mjit@silicon-docker/1450027
-    # http://ci.rvm.jp/results/trunk-mjit@silicon-docker/1469867
-    skip 'this randomly fails with MJIT' if RubyVM::MJIT.enabled?
-
     bug8004 = '[ruby-core:53103] [Bug #8004]'
     Dir.mktmpdir do |d|
       open("#{d}/tmp_script.cmd", "w") {|f| f.puts ": ;"; f.chmod(0755)}