Fix ordering of pthread_create(pthread_t *thread)

This change fixes a bug where signal_latency_async_test would flake less
than 1/1000 of the time. What was happening was pthread_kill(sender_thr)
would return EFAULT. This was because pthread_create() was not returning
the thread object pointer until after clone() had been called. So it was
actually possible for the main thread to stall after calling clone() and
during that time the receiver would launch and receive a signal from the
sender thread, and then fail when it tried to send a pong. I thought I'd
use a barrier at first, in the test, to synchronize thread creation, but
I firmly believe that pthread_create() was to blame and now that's fixed

libc/thread/pthread_create.c

@@ -199,14 +199,12 @@ static errno_t pthread_create_impl(pthread_t *thread,
                                    const pthread_attr_t *attr,
                                    void *(*start_routine)(void *), void *arg,
                                    sigset_t oldsigs) {
-  int rc, e = errno;
+  errno_t err;
   struct PosixThread *pt;
 
   // create posix thread object
-  if (!(pt = calloc(1, sizeof(struct PosixThread)))) {
-    errno = e;
+  if (!(pt = calloc(1, sizeof(struct PosixThread))))
     return EAGAIN;
-  }
   dll_init(&pt->list);
   pt->pt_locale = &__global_locale;
   pt->pt_start = start_routine;
@@ -215,7 +213,6 @@ static errno_t pthread_create_impl(pthread_t *thread,
   // create thread local storage memory
   if (!(pt->pt_tls = _mktls(&pt->tib))) {
     free(pt);
-    errno = e;
     return EAGAIN;
   }
 
@@ -232,9 +229,9 @@ static errno_t pthread_create_impl(pthread_t *thread,
     // caller supplied their own stack
     // assume they know what they're doing as much as possible
     if (IsOpenbsd()) {
-      if ((rc = FixupCustomStackOnOpenbsd(&pt->pt_attr))) {
+      if (!FixupCustomStackOnOpenbsd(&pt->pt_attr)) {
         _pthread_free(pt);
-        return rc;
+        return EPERM;
       }
     }
   } else {
@@ -259,7 +256,7 @@ static errno_t pthread_create_impl(pthread_t *thread,
       if (!(pt->pt_attr.__sigaltstackaddr =
                 malloc(pt->pt_attr.__sigaltstacksize))) {
         _pthread_free(pt);
-        return errno;
+        return EAGAIN;
       }
       pt->pt_flags |= PT_OWNSIGALTSTACK;
     }
@@ -282,35 +279,41 @@ static errno_t pthread_create_impl(pthread_t *thread,
                             memory_order_relaxed);
       break;
     default:
-      _pthread_free(pt);
-      return EINVAL;
+      // pthread_attr_setdetachstate() makes this impossible
+      __builtin_unreachable();
   }
 
+  // if pthread_attr_setdetachstate() was used then it's possible for
+  // the `pt` object to be freed before this clone call has returned!
+  atomic_store_explicit(&pt->pt_refs, 1, memory_order_relaxed);
+
   // add thread to global list
   // we add it to the beginning since zombies go at the end
   _pthread_lock();
   dll_make_first(&_pthread_list, &pt->list);
   _pthread_unlock();
 
-  // if pthread_attr_setdetachstate() was used then it's possible for
-  // the `pt` object to be freed before this clone call has returned!
-  _pthread_ref(pt);
+  // we don't normally do this, but it's important to write the result
+  // memory before spawning the thread, so it's visible to the threads
+  *thread = (pthread_t)pt;
 
   // launch PosixThread(pt) in new thread
-  if ((rc = clone(
+  if ((err = clone(
            PosixThread, pt->pt_attr.__stackaddr, pt->pt_attr.__stacksize,
            CLONE_VM | CLONE_THREAD | CLONE_FS | CLONE_FILES | CLONE_SIGHAND |
                CLONE_SYSVSEM | CLONE_SETTLS | CLONE_PARENT_SETTID |
                CLONE_CHILD_SETTID | CLONE_CHILD_CLEARTID,
            pt, &pt->tib->tib_ptid, __adj_tls(pt->tib), &pt->tib->tib_ctid))) {
+    *thread = 0;  // posix doesn't require we do this
     _pthread_lock();
     dll_remove(&_pthread_list, &pt->list);
     _pthread_unlock();
     _pthread_free(pt);
-    return rc;
+    if (err == ENOMEM)
+      err = EAGAIN;
+    return err;
   }
 
-  *thread = (pthread_t)pt;
   return 0;
 }
 
@@ -359,7 +362,7 @@ static const char *DescribeHandle(char buf[12], errno_t err, pthread_t *th) {
  *                 └──────────────┘
  *
  * @param thread is used to output the thread id upon success, which
- *     must be non-null
+ *     must be non-null; upon failure, its value is undefined
  * @param attr points to launch configuration, or may be null
  *     to use sensible defaults; it must be initialized using
  *     pthread_attr_init()
@@ -375,6 +378,7 @@ static const char *DescribeHandle(char buf[12], errno_t err, pthread_t *th) {
 errno_t pthread_create(pthread_t *thread, const pthread_attr_t *attr,
                        void *(*start_routine)(void *), void *arg) {
   errno_t err;
+  errno_t olderr = errno;
   _pthread_decimate(kPosixThreadZombie);
   BLOCK_SIGNALS;
   err = pthread_create_impl(thread, attr, start_routine, arg, _SigMask);
@@ -382,7 +386,10 @@ errno_t pthread_create(pthread_t *thread, const pthread_attr_t *attr,
   STRACE("pthread_create([%s], %p, %t, %p) → %s",
          DescribeHandle(alloca(12), err, thread), attr, start_routine, arg,
          DescribeErrno(err));
-  if (!err)
+  if (!err) {
     _pthread_unref(*(struct PosixThread **)thread);
+  } else {
+    errno = olderr;
+  }
   return err;
 }