Fix deadlock on pthread_atfork with glibc >= 2.28 (#31)

pthread_atfork cannot be safely entered from atfork handlers - the
behavior is generally deemed as undefined, and while it worked on glibc
2.27 and older, it no longer works since a refactoring in 2.28 to
simplify the atfork logic (see
https://sourceware.org/git/?p=glibc.git;a=commit;h=27761a1042daf01987e7d79636d0c41511c6df3c).

This change may only be safe with glibc (since glibc takes care of
reregistering the atfork handlers in the forked process) and additional
care may be needed if supporting other libc implementations is desired.

Behavior is undefined: https://elias.rhi.hi.is/libc/Threads-and-Fork.html
In depth discussion: https://sourceware.org/bugzilla/show_bug.cgi?id=24595#c1
Fix recommendation: https://bugzilla.redhat.com/show_bug.cgi?id=1888660

Author: iXce

src/posix/manager.c

@@ -126,22 +126,8 @@ worker_idle_threshold_per_cpu(void)
     return 2;
 }
 
-#if !defined(__ANDROID__)
-static void
-manager_reinit(void)
-{
-    if (manager_init() < 0)
-        abort();
-
-    for (size_t i = 0; i < PTHREAD_WORKQUEUE_MAX; i++) {
-        wqlist[i] = NULL;
-        ocwq[i] = NULL;
-    }
-}
-#endif
-
 int
-manager_init(void)
+_manager_init_common(void)
 {
     wqlist_has_manager = 0;
     pthread_cond_init(&wqlist_has_work, NULL);
@@ -183,6 +169,31 @@ manager_init(void)
         worker_min = cpu_count > 1 ? cpu_count : 2;
     worker_idle_threshold = (PWQ_ACTIVE_CPU > 0) ? (PWQ_ACTIVE_CPU) : worker_idle_threshold_per_cpu();
 
+    return (0);
+}
+
+#if !defined(__ANDROID__)
+static void
+manager_reinit(void)
+{
+    if (_manager_init_common() < 0)
+        abort();
+
+    for (size_t i = 0; i < PTHREAD_WORKQUEUE_MAX; i++) {
+        wqlist[i] = NULL;
+        ocwq[i] = NULL;
+    }
+}
+#endif
+
+int
+manager_init(void)
+{
+    int status = _manager_init_common();
+    if (status != 0) {
+        return status;
+    }
+
 /* FIXME: should test for symbol instead of for Android */
 #if !defined(__ANDROID__)
     if (pthread_atfork(NULL, NULL, manager_reinit) < 0) {