ext/socket: Set raddrinfo thread as detached before thread start (#15142)

We were seeing segfaults when calling `pthread_detach`. Apparently in
some versions of glibc there is a race between when this is called
(usually right after starting a thread) and a short-lived thread's
shutdown routine. The bug has been reported to glibc:

https://sourceware.org/bugzilla/show_bug.cgi?id=19951

I haven't been able to reproduce it on my Linux desktop but apparently
it's easier to reproduce on certain kinds of servers.

As a workaround, we can set the thread's detach state before thread
start. I don't know of a platform that doesn't have
`pthread_attr_setdetachstate`, but to be safe we check for it in
`extconf.rb` and use `pthread_detach` as a backup if it isn't available.

Fixes [Bug #21679]

3 files changed, 38 insertions, 4 deletions

diff --git a/ext/socket/extconf.rb b/ext/socket/extconf.rb
index d44ce31b0a..a814e21c3a 100644
--- a/ext/socket/extconf.rb
+++ b/ext/socket/extconf.rb
@@ -704,6 +704,7 @@ SRC
 
   have_func("pthread_create")
   have_func("pthread_detach")
+  have_func("pthread_attr_setdetachstate")
 
   $VPATH << '$(topdir)' << '$(top_srcdir)'
   create_makefile("socket")
diff --git a/ext/socket/ipsocket.c b/ext/socket/ipsocket.c
index fa41d89936..a95f5767d2 100644
--- a/ext/socket/ipsocket.c
+++ b/ext/socket/ipsocket.c
@@ -705,7 +705,6 @@ init_fast_fallback_inetsock_internal(VALUE v)
             if (raddrinfo_pthread_create(&threads[i], fork_safe_do_fast_fallback_getaddrinfo, arg->getaddrinfo_entries[i]) != 0) {
                 rsock_raise_resolution_error("getaddrinfo(3)", EAI_AGAIN);
             }
-            pthread_detach(threads[i]);
         }
 
         if (NIL_P(resolv_timeout)) {
diff --git a/ext/socket/raddrinfo.c b/ext/socket/raddrinfo.c
index 22ab34a073..e4e40e591d 100644
--- a/ext/socket/raddrinfo.c
+++ b/ext/socket/raddrinfo.c
@@ -496,13 +496,49 @@ int
 raddrinfo_pthread_create(pthread_t *th, void *(*start_routine) (void *), void *arg)
 {
     int limit = 3, ret;
+    int saved_errno;
+#ifdef HAVE_PTHREAD_ATTR_SETDETACHSTATE
+    pthread_attr_t attr;
+    pthread_attr_t *attr_p = &attr;
+    int err;
+    int init_retries = 0;
+    int init_retries_max = 3;
+retry_attr_init:
+    if ((err = pthread_attr_init(attr_p)) != 0) {
+        if (err == ENOMEM && init_retries < init_retries_max) {
+            init_retries++;
+            rb_gc();
+            goto retry_attr_init;
+        }
+        return err;
+    }
+    if ((err = pthread_attr_setdetachstate(attr_p, PTHREAD_CREATE_DETACHED)) != 0) {
+        saved_errno = errno;
+        pthread_attr_destroy(attr_p);
+        errno = saved_errno;
+        return err; // EINVAL - shouldn't happen
+    }
+#else
+    pthread_attr_t *attr_p = NULL;
+#endif
     do {
         // It is said that pthread_create may fail spuriously, so we follow the JDK and retry several times.
         //
         // https://bugs.openjdk.org/browse/JDK-8268605
         // https://github.com/openjdk/jdk/commit/e35005d5ce383ddd108096a3079b17cb0bcf76f1
-        ret = pthread_create(th, 0, start_routine, arg);
+        ret = pthread_create(th, attr_p, start_routine, arg);
     } while (ret == EAGAIN && limit-- > 0);
+#ifdef HAVE_PTHREAD_ATTR_SETDETACHSTATE
+    saved_errno = errno;
+    pthread_attr_destroy(attr_p);
+    if (ret != 0) {
+        errno = saved_errno;
+    }
+#else
+    if (ret == 0) {
+        pthread_detach(th); // this can race with shutdown routine of thread in some glibc versions
+    }
+#endif
     return ret;
 }
 
@@ -534,7 +570,6 @@ start:
         errno = err;
         return EAI_SYSTEM;
     }
-    pthread_detach(th);
 
     rb_thread_call_without_gvl2(wait_getaddrinfo, arg, cancel_getaddrinfo, arg);
 
@@ -770,7 +805,6 @@ start:
         errno = err;
         return EAI_SYSTEM;
     }
-    pthread_detach(th);
 
     rb_thread_call_without_gvl2(wait_getnameinfo, arg, cancel_getnameinfo, arg);