pthread: use recursive mutexes to allow callbacks to enqueue new c-ares queries. Regression from 1.31.0.

src/lib/ares__threads.c

@@ -219,18 +219,30 @@ struct ares__thread_mutex {
 
 ares__thread_mutex_t *ares__thread_mutex_create(void)
 {
+  pthread_mutexattr_t   attr;
   ares__thread_mutex_t *mut = ares_malloc_zero(sizeof(*mut));
   if (mut == NULL) {
     return NULL;
   }
 
-  if (pthread_mutex_init(&mut->mutex, NULL) != 0) {
-    goto fail;
+  if (pthread_mutexattr_init(&attr) != 0) {
+    ares_free(mut); /* LCOV_EXCL_LINE: UntestablePath */
+    return NULL;    /* LCOV_EXCL_LINE: UntestablePath */
   }
 
+  if (pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE) != 0) {
+    goto fail; /* LCOV_EXCL_LINE: UntestablePath */
+  }
+
+  if (pthread_mutex_init(&mut->mutex, &attr) != 0) {
+    goto fail; /* LCOV_EXCL_LINE: UntestablePath */
+  }
+
+  pthread_mutexattr_destroy(&attr);
   return mut;
 
 fail:
+  pthread_mutexattr_destroy(&attr);
   ares_free(mut);
   return NULL;
 }