avoid taking reopen_lock for every write

src/writer.c

@@ -185,10 +185,14 @@ wrt_write(tx_t *tx)
     CHECK_OBJ_NOTNULL(tx, TX_MAGIC);
     assert(tx->state == TX_SUBMITTED);
 
-    AZ(pthread_mutex_lock(&reopen_lock));
-    if (reopen && fo != stdout) {
+    while (reopen && fo != stdout) {
         int errnum;
 
+        AZ(pthread_mutex_lock(&reopen_lock));
+        if (! reopen) {
+            AZ(pthread_mutex_unlock(&reopen_lock));
+            break;
+        }
         wrt_return_freelist();
         if (fflush(fo) != 0)
             LOG_Log(LOG_ERR, "Cannot flush to %s, DATA DISCARDED: %s",
@@ -204,8 +208,9 @@ wrt_write(tx_t *tx)
             exit(EXIT_FAILURE);
         }
         reopen = 0;
+        AZ(pthread_mutex_unlock(&reopen_lock));
+        break;
     }
-    AZ(pthread_mutex_unlock(&reopen_lock));
 
     VRMB();
     os = FMT_Format(tx, &len);