tarfile.open() with mode 'x' created files without an end of archive marker.
diff --git a/Lib/tarfile.py b/Lib/tarfile.py
index 0f1c825..bf2234f 100755
--- a/Lib/tarfile.py
+++ b/Lib/tarfile.py
@@ -1484,7 +1484,7 @@
                     except HeaderError as e:
                         raise ReadError(str(e))
 
-            if self.mode in "aw":
+            if self.mode in ("a", "w", "x"):
                 self._loaded = True
 
                 if self.pax_headers:
@@ -1716,7 +1716,7 @@
 
         self.closed = True
         try:
-            if self.mode in "aw":
+            if self.mode in ("a", "w", "x"):
                 self.fileobj.write(NUL * (BLOCKSIZE * 2))
                 self.offset += (BLOCKSIZE * 2)
                 # fill up the end with zero-blocks