Move repack() state checks under the lock and merge the handle errors

The fp/_writing/_fileRefCnt checks guarded state that repack() then mutates
inside `with self._lock`, so the check and the `_writing = True` it gates were
not atomic. Move them into the lock. The writing-handle and reading-handle
errors describe the same condition, so state it once.

Author: fedonman

Doc/library/zipfile.rst

@@ -583,7 +583,7 @@ ZipFile objects
    Rewrites the archive to remove unreferenced local file entries, shrinking
    its file size.  The archive must be opened with mode ``'a'``, and any file
    object returned by :meth:`ZipFile.open` must be closed first, since
-   repacking moves the member data those objects read from.
+   repacking moves the member data such objects refer to.
 
    If *removed* is provided, it must be a sequence of :class:`ZipInfo` objects
    representing the recently removed members, and only their corresponding

Lib/zipfile/__init__.py

@@ -2397,20 +2397,16 @@ def repack(self, removed=None, *, strict_descriptor=True,
         truncation."""
         if self.mode != 'a':
             raise ValueError("repack() requires mode 'a'")
-        if not self.fp:
-            raise ValueError(
-                "Attempt to write to ZIP archive that was already closed")
-        if self._writing:
-            raise ValueError(
-                "Can't write to ZIP archive while an open writing handle exists"
-            )
-        if self._fileRefCnt > 1:
-            raise ValueError(
-                "Can't repack the ZIP archive while an open reading handle "
-                "exists. Close the reading handle before repacking."
-            )
 
         with self._lock:
+            if not self.fp:
+                raise ValueError(
+                    "Attempt to write to ZIP archive that was already closed")
+            if self._writing or self._fileRefCnt > 1:
+                raise ValueError(
+                    "Can't repack ZIP archive while an open handle exists"
+                )
+
             self._writing = True
             try:
                 repacker = _ZipRepacker(