Issue #12841: Fix tarfile extraction of non-existent uids/gids.

tarfile unnecessarily checked the existence of numerical user and group ids on
extraction. If one of them did not exist the respective id of the current user
(i.e. root) was used for the file and ownership information was lost. (Patch
by Sebastien Luttringer)

Lib/tarfile.py

@@ -2264,17 +2264,11 @@ class TarFile(object):
             try:
                 g = grp.getgrnam(tarinfo.gname)[2]
             except KeyError:
-                try:
-                    g = grp.getgrgid(tarinfo.gid)[2]
-                except KeyError:
-                    g = os.getgid()
+                g = tarinfo.gid
             try:
                 u = pwd.getpwnam(tarinfo.uname)[2]
             except KeyError:
-                try:
-                    u = pwd.getpwuid(tarinfo.uid)[2]
-                except KeyError:
-                    u = os.getuid()
+                u = tarinfo.uid
             try:
                 if tarinfo.issym() and hasattr(os, "lchown"):
                     os.lchown(targetpath, u, g)

Misc/NEWS

@@ -40,6 +40,11 @@ Core and Builtins
 Library
 -------
 
+- Issue #12841: tarfile unnecessarily checked the existence of numerical user
+  and group ids on extraction. If one of them did not exist the respective id
+  of the current user (i.e. root) was used for the file and ownership
+  information was lost.
+
 - Issue #10946: The distutils commands bdist_dumb, bdist_wininst and bdist_msi
   now respect a --skip-build option given to bdist.