[3.9] bpo-45863: tarfile: don't zero out header fields unnecessarily (GH-29693)

Lib/tarfile.py

@@ -887,15 +887,24 @@ def create_pax_header(self, info, encoding):
         # Test number fields for values that exceed the field limit or values
         # that like to be stored as float.
         for name, digits in (("uid", 8), ("gid", 8), ("size", 12), ("mtime", 12)):
-            if name in pax_headers:
-                # The pax header has priority. Avoid overflow.
-                info[name] = 0
-                continue
+            needs_pax = False
 
             val = info[name]
-            if not 0 <= val < 8 ** (digits - 1) or isinstance(val, float):
-                pax_headers[name] = str(val)
+            val_is_float = isinstance(val, float)
+            val_int = round(val) if val_is_float else val
+            if not 0 <= val_int < 8 ** (digits - 1):
+                # Avoid overflow.
                 info[name] = 0
+                needs_pax = True
+            elif val_is_float:
+                # Put rounded value in ustar header, and full
+                # precision value in pax header.
+                info[name] = val_int
+                needs_pax = True
+
+            # The existing pax header has priority.
+            if needs_pax and name not in pax_headers:
+                pax_headers[name] = str(val)
 
         # Create a pax extended header if necessary.
         if pax_headers:

Lib/test/test_tarfile.py

@@ -1898,6 +1898,61 @@ def test_pax_extended_header(self):
         finally:
             tar.close()
 
+    def test_create_pax_header(self):
+        # The ustar header should contain values that can be
+        # represented reasonably, even if a better (e.g. higher
+        # precision) version is set in the pax header.
+        # Issue #45863
+
+        # values that should be kept
+        t = tarfile.TarInfo()
+        t.name = "foo"
+        t.mtime = 1000.1
+        t.size = 100
+        t.uid = 123
+        t.gid = 124
+        info = t.get_info()
+        header = t.create_pax_header(info, encoding="iso8859-1")
+        self.assertEqual(info['name'], "foo")
+        # mtime should be rounded to nearest second
+        self.assertIsInstance(info['mtime'], int)
+        self.assertEqual(info['mtime'], 1000)
+        self.assertEqual(info['size'], 100)
+        self.assertEqual(info['uid'], 123)
+        self.assertEqual(info['gid'], 124)
+        self.assertEqual(header,
+            b'././@PaxHeader' + bytes(86) \
+            + b'0000000\x000000000\x000000000\x0000000000020\x0000000000000\x00010205\x00 x' \
+            + bytes(100) + b'ustar\x0000'+ bytes(247) \
+            + b'16 mtime=1000.1\n' + bytes(496) + b'foo' + bytes(97) \
+            + b'0000644\x000000173\x000000174\x0000000000144\x0000000001750\x00006516\x00 0' \
+            + bytes(100) + b'ustar\x0000' + bytes(247))
+
+        # values that should be changed
+        t = tarfile.TarInfo()
+        t.name = "foo\u3374" # can't be represented in ascii
+        t.mtime = 10**10 # too big
+        t.size = 10**10 # too big
+        t.uid = 8**8 # too big
+        t.gid = 8**8+1 # too big
+        info = t.get_info()
+        header = t.create_pax_header(info, encoding="iso8859-1")
+        # name is kept as-is in info but should be added to pax header
+        self.assertEqual(info['name'], "foo\u3374")
+        self.assertEqual(info['mtime'], 0)
+        self.assertEqual(info['size'], 0)
+        self.assertEqual(info['uid'], 0)
+        self.assertEqual(info['gid'], 0)
+        self.assertEqual(header,
+            b'././@PaxHeader' + bytes(86) \
+            + b'0000000\x000000000\x000000000\x0000000000130\x0000000000000\x00010207\x00 x' \
+            + bytes(100) + b'ustar\x0000' + bytes(247) \
+            + b'15 path=foo\xe3\x8d\xb4\n16 uid=16777216\n' \
+            + b'16 gid=16777217\n20 size=10000000000\n' \
+            + b'21 mtime=10000000000\n'+ bytes(424) + b'foo?' + bytes(96) \
+            + b'0000644\x000000000\x000000000\x0000000000000\x0000000000000\x00006540\x00 0' \
+            + bytes(100) + b'ustar\x0000' + bytes(247))
+
 
 class UnicodeTest:
 

Misc/NEWS.d/next/Library/2022-02-09-00-53-23.bpo-45863.zqQXVv.rst

@@ -0,0 +1 @@
+When the :mod:`tarfile` module creates a pax format archive, it will put an integer representation of timestamps in the ustar header (if possible) for the benefit of older unarchivers, in addition to the existing full-precision timestamps in the pax extended header.