bpo-18819: tarfile: only set device fields for device files (GH-18080)

The GNU docs describe the `devmajor` and `devminor` fields of the tar
header struct only in the context of character and block special files,
suggesting that in other cases they are not populated. Typical utilities
behave accordingly; this patch teaches `tarfile` to do the same.

Lib/test/test_tarfile.py

@@ -1549,6 +1549,52 @@ class GNUWriteTest(unittest.TestCase):
                    ("longlnk/" * 127) + "longlink_")
 
 
+class DeviceHeaderTest(WriteTestBase, unittest.TestCase):
+
+    prefix = "w:"
+
+    def test_headers_written_only_for_device_files(self):
+        # Regression test for bpo-18819.
+        tempdir = os.path.join(TEMPDIR, "device_header_test")
+        os.mkdir(tempdir)
+        try:
+            tar = tarfile.open(tmpname, self.mode)
+            try:
+                input_blk = tarfile.TarInfo(name="my_block_device")
+                input_reg = tarfile.TarInfo(name="my_regular_file")
+                input_blk.type = tarfile.BLKTYPE
+                input_reg.type = tarfile.REGTYPE
+                tar.addfile(input_blk)
+                tar.addfile(input_reg)
+            finally:
+                tar.close()
+
+            # devmajor and devminor should be *interpreted* as 0 in both...
+            tar = tarfile.open(tmpname, "r")
+            try:
+                output_blk = tar.getmember("my_block_device")
+                output_reg = tar.getmember("my_regular_file")
+            finally:
+                tar.close()
+            self.assertEqual(output_blk.devmajor, 0)
+            self.assertEqual(output_blk.devminor, 0)
+            self.assertEqual(output_reg.devmajor, 0)
+            self.assertEqual(output_reg.devminor, 0)
+
+            # ...but the fields should not actually be set on regular files:
+            with open(tmpname, "rb") as infile:
+                buf = infile.read()
+            buf_blk = buf[output_blk.offset:output_blk.offset_data]
+            buf_reg = buf[output_reg.offset:output_reg.offset_data]
+            # See `struct posixheader` in GNU docs for byte offsets:
+            # <https://www.gnu.org/software/tar/manual/html_node/Standard.html>
+            device_headers = slice(329, 329 + 16)
+            self.assertEqual(buf_blk[device_headers], b"0000000\0" * 2)
+            self.assertEqual(buf_reg[device_headers], b"\0" * 16)
+        finally:
+            support.rmtree(tempdir)
+
+
 class CreateTest(WriteTestBase, unittest.TestCase):
 
     prefix = "x:"