Support for SEEK_HOLE/SEEK_DATA

Lib/test/test_posix.py

 "Test posix functions"
 
 from test import support
 
 # Skip these tests if there is no posix module.
 posix = support.import_module('posix')
 
 import errno
 import sys
 import time
@@ skipping 990 matching lines @@
         b |= r
         self.assertIs(b, l)
         self.assertEqual(l.count(), 3)
 
     def test_rtld_constants(self):
         # check presence of major RTLD_* constants
         posix.RTLD_LAZY
         posix.RTLD_NOW
         posix.RTLD_GLOBAL
         posix.RTLD_LOCAL
+
+    @unittest.skipUnless(hasattr(os, 'SEEK_HOLE'),
+                         "test needs an OS that reports file holes")
+    def test_fs_holes(self) :
+        # Even if the filesystem doesn't report holes,
+        # if the OS supports it the SEEK_* constants
+        # will be defined and will have a consistent
+        # behaviour:
+        # os.SEEK_DATA = current position
+        # os.SEEK_HOLE = end of file position
+        with open(support.TESTFN, 'r+b') as fp :
+            fp.write(b"hello")
+            fp.flush()
+            size = fp.tell()
+            fno = fp.fileno()
+            for i in range(size) :
+                self.assertEqual(i, os.lseek(fno, i, os.SEEK_DATA))
+                self.assertLessEqual(size, os.lseek(fno, i, os.SEEK_HOLE))
+            self.assertRaises(OSError, os.lseek, fno, size, os.SEEK_DATA)
+            self.assertRaises(OSError, os.lseek, fno, size, os.SEEK_HOLE)
 
 class PosixGroupsTester(unittest.TestCase):
 
     def setUp(self):
         if posix.getuid() != 0:
             raise unittest.SkipTest("not enough privileges")
         if not hasattr(posix, 'getgroups'):
             raise unittest.SkipTest("need posix.getgroups")
         if sys.platform == 'darwin':
             raise unittest.SkipTest("getgroups(2) is broken on OSX")
@@ skipping 24 matching lines @@
             self.assertListEqual(groups, posix.getgroups())
 
 def test_main():
     try:
         support.run_unittest(PosixTester, PosixGroupsTester)
     finally:
         support.reap_children()
 
 if __name__ == '__main__':
     test_main()