Close #10142: Support for SEEK_HOLE/SEEK_DATA

3 files changed, 24 insertions, 9 deletions

diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index f66290f..41402b1 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -306,6 +306,7 @@ class IOBase(metaclass=abc.ABCMeta):
         * 0 -- start of stream (the default); offset should be zero or positive
         * 1 -- current stream position; offset may be negative
         * 2 -- end of stream; offset is usually negative
+        Some operating systems / file systems could provide additional values.
 
         Return an int indicating the new absolute position.
         """
@@ -866,7 +867,7 @@ class BytesIO(BufferedIOBase):
         elif whence == 2:
             self._pos = max(0, len(self._buffer) + pos)
         else:
-            raise ValueError("invalid whence value")
+            raise ValueError("unsupported whence value")
         return self._pos
 
     def tell(self):
@@ -1041,8 +1042,6 @@ class BufferedReader(_BufferedIOMixin):
         return _BufferedIOMixin.tell(self) - len(self._read_buf) + self._read_pos
 
     def seek(self, pos, whence=0):
-        if not (0 <= whence <= 2):
-            raise ValueError("invalid whence value")
         with self._read_lock:
             if whence == 1:
                 pos -= len(self._read_buf) - self._read_pos
@@ -1138,8 +1137,6 @@ class BufferedWriter(_BufferedIOMixin):
         return _BufferedIOMixin.tell(self) + len(self._write_buf)
 
     def seek(self, pos, whence=0):
-        if not (0 <= whence <= 2):
-            raise ValueError("invalid whence")
         with self._write_lock:
             self._flush_unlocked()
             return _BufferedIOMixin.seek(self, pos, whence)
@@ -1235,8 +1232,6 @@ class BufferedRandom(BufferedWriter, BufferedReader):
         BufferedWriter.__init__(self, raw, buffer_size, max_buffer_size)
 
     def seek(self, pos, whence=0):
-        if not (0 <= whence <= 2):
-            raise ValueError("invalid whence")
         self.flush()
         if self._read_buf:
             # Undo read ahead.
@@ -1852,8 +1847,7 @@ class TextIOWrapper(TextIOBase):
                 self._decoder.reset()
             return position
         if whence != 0:
-            raise ValueError("invalid whence (%r, should be 0, 1 or 2)" %
-                             (whence,))
+            raise ValueError("unsupported whence (%r)" % (whence,))
         if cookie < 0:
             raise ValueError("negative seek position %r" % (cookie,))
         self.flush()
diff --git a/Lib/os.py b/Lib/os.py
index 2c7d954..2403f30 100644
--- a/Lib/os.py
+++ b/Lib/os.py
@@ -116,6 +116,7 @@ del _names
 
 # Python uses fixed values for the SEEK_ constants; they are mapped
 # to native constants if necessary in posixmodule.c
+# Other possible SEEK values are directly imported from posixmodule.c
 SEEK_SET = 0
 SEEK_CUR = 1
 SEEK_END = 2
diff --git a/Lib/test/test_posix.py b/Lib/test/test_posix.py
index 142dddd..c8798a9 100644
--- a/Lib/test/test_posix.py
+++ b/Lib/test/test_posix.py
@@ -1009,6 +1009,26 @@ class PosixTester(unittest.TestCase):
         posix.RTLD_GLOBAL
         posix.RTLD_LOCAL
 
+    @unittest.skipUnless('PC_MIN_HOLE_SIZE' in os.pathconf_names,
+                         "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):