- Issue #6939: Fix file I/O objects in the `io` module to keep the original

  file position when calling `truncate()`.  It would previously change the
  file position to the given argument, which goes against the tradition of
  ftruncate() and other truncation APIs.  Patch by Pascal Chambon.

Lib/_pyio.py

@@ -867,7 +867,7 @@ class BytesIO(BufferedIOBase):
         elif pos < 0:
             raise ValueError("negative truncate position %r" % (pos,))
         del self._buffer[pos:]
-        return self.seek(pos)
+        return pos
 
     def readable(self):
         return True
@@ -1226,8 +1226,7 @@ class BufferedRandom(BufferedWriter, BufferedReader):
         if pos is None:
             pos = self.tell()
         # Use seek to flush the read buffer.
-        self.seek(pos)
-        return BufferedWriter.truncate(self)
+        return BufferedWriter.truncate(self, pos)
 
     def read(self, n=None):
         if n is None:
@@ -1727,8 +1726,7 @@ class TextIOWrapper(TextIOBase):
         self.flush()
         if pos is None:
             pos = self.tell()
-        self.seek(pos)
-        return self.buffer.truncate()
+        return self.buffer.truncate(pos)
 
     def detach(self):
         if self.buffer is None:

Lib/test/test_fileio.py

@@ -329,6 +329,17 @@ class OtherFileTests(unittest.TestCase):
             f.close()
             self.fail("no error for invalid mode: %s" % bad_mode)
 
+    def testTruncate(self):
+        f = _FileIO(TESTFN, 'w')
+        f.write(bytes(bytearray(range(10))))
+        self.assertEqual(f.tell(), 10)
+        f.truncate(5)
+        self.assertEqual(f.tell(), 10)
+        self.assertEqual(f.seek(0, os.SEEK_END), 5)
+        f.truncate(15)
+        self.assertEqual(f.tell(), 5)
+        self.assertEqual(f.seek(0, os.SEEK_END), 15)
+
     def testTruncateOnWindows(self):
         def bug801631():
             # SF bug <http://www.python.org/sf/801631>

Lib/test/test_io.py

@@ -233,6 +233,11 @@ class IOTest(unittest.TestCase):
         support.unlink(support.TESTFN)
 
     def write_ops(self, f):
+        self.assertEqual(f.write(b"blah."), 5)
+        f.truncate(0)
+        self.assertEqual(f.tell(), 5)
+        f.seek(0)
+
         self.assertEqual(f.write(b"blah."), 5)
         self.assertEqual(f.seek(0), 0)
         self.assertEqual(f.write(b"Hello."), 6)
@@ -244,8 +249,9 @@ class IOTest(unittest.TestCase):
         self.assertEqual(f.write(b"h"), 1)
         self.assertEqual(f.seek(-1, 2), 13)
         self.assertEqual(f.tell(), 13)
+
         self.assertEqual(f.truncate(12), 12)
-        self.assertEqual(f.tell(), 12)
+        self.assertEqual(f.tell(), 13)
         self.assertRaises(TypeError, f.seek, 0.0)
 
     def read_ops(self, f, buffered=False):
@@ -290,7 +296,7 @@ class IOTest(unittest.TestCase):
         self.assertEqual(f.tell(), self.LARGE + 2)
         self.assertEqual(f.seek(0, 2), self.LARGE + 2)
         self.assertEqual(f.truncate(self.LARGE + 1), self.LARGE + 1)
-        self.assertEqual(f.tell(), self.LARGE + 1)
+        self.assertEqual(f.tell(), self.LARGE + 2)
         self.assertEqual(f.seek(0, 2), self.LARGE + 1)
         self.assertEqual(f.seek(-1, 2), self.LARGE)
         self.assertEqual(f.read(2), b"x")
@@ -988,7 +994,7 @@ class BufferedWriterTest(unittest.TestCase, CommonBufferedTests):
             bufio = self.tp(raw, 8)
             bufio.write(b"abcdef")
             self.assertEqual(bufio.truncate(3), 3)
-            self.assertEqual(bufio.tell(), 3)
+            self.assertEqual(bufio.tell(), 6)
         with self.open(support.TESTFN, "rb", buffering=0) as f:
             self.assertEqual(f.read(), b"abc")
 
@@ -1374,6 +1380,14 @@ class BufferedRandomTest(BufferedReaderTest, BufferedWriterTest):
             self.assertEqual(s,
                 b"A" + b"B" * overwrite_size + b"A" * (9 - overwrite_size))
 
+    def test_truncate_after_read_or_write(self):
+        raw = self.BytesIO(b"A" * 10)
+        bufio = self.tp(raw, 100)
+        self.assertEqual(bufio.read(2), b"AA") # the read buffer gets filled
+        self.assertEqual(bufio.truncate(), 2)
+        self.assertEqual(bufio.write(b"BB"), 2) # the write buffer increases
+        self.assertEqual(bufio.truncate(), 4)
+
     def test_misbehaved_io(self):
         BufferedReaderTest.test_misbehaved_io(self)
         BufferedWriterTest.test_misbehaved_io(self)

Lib/test/test_largefile.py

@@ -125,7 +125,7 @@ class LargeFileTest(unittest.TestCase):
             f.seek(42)
             f.truncate(newsize)
             if self.new_io:
-                self.assertEqual(f.tell(), newsize)  # else wasn't truncated
+                self.assertEqual(f.tell(), 42)
             f.seek(0, 2)
             self.assertEqual(f.tell(), newsize)
             # XXX truncate(larger than true size) is ill-defined
@@ -133,7 +133,7 @@ class LargeFileTest(unittest.TestCase):
             f.seek(0)
             f.truncate(1)
             if self.new_io:
-                self.assertEqual(f.tell(), 1)       # else pointer moved
+                self.assertEqual(f.tell(), 0)       # else pointer moved
             f.seek(0)
             self.assertEqual(len(f.read()), 1)  # else wasn't truncated
 

Lib/test/test_memoryio.py

@@ -76,7 +76,7 @@ class MemoryTestMixin:
         self.assertEqual(f.seek(0), 0)
         self.assertEqual(f.write(t("h")), 1)
         self.assertEqual(f.truncate(12), 12)
-        self.assertEqual(f.tell(), 12)
+        self.assertEqual(f.tell(), 1)
 
     def test_write(self):
         buf = self.buftype("hello world\n")
@@ -127,7 +127,8 @@ class MemoryTestMixin:
         # truncate() accepts long objects
         self.assertEqual(memio.truncate(4L), 4)
         self.assertEqual(memio.getvalue(), buf[:4])
-        self.assertEqual(memio.tell(), 4)
+        self.assertEqual(memio.tell(), 6)
+        memio.seek(0, 2)
         memio.write(buf)
         self.assertEqual(memio.getvalue(), buf[:4] + buf)
         pos = memio.tell()

Misc/ACKS

@@ -124,6 +124,7 @@ Donn Cave
 Charles Cazabon
 Per Cederqvist
 Octavian Cerna
+Pascal Chambon
 Hye-Shik Chang
 Jeffrey Chang
 Mitch Chapman

Misc/NEWS

@@ -50,6 +50,11 @@ Core and Builtins
 Library
 -------
 
+- Issue #6939: Fix file I/O objects in the `io` module to keep the original
+  file position when calling `truncate()`.  It would previously change the
+  file position to the given argument, which goes against the tradition of
+  ftruncate() and other truncation APIs.  Patch by Pascal Chambon.
+
 - UserDict is now a new-style class.
 
 - Issue #7610: Reworked implementation of the internal

Modules/_io/bytesio.c

@@ -414,7 +414,7 @@ PyDoc_STRVAR(truncate_doc,
 "truncate([size]) -> int.  Truncate the file to at most size bytes.\n"
 "\n"
 "Size defaults to the current file position, as returned by tell().\n"
-"Returns the new size.  Imply an absolute seek to the position size.");
+"The current file position is unchanged.  Returns the new size.\n");
 
 static PyObject *
 bytesio_truncate(bytesio *self, PyObject *args)
@@ -453,7 +453,6 @@ bytesio_truncate(bytesio *self, PyObject *args)
         if (resize_buffer(self, size) < 0)
             return NULL;
     }
-    self->pos = size;
 
     return PyLong_FromSsize_t(size);
 }

Modules/_io/fileio.c

@@ -758,8 +758,10 @@ fileio_tell(fileio *self, PyObject *args)
 static PyObject *
 fileio_truncate(fileio *self, PyObject *args)
 {
-	PyObject *posobj = NULL;
+	PyObject *posobj = NULL; /* the new size wanted by the user */
+#ifndef MS_WINDOWS
 	Py_off_t pos;
+#endif
 	int ret;
 	int fd;
 
@@ -774,58 +776,86 @@ fileio_truncate(fileio *self, PyObject *args)
 
 	if (posobj == Py_None || posobj == NULL) {
 		/* Get the current position. */
-                posobj = portable_lseek(fd, NULL, 1);
-                if (posobj == NULL)
+		posobj = portable_lseek(fd, NULL, 1);
+		if (posobj == NULL)
 			return NULL;
-        }
-        else {
-		/* Move to the position to be truncated. */
-                posobj = portable_lseek(fd, posobj, 0);
-        }
-	if (posobj == NULL)
-		return NULL;
-
-#if defined(HAVE_LARGEFILE_SUPPORT)
-	pos = PyLong_AsLongLong(posobj);
-#else
-	pos = PyLong_AsLong(posobj);
-#endif
-	if (pos == -1 && PyErr_Occurred())
-		return NULL;
+	}
+	else {
+		Py_INCREF(posobj);
+	}
 
 #ifdef MS_WINDOWS
 	/* MS _chsize doesn't work if newsize doesn't fit in 32 bits,
 	   so don't even try using it. */
 	{
+		PyObject *oldposobj, *tempposobj;
 		HANDLE hFile;
+	
+		/* we save the file pointer position */
+		oldposobj = portable_lseek(fd, NULL, 1); 
+		if (oldposobj == NULL) {
+			Py_DECREF(posobj);
+			return NULL;
+		}
+
+		/* we then move to the truncation position */
+		tempposobj = portable_lseek(fd, posobj, 0);
+		if (tempposobj == NULL) {
+			Py_DECREF(oldposobj);
+			Py_DECREF(posobj);
+			return NULL;
+		}
+		Py_DECREF(tempposobj);
 
 		/* Truncate.  Note that this may grow the file! */
 		Py_BEGIN_ALLOW_THREADS
 		errno = 0;
 		hFile = (HANDLE)_get_osfhandle(fd);
-		ret = hFile == (HANDLE)-1;
+		ret = hFile == (HANDLE)-1; /* testing for INVALID_HANDLE value */
 		if (ret == 0) {
 			ret = SetEndOfFile(hFile) == 0;
 			if (ret)
 				errno = EACCES;
 		}
 		Py_END_ALLOW_THREADS
+
+		/* we restore the file pointer position in any case */
+		tempposobj = portable_lseek(fd, oldposobj, 0);
+		Py_DECREF(oldposobj);
+		if (tempposobj == NULL) {
+			Py_DECREF(posobj);
+			return NULL;
+		}
+		Py_DECREF(tempposobj);
 	}
 #else
+
+#if defined(HAVE_LARGEFILE_SUPPORT)
+	pos = PyLong_AsLongLong(posobj);
+#else
+	pos = PyLong_AsLong(posobj);
+#endif
+	if (PyErr_Occurred()){
+		Py_DECREF(posobj);
+		return NULL;
+	}
+
 	Py_BEGIN_ALLOW_THREADS
 	errno = 0;
 	ret = ftruncate(fd, pos);
 	Py_END_ALLOW_THREADS
+
 #endif /* !MS_WINDOWS */
 
 	if (ret != 0) {
+		Py_DECREF(posobj);
 		PyErr_SetFromErrno(PyExc_IOError);
 		return NULL;
 	}
 
 	return posobj;
 }
-#endif
+#endif /* HAVE_FTRUNCATE */
 
 static char *
 mode_string(fileio *self)

Modules/_io/iobase.c

@@ -102,8 +102,8 @@ iobase_tell(PyObject *self, PyObject *args)
 PyDoc_STRVAR(iobase_truncate_doc,
     "Truncate file to size bytes.\n"
     "\n"
-    "Size defaults to the current IO position as reported by tell().  Return\n"
-    "the new size.");
+    "File pointer is left unchanged.  Size defaults to the current IO\n"
+    "position as reported by tell().  Returns the new size.");
 
 static PyObject *
 iobase_truncate(PyObject *self, PyObject *args)

Modules/_io/stringio.c

@@ -350,7 +350,7 @@ PyDoc_STRVAR(stringio_truncate_doc,
     "Truncate size to pos.\n"
     "\n"
     "The pos argument defaults to the current file position, as\n"
-    "returned by tell().  Imply an absolute seek to pos.\n"
+    "returned by tell().  The current file position is unchanged.\n"
     "Returns the new absolute position.\n");
 
 static PyObject *
@@ -390,7 +390,6 @@ stringio_truncate(stringio *self, PyObject *args)
             return NULL;
         self->string_size = size;
     }
-    self->pos = size;
 
     return PyLong_FromSsize_t(size);
 }

Modules/_io/textio.c

@@ -2313,15 +2313,7 @@ textiowrapper_truncate(textio *self, PyObject *args)
         return NULL;
     Py_DECREF(res);
 
-    if (pos != Py_None) {
-        res = PyObject_CallMethodObjArgs((PyObject *) self,
-                                          _PyIO_str_seek, pos, NULL);
-        if (res == NULL)
-            return NULL;
-        Py_DECREF(res);
-    }
-
-    return PyObject_CallMethodObjArgs(self->buffer, _PyIO_str_truncate, NULL);
+    return PyObject_CallMethodObjArgs(self->buffer, _PyIO_str_truncate, pos, NULL);
 }
 
 static PyObject *