- 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.

12 files changed, 97 insertions, 47 deletions

diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index 49fbe19..a1c21d3 100644
--- a/Lib/_pyio.py
+++ b/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:
diff --git a/Lib/test/test_fileio.py b/Lib/test/test_fileio.py
index 91c5251..cdc3123 100644
--- a/Lib/test/test_fileio.py
+++ b/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>
diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 6f136f2..b91c0a1 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -234,6 +234,11 @@ class IOTest(unittest.TestCase):
 
     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)
         self.assertEqual(f.tell(), 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)
diff --git a/Lib/test/test_largefile.py b/Lib/test/test_largefile.py
index c4faead..16da8d8 100644
--- a/Lib/test/test_largefile.py
+++ b/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
 
diff --git a/Lib/test/test_memoryio.py b/Lib/test/test_memoryio.py
index c2190b6..d06f1c9 100644
--- a/Lib/test/test_memoryio.py
+++ b/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()
diff --git a/Misc/ACKS b/Misc/ACKS
index 877c326..3eb9c49 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -124,6 +124,7 @@ Donn Cave
 Charles Cazabon
 Per Cederqvist
 Octavian Cerna
+Pascal Chambon
 Hye-Shik Chang
 Jeffrey Chang
 Mitch Chapman
diff --git a/Misc/NEWS b/Misc/NEWS
index fd99685..5f64b8a 100644
--- a/Misc/NEWS
+++ b/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
diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index d25cb67..25ff247 100644
--- a/Modules/_io/bytesio.c
+++ b/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);
 }
diff --git a/Modules/_io/fileio.c b/Modules/_io/fileio.c
index 164fe5b..59cd3f7 100644
--- a/Modules/_io/fileio.c
+++ b/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)
diff --git a/Modules/_io/iobase.c b/Modules/_io/iobase.c
index 3732f46..3344847 100644
--- a/Modules/_io/iobase.c
+++ b/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)
diff --git a/Modules/_io/stringio.c b/Modules/_io/stringio.c
index a5bfd79..505bde0 100644
--- a/Modules/_io/stringio.c
+++ b/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);
 }
diff --git a/Modules/_io/textio.c b/Modules/_io/textio.c
index 91fecae..90a4c09 100644
--- a/Modules/_io/textio.c
+++ b/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 *