Change Windows file.truncate() to (a) restore the original file position,

and (b) stop trying to prevent file growth.

Beef up the file.truncate() docs.

Change test_largefile.py to stop assuming that f.truncate() moves the
file pointer to the truncation point, and to verify instead that it leaves
the file position alone.  Remove the test for what happens when a
specified size exceeds the original file size (it's ill-defined, according
to the Single Unix Spec).

Doc/lib/libstdtypes.tex

@@ -1152,9 +1152,14 @@ Files have the following methods:
 \end{methoddesc}
 
 \begin{methoddesc}[file]{truncate}{\optional{size}}
-  Truncate the file's size.  If the optional \var{size} argument
+  Truncate the file's size.  If the optional \var{size} argument is
   present, the file is truncated to (at most) that size.  The size
-  defaults to the current position.
+  defaults to the current position.  The current file position is
+  not changed.  Note that if a specified size exceeds the file's
+  current size, the result is platform-dependent:  possibilities
+  include that file may remain unchanged, increase to the specified
+  size as if zero-filled, or increase to the specified size with
+  undefined new content.
   Availability:  Windows, many \UNIX variants.
 \end{methoddesc}
 

Lib/test/test_largefile.py

@@ -133,24 +133,30 @@ if hasattr(f, 'truncate'):
         print 'try truncate'
     f = open(name, 'r+b')
     f.seek(0, 2)
-    expect(f.tell(), size+1)
+    expect(f.tell(), size+1)    # else we've lost track of the true size
     # Cut it back via seek + truncate with no argument.
     newsize = size - 10
     f.seek(newsize)
     f.truncate()
-    expect(f.tell(), newsize)
-    # Ensure that truncate(bigger than true size) doesn't grow the file.
-    f.truncate(size)
-    expect(f.tell(), newsize)
+    expect(f.tell(), newsize)   # else pointer moved
+    f.seek(0, 2)
+    expect(f.tell(), newsize)   # else wasn't truncated
     # Ensure that truncate(smaller than true size) shrinks the file.
     newsize -= 1
-    f.seek(0)
+    f.seek(42)
     f.truncate(newsize)
-    expect(f.tell(), newsize)
+    expect(f.tell(), 42)        # else pointer moved
+    f.seek(0, 2)
+    expect(f.tell(), newsize)   # else wasn't truncated
+
+    # XXX truncate(larger than true size) is ill-defined across platforms
+
     # cut it waaaaay back
-    f.truncate(1)
     f.seek(0)
-    expect(len(f.read()), 1)
+    f.truncate(1)
+    expect(f.tell(), 0)         # else pointer moved
+    expect(len(f.read()), 1)    # else wasn't truncated
+
     f.close()
 
 os.unlink(name)

Objects/fileobject.c

@@ -415,32 +415,36 @@ file_truncate(PyFileObject *f, PyObject *args)
 
 #ifdef MS_WIN32
 	/* MS _chsize doesn't work if newsize doesn't fit in 32 bits,
-	   so don't even try using it.  truncate() should never grow the
-	   file, but MS SetEndOfFile will grow a file, so we need to
-	   compare the specified newsize to the actual size.  Some
-	   optimization could be done here when newsizeobj is NULL. */
+	   so don't even try using it. */
 	{
-		Py_off_t currentEOF;	/* actual size */
+		Py_off_t current;	/* current file position */
 		HANDLE hFile;
 		int error;
 
-		/* First move to EOF, and set currentEOF to the size. */
+		/* current <- current file postion. */
+		if (newsizeobj == NULL)
+			current = newsize;
+		else {
+			Py_BEGIN_ALLOW_THREADS
 			errno = 0;
-		if (_portable_fseek(f->f_fp, 0, SEEK_END) != 0)
-			goto onioerror;
-		errno = 0;
-		currentEOF = _portable_ftell(f->f_fp);
-		if (currentEOF == -1)
+			current = _portable_ftell(f->f_fp);
+			Py_END_ALLOW_THREADS
+			if (current == -1)
 				goto onioerror;
+		}
 
-		if (newsize > currentEOF)
-			newsize = currentEOF;	/* never grow the file */
-
-		/* Move to newsize, and truncate the file there. */
-		if (newsize != currentEOF) {
+		/* Move to newsize. */
+		if (current != newsize) {
+			Py_BEGIN_ALLOW_THREADS
 			errno = 0;
-			if (_portable_fseek(f->f_fp, newsize, SEEK_SET) != 0)
+			error = _portable_fseek(f->f_fp, newsize, SEEK_SET)
+				!= 0;
+			Py_END_ALLOW_THREADS
+			if (error)
 				goto onioerror;
+		}
+
+		/* Truncate.  Note that this may grow the file! */
 		Py_BEGIN_ALLOW_THREADS
 		errno = 0;
 		hFile = (HANDLE)_get_osfhandle(fileno(f->f_fp));
@@ -453,8 +457,17 @@ file_truncate(PyFileObject *f, PyObject *args)
 		Py_END_ALLOW_THREADS
 		if (error)
 			goto onioerror;
-		}
 
+		/* Restore original file position. */
+		if (current != newsize) {
+			Py_BEGIN_ALLOW_THREADS
+			errno = 0;
+			error = _portable_fseek(f->f_fp, current, SEEK_SET)
+				!= 0;
+			Py_END_ALLOW_THREADS
+			if (error)
+				goto onioerror;
+		}
 	}
 #else
 	Py_BEGIN_ALLOW_THREADS