file_truncate(): provide full "large file" support on Windows, by

dropping MS's inadequate _chsize() function.  This was inspired by
SF patch 498109 ("fileobject truncate support for win32"), which I
rejected.

libstdtypes.tex:  Someone who knows should update the availability
blurb.  For example, if it's available on Linux, it would be good to
say so.

test_largefile:  Uncommented the file.truncate() tests, and reworked to
do more.  The old comment about "permission errors" in the truncation
tests under Windows was almost certainly due to that the file wasn't open
for *write* access at this point, so of course MS wouldn't let you
truncate it.  I'd be appalled if a Unixish system did.

CAUTION:  Someone should run this test on Linux (etc) too.  The
truncation part was commented out before.  Note that test_largefile isn't
run by default.

Doc/lib/libstdtypes.tex

@@ -1154,9 +1154,8 @@ Files have the following methods:
 \begin{methoddesc}[file]{truncate}{\optional{size}}
   Truncate the file's size.  If the optional \var{size} argument
   present, the file is truncated to (at most) that size.  The size
-  defaults to the current position.  Availability of this function
-  depends on the operating system version (for example, not all
-  \UNIX{} versions support this operation).
+  defaults to the current position.
+  Availability:  Windows, many \UNIX variants.
 \end{methoddesc}
 
 \begin{methoddesc}[file]{write}{str}

Lib/test/test_largefile.py

@@ -128,20 +128,29 @@ expect(os.lseek(f.fileno(), size, 0), size)
 expect(f.read(1), 'a') # the 'a' that was written at the end of the file above
 f.close()
 
-
-# XXX add tests for truncate if it exists
-# XXX has truncate ever worked on Windows? specifically on WinNT I get:
-#     "IOError: [Errno 13] Permission denied"
-##try:
-##      newsize = size - 10
-##      f.seek(newsize)
-##      f.truncate()
-##      expect(f.tell(), newsize)
-##      newsize = newsize - 1
-##      f.seek(0)
-##      f.truncate(newsize)
-##      expect(f.tell(), newsize)
-##except AttributeError:
-##      pass
+if hasattr(f, 'truncate'):
+    if test_support.verbose:
+        print 'try truncate'
+    f = open(name, 'r+b')
+    f.seek(0, 2)
+    expect(f.tell(), size+1)
+    # 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)
+    # Ensure that truncate(smaller than true size) shrinks the file.
+    newsize -= 1
+    f.seek(0)
+    f.truncate(newsize)
+    expect(f.tell(), newsize)
+    # cut it waaaaay back
+    f.truncate(1)
+    f.seek(0)
+    expect(len(f.read()), 1)
+    f.close()
 
 os.unlink(name)

Misc/NEWS

@@ -84,6 +84,10 @@ Tests
 
 Windows
 
+- file.truncate([newsize]) now works on Windows for all newsize values.
+  It used to fail if newsize didn't fit in 32 bits, reflecting a
+  limitation of MS _chsize (which is no longer used).
+
 - os.waitpid() is now implemented for Windows, and can be used to block
   until a specified process exits.  This is similar to, but not exactly
   the same as, os.waitpid() on POSIX systems.  If you're waiting for

Objects/fileobject.c

@@ -10,8 +10,10 @@
 
 #ifdef MS_WIN32
 #define fileno _fileno
-/* can (almost fully) duplicate with _chsize, see file_truncate */
+/* can simulate truncate with Win32 API functions; see file_truncate */
 #define HAVE_FTRUNCATE
+#define WINDOWS_LEAN_AND_MEAN
+#include <windows.h>
 #endif
 
 #ifdef macintosh
@@ -379,6 +381,9 @@ file_truncate(PyFileObject *f, PyObject *args)
 	newsizeobj = NULL;
 	if (!PyArg_ParseTuple(args, "|O:truncate", &newsizeobj))
 		return NULL;
+
+	/* Set newsize to current postion if newsizeobj NULL, else to the
+	   specified value. */
 	if (newsizeobj != NULL) {
 #if !defined(HAVE_LARGEFILE_SUPPORT)
 		newsize = PyInt_AsLong(newsizeobj);
@@ -389,37 +394,67 @@ file_truncate(PyFileObject *f, PyObject *args)
 #endif
 		if (PyErr_Occurred())
 			return NULL;
-	} else {
-		/* Default to current position*/
+	}
+	else {
+		/* Default to current position. */
 		Py_BEGIN_ALLOW_THREADS
 		errno = 0;
 		newsize = _portable_ftell(f->f_fp);
 		Py_END_ALLOW_THREADS
-		if (newsize == -1) {
-		        PyErr_SetFromErrno(PyExc_IOError);
-			clearerr(f->f_fp);
-			return NULL;
-		}
+		if (newsize == -1)
+			goto onioerror;
 	}
+
+	/* Flush the file. */
 	Py_BEGIN_ALLOW_THREADS
 	errno = 0;
 	ret = fflush(f->f_fp);
 	Py_END_ALLOW_THREADS
-	if (ret != 0) goto onioerror;
+	if (ret != 0)
+		goto onioerror;
 
 #ifdef MS_WIN32
-	/* can use _chsize; if, however, the newsize overflows 32-bits then
-	   _chsize is *not* adequate; in this case, an OverflowError is raised */
-	if (newsize > LONG_MAX) {
-		PyErr_SetString(PyExc_OverflowError,
-			"the new size is too long for _chsize (it is limited to 32-bit values)");
-		return NULL;
-	} else {
+	/* 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. */
+	{
+		Py_off_t currentEOF;	/* actual size */
+		HANDLE hFile;
+		int error;
+
+		/* First move to EOF, and set currentEOF to the size. */
+		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)
+			goto onioerror;
+
+		if (newsize > currentEOF)
+			newsize = currentEOF;	/* never grow the file */
+
+		/* Move to newsize, and truncate the file there. */
+		if (newsize != currentEOF) {
+			errno = 0;
+			if (_portable_fseek(f->f_fp, newsize, SEEK_SET) != 0)
+				goto onioerror;
 			Py_BEGIN_ALLOW_THREADS
 			errno = 0;
-		ret = _chsize(fileno(f->f_fp), (long)newsize);
+			hFile = (HANDLE)_get_osfhandle(fileno(f->f_fp));
+			error = hFile == (HANDLE)-1;
+			if (!error) {
+				error = SetEndOfFile(hFile) == 0;
+				if (error)
+					errno = EACCES;
+			}
 			Py_END_ALLOW_THREADS
-		if (ret != 0) goto onioerror;
+			if (error)
+				goto onioerror;
+		}
+
 	}
 #else
 	Py_BEGIN_ALLOW_THREADS