[Mac] Create transparent cursor when given empty custom data.

webkit/glue/webcursor_mac.mm

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "webkit/glue/webcursor.h"
 
 #import <AppKit/AppKit.h>
 #include <Carbon/Carbon.h>
 
 #include "base/logging.h"
@@ skipping 22 matching lines @@
   DCHECK(cursor_image);
   return [[[NSCursor alloc] initWithImage:cursor_image
                                   hotSpot:NSMakePoint(x, y)] autorelease];
 }
 
 CGImageRef CreateCGImageFromCustomData(const std::vector<char>& custom_data,
                                        const gfx::Size& custom_size) {
   base::mac::ScopedCFTypeRef<CGColorSpaceRef> cg_color(
       CGColorSpaceCreateDeviceRGB());
   // This is safe since we're not going to draw into the context we're creating.
-  void* data = const_cast<char*>(&custom_data[0]);
+  void* data = NULL;
+  if (!custom_data.empty())
+    data = const_cast<char*>(&custom_data[0]);
   // The settings here match SetCustomData() below; keep in sync.
   base::mac::ScopedCFTypeRef<CGContextRef> context(
       CGBitmapContextCreate(data,

[Avi (use Gerrit), 2011/06/29 00:22:47]

You can do that, pass in a null data for a 0x0 image?

[Scott Hess - ex-Googler, 2011/06/29 01:06:13]

Docs say that passing NULL means that Quartz allocates the memory.

I tested with NULL - but the size was non-zero.  In that case it worked as
expected.  But I just went back and instrumented the code to pass a size of
{0,0}, and in that case CGBitmapContextCreate() returns NULL, and the exception
stuff from the old comment happens.

I tried revising so that |cursor_image| gets no representation in case of NULL
or 0x0, but no go.  Nothing bad happens, but the cursor doesn't change at all. 
The docs seem to imply that a zero-size image is not going to generally work
well, in any case.

I do not have enough insight into the overall system to be able to offhand say
if we can get an actual {0,0} size, or if the empty image data with a non-zero
size is expected.  The transparent cursor seems right, it would be easy enough
to pass NULL for data (system allocated transparent backing) and {1,1} for size
in that case.  Or I could back off and instrument RWHVM further to dump the size
into breakpad along with the type.

[I can't for the life of me figure out how to snag the URL for this crash, that
would rock.]

                             custom_size.width(),
                             custom_size.height(),
                             8,
                             custom_size.width()*4,
                             cg_color.get(),
                             kCGImageAlphaPremultipliedLast |
                             kCGBitmapByteOrder32Big));
   return CGBitmapContextCreateImage(context.get());
 }
 
 NSCursor* CreateCustomCursor(const std::vector<char>& custom_data,
                              const gfx::Size& custom_size,
                              const gfx::Point& hotspot) {
-  // CG throws a cocoa exception if we try to create an empty image, which
-  // results in an infinite loop.  This CHECK ensures that we crash instead.
-  CHECK(!custom_data.empty());
-
   base::mac::ScopedCFTypeRef<CGImageRef> cg_image(
       CreateCGImageFromCustomData(custom_data, custom_size));
 
   NSBitmapImageRep* ns_bitmap =
       [[NSBitmapImageRep alloc] initWithCGImage:cg_image.get()];
   NSImage* cursor_image = [[NSImage alloc] init];
   DCHECK(cursor_image);
   [cursor_image addRepresentation:ns_bitmap];
   [ns_bitmap release];
 
@@ skipping 331 matching lines @@
   return true;
 }
 
 void WebCursor::CleanupPlatformData() {
   return;
 }
 
 void WebCursor::CopyPlatformData(const WebCursor& other) {
   return;
 }