Mac File Dialogs: Fix crash due to adding nil value to an NSMutableSet.

ui/shell_dialogs/select_file_dialog_mac.mm

 // Copyright (c) 2012 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 "ui/shell_dialogs/select_file_dialog_mac.h"
 
 #include <CoreServices/CoreServices.h>
 #include <stddef.h>
 
 #include <vector>
@@ skipping 11 matching lines @@
 #include "ui/base/l10n/l10n_util_mac.h"
 #include "ui/strings/grit/ui_strings.h"
 
 namespace {
 
 const int kFileTypePopupTag = 1234;
 
 CFStringRef CreateUTIFromExtension(const base::FilePath::StringType& ext) {
   base::ScopedCFTypeRef<CFStringRef> ext_cf(base::SysUTF8ToCFStringRef(ext));
   return UTTypeCreatePreferredIdentifierForTag(
       kUTTagClassFilenameExtension, ext_cf.get(), NULL);

[Avi (use Gerrit), 2016/07/21 14:25:20]

The documentation
(https://developer.apple.com/library/mac/documentation/MobileCoreServices/Refe...)
says that this should only return NULL if the tag class is unknown, not if the
tag is unknown.

Please file a radar with Apple.

[karandeepb, 2016/07/22 09:17:16]

Filed rdar://27490414.  Although can't reproduce this manually and this is a
fairly rare crash.

 }
 
 NSString* GetDescriptionFromExtension(const base::FilePath::StringType& ext) {
   base::ScopedCFTypeRef<CFStringRef> uti(CreateUTIFromExtension(ext));
   base::ScopedCFTypeRef<CFStringRef> description(
       UTTypeCopyDescription(uti.get()));
 
   if (description && CFStringGetLength(description))
     return [[base::mac::CFToNSCast(description.get()) retain] autorelease];
 
@@ skipping 260 matching lines @@
     DCHECK_NE(0u, [type_description length]);
     [popup addItemWithTitle:type_description];
 
     // Populate file_type_lists.
     // Set to store different extensions in the current extension group.
     NSMutableSet* file_type_set = [NSMutableSet set];
     for (const base::FilePath::StringType& ext : ext_list) {
       if (ext == default_extension)
         default_extension_index = i;
 
+      // Crash reports suggest that CreateUTIFromExtension may return nil. Hence
+      // we nil check before adding to |file_type_set|. See crbug.com/630101.
       base::ScopedCFTypeRef<CFStringRef> uti(CreateUTIFromExtension(ext));
-      [file_type_set addObject:base::mac::CFToNSCast(uti.get())];
+      if (uti)
+        [file_type_set addObject:base::mac::CFToNSCast(uti.get())];
 
       // Always allow the extension itself, in case the UTI doesn't map
       // back to the original extension correctly. This occurs with dynamic
       // UTIs on 10.7 and 10.8.
       // See http://crbug.com/148840, http://openradar.me/12316273
       base::ScopedCFTypeRef<CFStringRef> ext_cf(
           base::SysUTF8ToCFStringRef(ext));
       [file_type_set addObject:base::mac::CFToNSCast(ext_cf.get())];
     }
     [file_type_lists addObject:[file_type_set allObjects]];
@@ skipping 117 matching lines @@
     // For save dialogs, this causes the first item in the allowedFileTypes
     // array to be used as the extension for the save panel.
     [dialog_ setAllowedFileTypes:[fileTypeLists_ objectAtIndex:index]];
   } else {
     // The user selected "All files" option.
     [dialog_ setAllowedFileTypes:nil];
   }
 }
 
 @end