For the immediate milestone, exclude History and Thumbnails from being backed...

base/mac_util.mm

 // Copyright (c) 2008-2009 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 "base/mac_util.h"
 
 #import <Cocoa/Cocoa.h>
 
 #include "base/file_path.h"
 #include "base/logging.h"
@@ skipping 219 matching lines @@
         !it->compare(it->length() - kExtLength, kExtLength, kExt, kExtLength))
       return FilePath(bundle_name);
 
     // Separate this component from the next one.
     bundle_name += '/';
   }
 
   return FilePath();
 }
 
+bool SetFileBackupExclusion(const FilePath& file_path, bool exclude) {
+  NSString* filePath =

[TVL, 2009/11/11 02:51:02]

CHECK/DCHECK for file_path.IsAbsolute()?

+      [NSString stringWithUTF8String:file_path.value().c_str()];
+  NSURL* url = [NSURL fileURLWithPath:filePath];
+  // Note that we always set CSBackupSetItemExcluded's excludeByPath param
+  // to true.  This prevents a problem with toggling the setting: if the file
+  // is excluded with excludeByPath set to true then excludeByPath must
+  // also be true when un-excluding the file, otherwise the un-excluding
+  // will be ignored.
+  bool success =
+      CSBackupSetItemExcluded((CFURLRef)url, exclude, true) == noErr;

[TVL, 2009/11/11 02:51:02]

drive by: don't the docs talk about needing to run as admin?  (To change the
backup exclusion status of a path, your application must be running with
administrator privileges.)  does that mean for non admins, we'll still back
stuff up?

+  if (!success)
+    LOG(WARNING) << "Failed to set backup excluson for file '"
+                 << file_path.value().c_str() << "'.  Continuing.";
+  return success;
+}
+
+
 }  // namespace mac_util