MachMessageServer: invert the request buffer allocation logic

util/mach/mach_message_server.cc

 // Copyright 2014 The Crashpad Authors. All rights reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
@@ skipping 94 matching lines @@
 
   if (receive_large == kReceiveLargeResize) {
     options |= MACH_RCV_LARGE;
   } else {
     options &= ~MACH_RCV_LARGE;
   }
 
   mach_msg_size_t trailer_alloc = REQUESTED_TRAILER_SIZE(options);
   mach_msg_size_t max_request_size = interface->MachMessageServerRequestSize();
   mach_msg_size_t request_alloc = round_page(max_request_size + trailer_alloc);
+
+  // mach_msg_server() and mach_msg_server_once() invert this condition, but
+  // their interpretation is incorrect. When it is desirable to retry a receive
+  // attempt that returns MACH_RCV_TOO_LARGE with a larger receive buffer, it is
+  // also desirable to use the full receive buffer rounded up to a page size for
+  // the initial receive attempt. On the other hand, when this behavior is not
+  // requested, there is no reason to attempt receiving messages any larger than
+  // expected.
   mach_msg_size_t request_size = (receive_large == kReceiveLargeResize)
-                                     ? request_alloc
-                                     : max_request_size + trailer_alloc;
+                                     ? max_request_size + trailer_alloc
+                                     : request_alloc;
 
   mach_msg_size_t max_reply_size = interface->MachMessageServerReplySize();
 
   // mach_msg_server() and mach_msg_server_once() would consider whether
   // |options| contains MACH_SEND_TRAILER and include MAX_TRAILER_SIZE in this
   // computation if it does, but that option is ineffective on OS X.
   mach_msg_size_t reply_alloc = round_page(max_reply_size);
 
   kern_return_t kr;
 
@@ skipping 148 matching lines @@
         // persistent mode, and just return success when not in persistent mode.
         return (persistent == kPersistent) ? MACH_RCV_TIMED_OUT : kr;
       }
     }
   } while (persistent == kPersistent);
 
   return kr;
 }
 
 }  // namespace crashpad