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Re: dropping incoming DO error
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Re: dropping incoming DO error

On Nov 1, 2007, at 8:45 PM, Steve Gehrman wrote:
Anyone know what causes this? Anyone know how I can figure out where it's coming from?

*** -[NSMachPort handlePortMessage:]: dropping incoming DO message because the connection or ports are invalid


I can take a stab at this ...

Once one side of a DO connection sends a message to the other side, a race begins. Or, three races. While the message is "in- flight" (stored in the kernel for this NSMachPort case), the process which is going to receive it might decide, for whatever reason, to shutdown the connection. Thus, when the message is pulled from the kernel, the connection is already invalid and data structures torn down, and there's nothing to be done with the message.

The second/third races are more interesting and likely. While the message is in-flight, the sending process might decide to invalidate the connection, or terminate. Either of those will invalidate the sender's Mach ports in the message. Plus, port death notification messages get generated by the kernel and sent to the process with the other end of the connection (since it is interested).

When the receiving process gets those port death notifications it is going to act on them by invalidating the connection and tearing down data structures. Then the message might be pulled from the kernel. So this is kind of a variation on the first race but still interesting in its own right.

Or, the message that was sent might be pulled from the kernel first before those port death notifications, but still all is not well. When a port goes invalid, the kernel scribbles MACH_PORT_DEAD over the previous port identifier for that port in all messages still waiting in the kernel. When the receiving process pulls the message from the kernel, it sees MACH_PORT_DEAD rather than the sender's port identifier. Well, the sender's port is part of the information that the lowest layers use to figure out which connection the message is destined for, since of course a process can have many connections to other processes. The received message lacks the information needed to do the mapping, so it must be dropped (and likely, connection invalidation is imminent in any case). And of course one could be using multiple threads and more than one thread could be doing some of these steps.

All these races can potentially apply to all versions of Mac OS X, if the timing is "right".

So one can imagine how this might occur pretty easily in the termination case. A sender sends, say, a oneway DO message to another process and immediately quits. Maybe it is sending a "goodbye" message. There is then a race between the receipt of the in-flight message on the other side and the death of those ports in the sender; the sender is inadvertently (probably!) screwing it's own last message by terminating "too quickly". And the insidious thing is that this can work sometimes, and not others, depending on the timing. Or it can work in one OS release, and not another, because, say, performance of the something during the shutdown improves (or the kernel gets faster) and the sender now goes away a little quicker than it used to. Or it might not happen on a slower machine but does on a faster (or multicore) machine. Or whatever.

As for figuring out where it is coming from ...
So the thing to look for I suppose is where you have processes terminating, or manual connection invalidations going on. Do a thought experiment where you imagine messages that are sent back and forth remain in-flight for an hour, and think about whether you're doing something that might cause this race to crop up. (Assuming you saw this message with one of your own apps.)

Chris Kane Cocoa Frameworks, Apple


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 >dropping incoming DO error (From: Steve Gehrman <email@hidden>)

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