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GNU cfengine

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6.2.5 Deadlocks and runaway loops

Whenever we allow concurrent processes to share a resource, we open ourselves up the possibilty of deadlock. This is a situation where two or more processes are locked in a vicious stalemate from which none can escape. Another problem is that it might be possible to start an infinite loop: cfagent starts itself.

Cfagent protects you from such loops to a large degree. It should not be possible to make such a loop by accident. The reason for this is the locking mechanism which prevents tasks being repeated too often. If you start a cfagent process which contains a shell-command to start cfagent again, this shell command will be locked, so it will not be possible to run it a second time. So while you might be able to start a second cfagent process, further processes will not be started and you will simply have wasted a little CPU time. When the first cfagent returns, the tasks which the second cfagent completed will not be repeated unless you have set the IfElapsed time or the ExpireAfter time to zero. In general, if you wish to avoid problems like this, you should not disable the locking mechanism by setting these two times to zero.

The possibility of deadlock arises in network connection. Cfengine will not attempt to use the network to copy a file which can be copied internally from some machine to itself. It will always replace the server= directive in a copy with `localhost' to avoid unnecessary network connections. This prevents one kind of deadlock which could occur: namely cfrun executes cfagent on host A (cfservd on host A is then blocked until this completes), but the host A configuration file contains a remote copy from itself to itself. This remote copy would then have to wait for cfservd to unblock, but this would be impossible since cfservd cannot unblock until it has the file. By avoiding remote copies to localhost, this possibility is avoided.


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