Hanging NSTask using waitUntilExit

Question

I need to use NSTask synchronously, however I find that occasionally my task hangs under the 'waitUntilExit' command. I wonder if there is a graceful way--an error handling method--to terminated the hanging task so I can re-launch another?

Answer 1

Note that if the task being run via NSTask fills the output pipe then the process will hang, effectively blocking waitUntilExit from returning.

You can prevent this situation by calling

[task.standardOutput.fileHandleForReading readDataToEndOfFile];

before calling

[task waitUntilExit];

This will cause the output pipe's data to be read until the process writing to the output pipe closes it.

Example code demonstrating the issue and various solutions:

https://github.com/lroathe/PipeTest

Answer 2

You could launch the task using -[task launch] and then poll periodically on its isRunning property to check whether it has already finished. If it has not finished after a given time interval, you can call -[task terminate] to terminate it. This requires that the task you start does not ignore the SIGTERM signal.

If however polling for task termination is too inefficient in your case, you could setup a dispatch source of type DISPATCH_SOURCE_TYPE_PROC after you have launched your task. This source then asynchronously calls its event block when the task terminates:

dispatch_source_create(DISPATCH_SOURCE_TYPE_PROC, task.processIdentifier, DISPATCH_PROC_EXIT, dispatch_get_main_queue());