How to kill a child process? (and read its output)

[dan-da]

The docs for tokio::process::Child::kill() give a simple example how to kill a child process using a select!(). The example uses Child::wait() and Child::kill() in the two select branches. The klll() only occurs if a message is received over a channel.

In my case, I need to read stdout from the child process. So I'm using Child::wait_with_output(). However this method takes self and thus the select!() won't compile.

here's a playground.

So, how to do it?

to be clear: happy path is that the child process completes normally and we read its output from stdout. But I also need to support the path that we receive a message while child process is executing and then kill the child process.

I am aware of Command::kill_on_drop(true), which can kill the child process in the background without need for a select!(). That is not a good solution in this case because the application must be certain that the child process has been terminated before proceeding and termnation may take some seconds. kill_on_drop() docs recommend using Child::kill() for such situations.

also, I am wondering if Child::wait_with_output() really has to take self or maybe it should take &mut self, as Child::wait() does.

[dan-da]

I was able to accomplish my goal by copying Child::wait_with_output() into my code and modifying it to take &mut child instead of self.

playground

This makes me believe it is a mistake for that method to take self. I will wait for comments here, and otherwise file an issue or maybe raise a PR.

[Darksonn]

It sounds like you solved your problem. The wait_with_output takes self to match the standard library. It can't be changed.

[Darksonn]

I'm happy to see additional examples, though. I do think that examples showing how to read part of the output into a buffer would be useful.

[dan-da]

It sounds like you solved your problem.

I only solved the problem by copy/pasting a tokio method into my application code and modifying it. That should not be necessary.

The wait_with_output takes self to match the standard library. It can't be changed.

Child::wait_with_output() and Child::kill() appear to be incompatible. in other words, tokio appears to provide no way to kill a child process if one must also read the output of that process. That seems a serious deficiency, if not an outright bug.

And that is only because wait_with_output() takes self instead of &mut self, when it clearly does not need to consume self.

If it is frozen due to policy (ie std lib compatibility) then perhaps a new method should be added.

I'm happy to see additional examples, though

Indeed. if someone can demonstrate how to kill a child process while waiting for its output using unmodiified tokio, then I won't need to file an issue.

If such an example can be made, it should be added to the Child::kill() docs, please.

[dan-da]

In theory it should be possible to use Child::wait() and read the process's output. I tried that initially and did not find a good solution.

Perhaps someone here can demonstrate how to do it...?