FEATURE REQUEST: Callback or blocking method to wait for exit of spawned process

[jakob-ledermann]

Currently there does not seem to be an easy way to wait for the spawned process to exit.

I do not see a better way to call the quit_cb callback, when the spawned process exits in https://github.com/jakob-ledermann/zellij/blob/445ee782e398aac34b4a4b7cedf386fa1d5c31f6/zellij-server/src/os_input_output.rs#L310

The unix implementation can be found here and uses .wait() to block until the child process has exited.

[andfoy]

Hi @jakob-ledermann, thanks for the suggestion. Right now the only way to test if a process is still "alive" is by trying to read from it in a blocking fashion, once the process finishes, the function will Err, which is similar to the functionality you are requesting for.

winpty-rs/src/pty/base.rs

Line 461 in 4799bdc

pub fn read(&self, length: u32, blocking: bool) -> Result<OsString, OsString> {

winpty-rs/src/pty/base.rs

Line 403 in 4799bdc

Ok(None) => Err(OsString::from("Standard out reached EOF")),

We could add a method wait or alike that could replicate the same functionality. However, from an implementation standpoint, I think it is clearer to handle the Err in the downstream function that performs the reading operation.

[jakob-ledermann]

I agree that reacting to the error from read is currently the prefered option.
Unfortunately reading and detecting the termination are quite far apart in that code base so using the error to read to detect the termination is not trivial.

Replicating a wait function, that functions by reading the output of the process gets into conflicts if the user needs to read and process the output of the process.

Microsofts dotnet does provide an event Exited for a process, which can be used to register a callback to react to the termination of the process. That gave me the idea, that something simliar should be possible.

The Callback is registered with the Windows API in
https://github.com/microsoft/referencesource/blob/51cf7850defa8a17d815b4700b67116e3fa283c2/System/services/monitoring/system/diagnosticts/Process.cs#L1403-L1422

So it looks like a method .wait_for_exit() might actually be a call to WaitForSingleObject with the OS-Handle to the launched Process.

As far as I see there is currently no way to implement this outside the library as the native process handle can not be obtained.
Using the PID to obtain this handle is also not the best idea, as the PID is invalidated as soon as the process terminates. So you do run the risk of a data race for short running processes.

[andfoy]

Using WaitForSingleObject sounds more reasonable, thanks for the suggestion! I'll implement it for the next release