std: Support consuming a Process without waiting

[alexcrichton]

Forking off a child which survives the parent is often a useful task, and is
currently not possible because the Process type will invoke wait() in its
destructor in order to prevent leaking resources. This function adds a new safe
method, daemonize, which can be used to consume an instance of Process which
will then not call wait in the destructor.

This new method is clearly documented as a leak of resources, but it must be
forcibly opted in to.

Closes #14467

[lilyball]

What makes it forceful?

[lilyball]

Do we actually want to leave stdin/stdout/stderr/extra_io open? I assumed we'd just want to avoid calling wait().

Also, this doesn't really daemonize the process. It just allows the child to outlive the Process object.

[lilyball]

To clarify, daemonization is typically assumed to involve things like ignoring SIGHUP, and perhaps adopting a new process group.

[alexcrichton]

This doesn't leave the I/O handles open, the function here consumes the type and the drop glue will take over after we canceled our own destructor, closing all the handles.

I suppose I use the term daemon for any long-running process, so I'm not too familiar with any specific connotations it has. Do you have a suggestion for an alternate name to use? The name detach was brought up as partially in conflict with the detach option on Command, so I was hoping to avoid that name.

[lilyball]

Ah, you're right, they'll close automatically. I was fooled by the fact that the Drop impl closes them explicitly, but it's doing that because it needs that to happen before wait().

How about orphan()? It's giving away its child instead of sticking around to take care of it ;)

[alexcrichton]

According to wikipedia, "a daemon is a computer program that runs as a background process, rather than being under the direct control of an interactive user", which sounds like what this is doing. What do others think about the name daemonize?

[lilyball]

daemon() is a function that detaches from the controlling terminal. Notably, it forks and gives the child a new process group. Basically, daemonizing is what Command::detached() does to the new child. The function you're adding does not muck with process groups, and it doesn't muck with SIGHUP (which is the other thing daemonizing usually tweaks).

[lilyball]

Perhaps Command::detached() should be renamed to Command::daemonized() and then your new function can be called detach().

[brson]

Regardless of the definition, I feel that 'daemonize' is fairly jargony (and clearly open to interpretation).

[alexcrichton]

Pushed with a renaming of the method to forget which may more accurately reflect what exactly it's doing (only forgetting to collect the status of the process).

[alexcrichton]

ping r?