4 ms·

If this works, why does the featured article exist?

tedunangst 3y ago
Not every process wants to be the leader of its own process group.
Animats 3y ago
I'm puzzled, too. The example is about spawning threads in Rust, not processes under UNIX. In Rust, if the main thread dies, all other threads get killed. So child threads outliving the main thread is not a problem. A harder problem is making sure that the program shuts down if some thread panics. There are several ways to do that, but it's not automatic. Incidentally, regular mutexes are "poisoned" if the thread holding them panics. Anyone trying to lock the mutex gets an error. However, parking_lot mutexes do not do this. If the thread holding one of those panics, the mutex unlocks. This may be unsafe.
- neandrake 3y ago
  I think the example is the code for a child process which must use a separate thread to block on stdin for the whole process. As soon as the parent process dies (no example code?), the connection to the child process’ stdin will be unblock, causing the child monitor thread to detect and panic.
- thecodedmessage 3y ago
  The example is for the child process. The threads in Rust is meant as an implementation detail. This is super unclear and should’ve been explained.
userbinator 3y ago
Not everyone knows everything, and people will reinvent using what they do know. The process group mechanism may be one of the less well-known parts of UNIX.
- petters 3y ago
  The article mentions > There’s process group mechanism, but it is one-level deep and is mostly reserved for the shell. So the author seems aware that they exist, at least
thayne 3y ago
Well for one thing, the child process could handle sighup, or just mask it and ignore it. And as another comment mentioned, it only goes one level, so it won't signal grandchildren. But, if you control the code of the child, as this article assumes, you could probably deal with that.
- c7DJTLrn 3y ago
  >Well for one thing, the child process could handle sighup I've come across programs that use SIGHUP for various things like config reloading when they could use USR1/USR2 instead. UNIX signals are kind of crap in my opinion. USR1/USR2 should probably not even exist, there should be some other way to 'message' processes without IPC.
- xenadu02 3y ago
  The article is wrong FWIW. By default a child is a member of the same process group as its parent no matter how many levels deep. Shells push each job into a separate group, making the shell interpreter the process group leader. Thats why suspend works, it sends that whole process group to the background. Depending on how an interpreter is setup a shell script might end up under its own progress group if the shell always creates one... usually when invoked as an interpreter it shouldn't be making a new one though.
  - thayne 3y ago
    well process groups aren't hierarchal. So if a child (or other descendant) process sets itself as a process group leader, so that it can kill it's children, it escapes your process group. So it isn't composable.
    - xenadu02 3y ago
      Process groups aren't hierarchical because it is a two-level namespace: process groups (jobs) are members of a session and all processes in a job must be members of the same session. All child processes inherit their parent's process group and session. If they didn't then such a program would not work correctly in any shell pipeline and a bunch of other behavior would break. I'm assuming all processes are under your control in this scenario. It should be very rare for any process that isn't a shell or daemon to create its own process group or session. Sessions can optionally have a controlling terminal, one (and only one) foreground process group, plus any number of background process groups. The foreground job is how the system decides who receives signals generated by the TTY. If the controlling TTY disconnects or is closed the session leader gets SIGHUP which it usually uses to signal all the process groups it created. If the session leader dies or exits then all processes in the foreground process group get a SIGHUP. If you exit an interactive shell or close your SSH session the system generally cleans everything up as you would expect. The processes involved generally don't implement any special handling code to make this happen: the session leader (your login shell) uses the standard Unix job control APIs to manage the session + process groups and the expected behavior falls out naturally. Unless there is a good reason to implement this differently: don't. Use the standard Unix job control APIs as they were designed to be used.
mananaysiempre 3y ago
Structured concurrency as commonly understood requires children to not outlive parents, recursively. For example, if A spawns B spawns C, then B cancels for C with timeout 30s, then 5s later A cancels B with timeout 5s, then the timeout for C must be effectively reduced by 20s[1]. But the issue here is more basic than timeouts. If we are to use process groups in the example, then B must be a process group leader for C and simultaneously belong to the process group led by A, which is impossible on Unix—that’s what TFA means by “[the] process group mechanism [is] one-level deep”. Close but no cigar. (On the other hand, no reason this can’t be an extension to—I would even say a rationalization of—the process group mechanism. It doesn’t exist right now, though.) [1] https://250bpm.com/blog:71/ https://250bpm.com/blog:71/

Not every process wants to be the leader of its own process group.

I'm puzzled, too. The example is about spawning threads in Rust, not processes under UNIX. In Rust, if the main thread dies, all other threads get killed. So child threads outliving the main thread is not a problem. A harder problem is making sure that the program shuts down if some thread panics. There are several ways to do that, but it's not automatic. Incidentally, regular mutexes are "poisoned" if the thread holding them panics. Anyone trying to lock the mutex gets an error. However, parking_lot mutexes do not do this. If the thread holding one of those panics, the mutex unlocks. This may be unsafe.

neandrake 3y ago
I think the example is the code for a child process which must use a separate thread to block on stdin for the whole process. As soon as the parent process dies (no example code?), the connection to the child process’ stdin will be unblock, causing the child monitor thread to detect and panic.
thecodedmessage 3y ago
The example is for the child process. The threads in Rust is meant as an implementation detail. This is super unclear and should’ve been explained.

userbinator 3y ago

Not everyone knows everything, and people will reinvent using what they do know. The process group mechanism may be one of the less well-known parts of UNIX.

petters 3y ago
The article mentions > There’s process group mechanism, but it is one-level deep and is mostly reserved for the shell. So the author seems aware that they exist, at least

thayne 3y ago

Well for one thing, the child process could handle sighup, or just mask it and ignore it. And as another comment mentioned, it only goes one level, so it won't signal grandchildren. But, if you control the code of the child, as this article assumes, you could probably deal with that.

c7DJTLrn 3y ago
>Well for one thing, the child process could handle sighup I've come across programs that use SIGHUP for various things like config reloading when they could use USR1/USR2 instead. UNIX signals are kind of crap in my opinion. USR1/USR2 should probably not even exist, there should be some other way to 'message' processes without IPC.
xenadu02 3y ago
The article is wrong FWIW. By default a child is a member of the same process group as its parent no matter how many levels deep. Shells push each job into a separate group, making the shell interpreter the process group leader. Thats why suspend works, it sends that whole process group to the background. Depending on how an interpreter is setup a shell script might end up under its own progress group if the shell always creates one... usually when invoked as an interpreter it shouldn't be making a new one though.
- thayne 3y ago
  well process groups aren't hierarchal. So if a child (or other descendant) process sets itself as a process group leader, so that it can kill it's children, it escapes your process group. So it isn't composable.
  - xenadu02 3y ago
    Process groups aren't hierarchical because it is a two-level namespace: process groups (jobs) are members of a session and all processes in a job must be members of the same session. All child processes inherit their parent's process group and session. If they didn't then such a program would not work correctly in any shell pipeline and a bunch of other behavior would break. I'm assuming all processes are under your control in this scenario. It should be very rare for any process that isn't a shell or daemon to create its own process group or session. Sessions can optionally have a controlling terminal, one (and only one) foreground process group, plus any number of background process groups. The foreground job is how the system decides who receives signals generated by the TTY. If the controlling TTY disconnects or is closed the session leader gets SIGHUP which it usually uses to signal all the process groups it created. If the session leader dies or exits then all processes in the foreground process group get a SIGHUP. If you exit an interactive shell or close your SSH session the system generally cleans everything up as you would expect. The processes involved generally don't implement any special handling code to make this happen: the session leader (your login shell) uses the standard Unix job control APIs to manage the session + process groups and the expected behavior falls out naturally. Unless there is a good reason to implement this differently: don't. Use the standard Unix job control APIs as they were designed to be used.

mananaysiempre 3y ago

Structured concurrency as commonly understood requires children to not outlive parents, recursively. For example, if A spawns B spawns C, then B cancels for C with timeout 30s, then 5s later A cancels B with timeout 5s, then the timeout for C must be effectively reduced by 20s[1]. But the issue here is more basic than timeouts. If we are to use process groups in the example, then B must be a process group leader for C and simultaneously belong to the process group led by A, which is impossible on Unix—that’s what TFA means by “[the] process group mechanism [is] one-level deep”. Close but no cigar. (On the other hand, no reason this can’t be an extension to—I would even say a rationalization of—the process group mechanism. It doesn’t exist right now, though.) [1] https://250bpm.com/blog:71/ https://250bpm.com/blog:71/