Minor doc update: "orphan" processes (#14271)

This patch makes two small changes to the documentation:

First, "zombie" processes are in a terminated state and will appear in
"ps" output in "Z" state.  The problem described in our documentation
seems to actually be about the "orphan" condition, in which a process
continues running after its parent terminates.  Most unix systems will
reparent the process with PID 1 but this can vary, but there is no
convention that the process should stop unless it has been wired using
eg. "prctl" to detect a terminated parent.  The distinction between
zombie and orphan is important because it means the child is still
active.

Second, our suggestion that the child process should detect stdin
closure is a good workaround but is not generally true for
applications that aren't doing their work on stdio.  I've tried to
clean up the text around the example shell script to clarify that
developers are free to use this technique to detect parent
termination, but it's not the rule for polite unix applications.
Hijacking stdin also comes with some drawbacks as is already
mentioned.

I'm not suggesting a different workaround at the moment—there is a
linux-specific technique of using "prctl" which is demonstrated in
https://groups.google.com/g/elixir-lang-core/c/yiepKrcEniU ,
however this isn't portable.

See #7495 and #9171

Author: adamwight

lib/elixir/lib/port.ex

@@ -142,22 +142,22 @@ defmodule Port do
   reimplementing core part of the Runtime System, such as the `:user` and
   `:shell` processes.
 
-  ## Zombie operating system processes
+  ## Orphan operating system processes
 
   A port can be closed via the `close/1` function or by sending a `{pid, :close}`
   message. However, if the VM crashes, a long-running program started by the port
   will have its stdin and stdout channels closed but **it won't be automatically
   terminated**.
 
-  While most Unix command line tools will exit once its communication channels
-  are closed, not all command line applications will do so. You can easily check
+  While some Unix command line tools will exit once its parent process
+  terminates, not all command line applications will do so. You can easily check
   this by starting the port and then shutting down the VM and inspecting your
   operating system to see if the port process is still running.
 
-  While we encourage graceful termination by detecting if stdin/stdout has been
-  closed, we do not always have control over how third-party software terminates.
-  In those cases, you can wrap the application in a script that checks for stdin.
-  Here is such script that has been verified to work on bash shells:
+  We do not always have control over how third-party software terminates.
+  If necessary, one workaround is to wrap the child application in a script that
+  checks whether stdin has been closed.  Here is such a script that has been
+  verified to work on bash shells:
 
       #!/usr/bin/env bash
 

lib/elixir/lib/system.ex

@@ -997,7 +997,7 @@ defmodule System do
   ports guarantee stdin/stdout devices will be closed but it does not
   automatically terminate the program. The documentation for the
   `Port` module describes this problem and possible solutions under
-  the "Zombie processes" section.
+  the "Orphan operating system processes" section.
 
   > #### Windows argument splitting and untrusted arguments {: .warning}
   >

lib/mix/lib/mix/tasks/cmd.ex

@@ -39,7 +39,7 @@ defmodule Mix.Tasks.Cmd do
 
     * `--cd` *(since v1.10.4)* - the directory to run the command in
 
-  ## Zombie operating system processes
+  ## Orphan operating system processes
 
   Beware that the Erlang VM does not terminate child processes
   when it shuts down. Therefore, if you use `mix cmd` to start
@@ -48,7 +48,7 @@ defmodule Mix.Tasks.Cmd do
 
   A solution is to make sure the child processes listen to the
   standard input and terminate when standard input is closed.
-  We discuss this topic at length in the "Zombie operating system processes"
+  We discuss this topic at length in the "Orphan operating system processes"
   of the `Port` module documentation.
   """