Re: $PPID not updated when the PPID changes (parent killed)

[Bart Schaefer <schaefer@brasslantern.com>]

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On Tue, May 18, 2021 at 7:53 AM Bart Schaefer <schaefer@brasslantern.com> wrote
>
> PPID is not a special variable.

Well, I have to take that back; it IS documented as special, but only
because it's readonly (which it also is in e.g. bash).

Anyway, as per Stephane Chazelas,
> POSIX specification of sh says:
>
> > Set by the shell to the decimal value of its parent process ID
> > during initialization of the shell
>
> [...] it would be more useful if
> $PPID reported the realtime value of the parent pid, but no
> other shell does it and that would break POSIX compliance, and we
> already have sysparam[ppid] as already noted.

POSIX doesn't say whether it should be readonly, as far as I can tell.
It is not readonly when bash is invoked as "sh" but remains readonly
in zsh's sh emulation, which might be considered a bug.

So ... hopefully the attached patch addresses the assorted issues raised.

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diff --git a/Doc/Zsh/params.yo b/Doc/Zsh/params.yo
index dc28a45ae..b514eb072 100644
--- a/Doc/Zsh/params.yo
+++ b/Doc/Zsh/params.yo
@@ -654,10 +654,11 @@ Same as tt(#).
 )
 vindex($)
 item(tt($) <S>)(
-The process ID of this shell.  Note that this indicates the original
-shell started by invoking tt(zsh); all processes forked from the shells
-without executing a new program, such as subshells started by
-tt(LPAR())var(...)tt(RPAR()), substitute the same value.
+The process ID of this shell, set when the shell initializes.  Processes
+forked from the shell without executing a new program, such as command
+substitutions and commands grouped with tt(LPAR())var(...)tt(RPAR()),
+are subshells that duplicate the current shell, and thus substitute the
+same value for tt($$) as their parent shell.
 )
 vindex(-)
 item(tt(-) <S>)(
@@ -817,9 +818,9 @@ The operating system, as determined at compile time.
 )
 vindex(PPID)
 item(tt(PPID) <S>)(
-The process ID of the parent of the shell.  As for tt($$), the
-value indicates the parent of the original shell and does not
-change in subshells.
+The process ID of the parent of the shell, set when the shell initializes.
+As with tt($$), the value does not change in subshells created as a
+duplicate of the current shell.
 )
 vindex(PWD)
 item(tt(PWD))(
diff --git a/Test/E03posix.ztst b/Test/E03posix.ztst
index 7db4c0c84..564afac29 100644
--- a/Test/E03posix.ztst
+++ b/Test/E03posix.ztst
@@ -161,3 +161,6 @@ F:POSIX has neither math functions nor floating point
 0f:Width of %s is computed in bytes not characters
 F:This is considered a bugfix in zsh
 ><  Stéphane>
+
+  PPID=foo
+-f:PPID is not a readonly variable
diff --git a/Doc/Zsh/mod_system.yo b/Doc/Zsh/mod_system.yo
index 8f525c576..399b6fe03 100644
--- a/Doc/Zsh/mod_system.yo
+++ b/Doc/Zsh/mod_system.yo
@@ -263,9 +263,9 @@ tt($$), which returns the process ID of the main shell process.
 )
 item(tt(ppid))(
 vindex(ppid, sysparams)
-Returns the process ID of the parent of the current process, even in
-subshells.  Compare tt($PPID), which returns the process ID of the parent
-of the main shell process.
+Returns the current process ID of the parent of the current process, even
+in subshells.  Compare tt($PPID), which returns the process ID of the
+initial parent of the main shell process.
 )
 item(tt(procsubstpid))(
 Returns the process ID of the last process started for process