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Re: leaks fd for internal functions but not external command


From: Sam Liddicott
Subject: Re: leaks fd for internal functions but not external command
Date: Tue, 23 Jul 2019 22:15:00 +0100

I'm very surprised that you continue to insist that it should be a *design*
decision that it should be hard for a script writer to be able to tell if a
handle will be left open or not.

What could be the rationale for such a design decision?

The vague justification you provide "there are plenty of things that depend
on whether or not a command is builtin, or whether it's run in the parent
shell" is true but more relevant to an implementation constraint than a
design decision.

I'm confident that most of these things you hint at are too *avoid* the
scripter needing to be aware of the difference between internal and
external commands.

A design decision may well be to leave a variable handle open, but what
*design* decision would add the proviso that it not be an external command?

I stress that the notable comparisons are not between 1 and 3; but between
1 and 2 or between 2 and 4.

Sam

On Tue, 23 Jul 2019, 21:57 Chet Ramey, <address@hidden> wrote:

> On 7/23/19 12:12 PM, Sam Liddicott wrote:
>
> > Given what you have explained as intentional, it the difference between 1
> > and 2, but it is best understood as a 4-way difference, outlined here:
> >
> > 1. {var} internal: fd remains open in parent
> > 2. {var} external: fd closed in parent
> > 3. numeric internal: fd closed in parent
> > 4. numeric external: fd closed in parent
> >
> > 1. {var} internal: fd remains open in parent
> > bash -c 'echo 1 {_}>&2 2>&1 1>&${_} {_}<&- ;
> >         echo done ; lsof -p $$ | grep CHR'
> >
> > 2. {var} external: fd closed in parent
> > bash -c '/bin/echo 1 {_}>&2 2>&1 1>&${_} {_}<&- ;
> >         echo done ; lsof -p $$ | grep CHR'
> >
> > 3. numeric internal: fd closed in parent
> > bash -c 'echo 1 10>&2 2>&1 1>&10 10<&- ;
> >         echo done ; lsof -p $$ | grep CHR'
> >
> > 4. numeric external: fd closed in parent
> > bash -c '/bin/echo 1 10>&2 2>&1 1>&10 10<&- ;
> >         echo done ; lsof -p $$ | grep CHR'
> >
> > You've indicated that {var} syntax leaves me an fd to do with what I
> wish.
>
> Yes. The question is whether the {_}<&- should close the file descriptor
> `permanently' or whether that close action should be undone because you're
> not using `exec' and the file descriptor was created using the variable
> assignment syntax. The latter is what bash does now.
>
> > You've also explained what bash is doing that makes this untrue if the
> > command was an external command.
>
> Because you never open the file descriptor in the parent shell:
> redirections happen in the child. The child process in case 2 (and 4)
> can do what it wants with the file descriptor, and its view of the
> file descriptor is the same as case 1 (and 3).
>
> > I don't believe that this behaviour is *intended( to depend on the
> > non-obvious detail of whether or not the command is external.
>
> Why? There are plenty of things that depend on whether or not a command is
> builtin, or whether it's run in the parent shell.
>
> >
> > Given the coding pattern of wrapping external commands with functions
> that
> > re-invoke using bash "command"; this can lead to unpredictable behaviours
> > when such wrappers are active.
> >
> > e.g. openssl() { LD_LIBRARY_PATH="$LD_LIBRARY_PATH:/extra" *command*
> > openssl "$@"; }
> >
> > If that function is defined, I get a handle leak*, if it isn't and main
> > openssl is called, I don't -- or from the other of view my handle got
> > closed without my knowledge, so I can't use it as I wish.
> >
> > Personally I would wish that "{var} internal" would also close the fd as
> it
> > does for numeric fd and for external {var} fd, because if I really wanted
> > to open an fd and have it hang around I would do a naked: exec {xxx}>&2 ;
> > type of thing.
>
> Sure. But there's not as good a reason to have that syntax otherwise. It's
> just syntactic sugar, a way for historical shells to use file descriptors
> greater than 9.
>
> > I also think that based on your description of what bash is doing, it
> might
> > be easier to fix by also closing in the parent, as I describe.
> >
> > It would bring full consistency and avoid hard to detect and hard to
> > code-around bugs.
> >
> > Ultimately, unless {var} external is intended to behave different to
> {var}
> > internal, then we have a consistency bug.
>
> The internal/external difference doesn't really have anything to do with
> the semantics of file descriptors or redirections per se; only that
> redirections are performed in the child. There are multiple variations in
> behavior depending on whether a command is executed in a subshell, and this
> is one of them.
>
> Ultimately, the difference is between cases 1 and 3 and retaining a handle
> to the file descriptor instead of closing it when the command finishes.
> Everything else is identical between the two commands. That was a design
> choice. I don't see changing it now.
>
> Chet
> --
> ``The lyf so short, the craft so long to lerne.'' - Chaucer
>                  ``Ars longa, vita brevis'' - Hippocrates
> Chet Ramey, UTech, CWRU    address@hidden    http://tiswww.cwru.edu/~chet/
>


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