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Re: leaks fd for internal functions but not external command


From: Chet Ramey
Subject: Re: leaks fd for internal functions but not external command
Date: Tue, 23 Jul 2019 16:57:57 -0400
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:60.0) Gecko/20100101 Thunderbird/60.8.0

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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