Piping interactive bash to a function stops the process

[JoshCheek]

I initially reported this issue here.

Description

When I pipe bash -i into a function (in this case, grep), the process gets stopped. @faho did some investigating and said:

The problem with bash -i seems to be that it grabs the terminal for itself - strace -e ioctl shows it doing one with TIOCSPGRP - which is the underlying syscall for tcsetpgrp on linux.
-- #1362 (comment)

My environment

$  fish --version
fish, version 2.7.1

$ echo $version
2.7.1

$ uname -a
Darwin Joshs-MacBook-Air-2.local 15.6.0 Darwin Kernel Version 15.6.0: Mon Aug 29 20:21:34 PDT 2016; root:xnu-3248.60.11~1/RELEASE_X86_64 x86_64

$ echo $TERM
xterm-256color

Demo / how to reproduce

# ===== Setup =====

# get a clean environment
$ sh -c 'env HOME=$(mktemp -d) fish'
Welcome to fish, the friendly interactive shell

# shorten the prompt
josh@Joshs-MacBook-Air-2 /U/josh> function fish_prompt; echo '> '; end


# ===== Demo the issue =====

# hangs (I'll press C-c)
> bash -i -c 'echo hello >&2' | grep whatever
Job 1, 'bash -i -c 'echo hello >&2' | g…' has stopped

# its state is "stopped"
> jobs
Job	Group	State	Command
1	24863	stopped	bash -i -c 'echo hello >&2' | grep whatever

# when I foreground it, it completes as expected
> fg
Send job 1, 'bash -i -c 'echo hello >&2' | grep whatever' to foreground
hello


# ===== Slight variations that don't exhibit the issue =====

# use the grep command instead of the function
> bash -i -c 'echo hello >&2' | command grep whatever
hello

# run bash without the -i flag
> bash -c 'echo hello >&2' | grep whatever
hello


# ===== Other potentially useful info =====

# the grep function
> type grep
grep is a function with definition
function grep
    command grep --color=auto $argv
end

[faho]

Okay, this is another issue related to #4540, #4653.

• Fish starts a "keepalive" process that gets its own process group.

• Fish starts bash in the keepalive's pgroup.

• Fish goes on to start grep, and because it is inside a function loses track of the old job and starts it in a new job and hence its own pgroup. It assigns control of the terminal to grep.

• With "-i", bash grabs control of the terminal for its pgroup, away from grep.

• Bash doesn't read from or write to the terminal

• grep does, and receives SIGTTOU, which stops it

• The pipeline hangs

[JoshCheek]

Oh, note that in my example the bash process writes to stderr, and grep writes to stdout. It's leftover context from the initial process I was debugging.

I just checked, and it doesn't seem to matter, though, this has the same issue:

$ bash -ic '' | grep x

[JoshCheek]

Another example to make the function thing clearer:

# Start: iterate over each line of input and print it out.
# There is no input, so it completes the iteration and exits
$ bash -ic '' | ruby -e '$stdin.each_line { |l| p l }'

# Now we put the RHS into a function
$ function x; ruby -e '$stdin.each_line { |l| p l }'; end

# When we pipe to that function instead, it hangs.
# When I press C-c, it sends an interrupt, Ruby dies, something gets stopped
$ bash -ic '' | x
^CTraceback (most recent call last):
	1: from -e:1:in `<main>'
-e:1:in `each_line': Interrupt
Job 6, 'bash -ic '' | x' has stopped

[benitogf]

hello not sure if it's the right thread to ask but is there a way to do something like diff <(echo "fish") <(echo "bash") ?

[faho]

hello not sure if it's the right thread to ask

@benitogf: Usually, you'd either open a new issue here (we do allow questions) or you'd ask on https://gitter.im/fish-shell/fish-shell.

Anyway, fish does not have the <(command) syntax ("process substitution"). Instead, it has the psub function, that allows it to be written as (command | psub). So this would translate to diff (echo fish | psub) (echo bash | psub).

[benitogf]

@faho thanks for the quick reply 🥇

[faho]

This seems to be fixed via #5219.