[ruby-core:97699] [Ruby master Bug#14413] `-n` and `-p` flags break when stdout is closed

Issue #14413 has been updated by josh.cheek (Josh Cheek).

File Screen Shot 2020-04-03 at 2.58.41 PM.png added

I saw flipflop was readded and was wanting to explain to people how to use it. I feel like if people knew how to use it, they wouldn't have wanted to remove it. So, I made this example (shell is fish, not sure if this example would work in bash):

``` shell
# a csv with a header
> cat ~/Downloads/Casos1' (3)'.csv | head -2
"ID de caso","Fecha de diagntico","Ciudad de ubicaci","Departamento o Distrito","Atenci**","Edad","Sexo","Tipo*","Pa﨎 de procedencia"
"1","06/03/2020","Bogot瓣,"BogotD.C.","Recuperado","19","F","Importado","Italia"


> cat ~/Downloads/Casos1' (3)'.csv |

# `-n` iterates over each line of input
# `-e` means the argument is the program, not a filename
ruby -ne '
  # `print` will print the current line.
  # `2..` is a flipflop beginning at 2, with no upper bound.
  # It will flip to true on line 2, and never flip back to false.
  # Hence, this prints all lines except for the first.
  print if 2..
' |

head -1
"1","06/03/2020","Bogot瓣,"BogotD.C.","Recuperado","19","F","Importado","Italia"
Traceback (most recent call last):
	2: from -e:6:in `<main>'
	1: from -e:6:in `print'
-e:6:in `write': Broken pipe @ io_write - <STDOUT> (Errno::EPIPE)
```

It does the correct behaviour (essentially `sed 1d`), but this example isn't compelling, because the error message means you would have to redirect stderr to `/dev/null`, which is hacky, and would also swallow real errors.


Perhaps another way to show this:

```shell
# desired
> seq 1000000 | sed -n '12,$p' | head -2
12
13

# actual
> seq 1000000 | ruby -ne 'print if 12..' | head -2
12
13
Traceback (most recent call last):
	2: from -e:1:in `<main>'
	1: from -e:1:in `print'
-e:1:in `write': Broken pipe @ io_write - <STDOUT> (Errno::EPIPE)
```

----------------------------------------
Bug #14413: `-n` and `-p` flags break when stdout is closed
https://bugs.ruby-lang.org/issues/14413#change-84905

* Author: josh.cheek (Josh Cheek)
* Status: Feedback
* Priority: Normal
* ruby -v: ruby 2.5.0preview1 (2017-10-10 trunk 60153) [x86_64-darwin16]
* Backport: 2.3: UNKNOWN, 2.4: UNKNOWN, 2.5: UNKNOWN
----------------------------------------
Ruby generally works well within a pipeline. The `-n` and `-p` flags are incredibly useful. However, it is common practice to use programs like `head` and `sed`, which will close the pipe after completing their job. This is convenient, because often it limits an expensive amount of output to a workable subset. I can figure out the pipeline for a subset of input, and then remove the limiting function.

However, Ruby explodes with `-e:1:in `write': Broken pipe @ io_write - <STDOUT> (Errno::EPIPE)`, when it writes the current line to stdout. When in a line oriented mode, and stdout closes, I think it should exit successfully (so it doesn't break bash scripts with `pipe fail`set) and silently (no error message), hence marking this a bug report rather than a feature request.

I've attached a screenshot to show that this is how every other program works, that I tried.

~~~
git clone https://github.com/jquery/esprima
cd esprima/test/fixtures/
ls                                                                             | head -1 # ls
find . -type f -name '*json'                                                   | head -1 # find
find . -type f -name '*json' | head -1 | xargs cat | jq .                      | head -1 # jq
find . -type f -name '*json' | grep -v JSX | grep -v tokenize                  | head -1 # grep
find . -type f -name '*json' | sed -E '/JSX|tokenize/d'                        | head -1 # sed
find . -type f -name '*json' | awk '/JSX|tokenize/ { next }; { print }'        | head -1 # awk
find . -type f -name '*json' | perl -e 'while(<>) { /JSX|tokenize/ || print }' | head -1 # perl
find . -type f -name '*json' | ruby -ne 'print unless /JSX|tokenize/'          | head -1 # ruby :(
~~~

---Files--------------------------------
Screen Shot 2018-01-27 at 7.27.49 PM.png (458 KB)
Screen Shot 2020-04-03 at 2.58.41 PM.png (158 KB)


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