Allow prompt() and confirm() to read from redirected stdin

[solb]

I love that Deno has built-in prompt() and confirm() functions because they make it easy to write command-line programs. Deno scripts make excellent replacements for Bourne shell scripts in many cases (and make things such as JSON processing trivial!).

However, the input functions' response to input redirection and piping hinders non-interactive use of command-line utilities. Although they have long returned null and false, respectively, I think the functions should instead retain their normal behavior regardless of whether the input is from a terminal.

As a simple example, the standard Unix utility rev lets one reverse strings from standard input:

$ rev
This is a line I typed.
.depyt I enil a si sihT
This is another line I typed.
.depyt I enil rehtona si sihT
$

It is most useful when used in a pipeline, like so:

$ fortune -l | rev
,slevel neves no tliub saw ti taht hcus saw htiriT saniM fo noihsaf eht roF
llaw hcae ni dna ,llaw a tes saw hcae tuoba dna ,llih a otni devled hcae
.etag a saw
"gniK eht fo nruteR ehT" ,neikloT .R.R.J --		

nehw ,4.4V ,"serutcurtS ataD dna slanretnI SMV" ni detouQ[	
].weivrevo metsys ot gnirrefer 	
$

One can trivially implement a rev clone in Deno by putting the following in an executable script named rev:

#!/usr/bin/env -S deno run --ext ts --check

let read;
while((read = prompt("")) != null)
	console.log([...read].reverse().join(""));

However, although the first use case works, the second exits immediately when prompt() returns null:

$ ./rev
 This is a line I typed.
.depyt I enil a si sihT
 This is another line I typed.
.depyt I enil rehtona si sihT

$ fortune -l | ./rev
$

The current behavior is achieved by the stdin.isTerminal() checks in runtime/js/41_prompt.js. They have been present since the input functions were introduced, having been added in response to a reviewer feedback in #7507 with no further discussion. I think that supporting this style of command-line utility is more important than matching browser behavior in this case.

I suspect that removing this special case would obviate other issues such as #22633.

[lucacasonato]

prompt, confirm, and input are meant for users. They should not block when called in CI for example. Changing this would be a breaking change.

If you want to collect input from a user or machine, I suggest you call Deno.stdin.readSync() to read stdin input.