Re: [PATCH] clean: explicitly `fflush` stdout

[Jeff King <peff@peff.net>]

On Wed, Apr 08, 2020 at 02:51:11PM -0700, Junio C Hamano wrote:

> > Agreed. I guess it didn't affect people on other platforms much because
> > stdout to a terminal is generally line-buffered on POSIX systems. But
> > it's better not to rely on that, plus it could create confusion if
> > somebody tried to manipulate the interactive operation through a pipe
> > (e.g., driving it from a GUI or something).
> 
> Hmph, I thought it was more common to do prompts etc. on the
> standard error stream, which tends to make the buffering of the
> output less of a problem, but apparently these prompts are given to
> the standard output.

The prompts in git-add--interactive.perl go to stdout, as do the ones
for "am --interactive". I think that's reasonable, if you think of it as
the main output of the program.

At one point the "am" ones actually went to /dev/tty, but IMHO that's a
mistake. In 99% of cases it behaves the same as looking at stdin/stdout
(because they're connected to the terminal anyway), and in the other 1%
it's just as likely to be the wrong thing as the right. Plus it makes
testing much harder.

> I am also OK to sprinkle fflush(stdout) but in
> the longer term, it would probably be a good idea to introduce a
> helper to "prompt then grab input" or "read user input" (if the
> former, we'd be able to bring consistency into "which between the
> standard output or the standard error does a prompt come?", and if
> the latter we'd do fflush(NULL) before reading), especially if there
> are many git subcommands that go interactive.

Agreed. There is git_prompt(), but that does the tty thing, which I
think we'd want to avoid in most cases. It's used by the credential
code, which really does want to use a terminal to do things like turn
off character echoing. Plus it's not the main function of the program,
but rather a side effect (so we have to avoid disrupting the main
stdin/stdout).

-Peff