Re: [PATCH 2/3] pager: factor out a helper to prepare a child process to run the pager

Jeff King [off-list ref] writes:

On Tue, Feb 16, 2016 at 03:06:56PM -0800, Junio C Hamano wrote:

quoted

diff --git a/pager.c b/pager.c
index 5dbcc5a..1406370 100644
--- a/pager.c
+++ b/pager.c

@@ -53,6 +53,23 @@ const char *git_pager(int stdout_is_tty)
 	return pager;
 }
 
+void prepare_pager_args(struct child_process *pager_process, ...)
+{
+	va_list ap;
+	const char *arg;
+
+	va_start(ap, pager_process);
+	while ((arg = va_arg(ap, const char *)))
+		argv_array_push(&pager_process->args, arg);
+	va_end(ap);
+
+	pager_process->use_shell = 1;
+	if (!getenv("LESS"))
+		argv_array_push(&pager_process->env_array, "LESS=FRX");
+	if (!getenv("LV"))
+		argv_array_push(&pager_process->env_array, "LV=-c");
+}

When reading this, I had to wonder what the "..." args were supposed to
be. I figured it out when I read the caller, but I wonder if a comment
would help. Also, we are expecting the pager here as the first argument,
so maybe:

  void prepare_pager_args(struct child_process *pager_process,
                          const char *pager, ...);

would be a better signature.

;-)

I had that originally, and then incorrectly fed pager to va_start()
which broke the whole thing.

That also made me wonder if we could simply
get away with:

  void prepare_pager_args(struct child_process *pager_process,
                          const char *pager);

and have callers argv_array_push() themselves afterwards.

After all that may be a nicer way to structure.

And if you put the git_pager() call inside prepare_pager_args (which I
agree would be cleaner), we just have:

  void prepare_pager_args(struct child_process *pager_process);

which is pretty self-explanatory (though it might need a new name; I'd
be tempted to call it init_pager_process() or something, and actually
have it do the child_process_init() to make sure it is working with a
sane clean slate).

Conceptually I am on the same page, but I am not sure how well that
interacts with what "git am -i" codepath wants to do, though.

One big difference between the "we'll feed our output to pager"
codepath and "we'll spawn a pager to let a file on the filesystem be
read" codepath is that the former needs to call git_pager() and
check the NULL-ness of the return value to decide that it does not
want to spawn a pager and let the standard output just go straight
to the outside world.  The latter, on the other hand, does want to
spawn something to cause the file to be presented to the end user
even git_pager() returns NULL.

And that is why I didn't make this helper call git_pager() itself.

On Tue, Feb 16, 2016 at 03:49:55PM -0800, Junio C Hamano wrote:

quoted

And if you put the git_pager() call inside prepare_pager_args (which I
agree would be cleaner), we just have:

  void prepare_pager_args(struct child_process *pager_process);

which is pretty self-explanatory (though it might need a new name; I'd
be tempted to call it init_pager_process() or something, and actually
have it do the child_process_init() to make sure it is working with a
sane clean slate).

Conceptually I am on the same page, but I am not sure how well that
interacts with what "git am -i" codepath wants to do, though.

One big difference between the "we'll feed our output to pager"
codepath and "we'll spawn a pager to let a file on the filesystem be
read" codepath is that the former needs to call git_pager() and
check the NULL-ness of the return value to decide that it does not
want to spawn a pager and let the standard output just go straight
to the outside world.  The latter, on the other hand, does want to
spawn something to cause the file to be presented to the end user
even git_pager() returns NULL.

And that is why I didn't make this helper call git_pager() itself.

That makes sense. I didn't dig into it carefully. I saw the "pager=cat"
thing in the context of your diff to git-am, and assumed it was weird
fallback that should be done by the regular pager infrastructure. But
it's the exact thing you're talking about here.

So of all of the things I suggested, I think the non-varargs one that
takes "pager" as a string makes the most sense.

-Peff