threads / patch / 47618

patchdiff.c: flush stdout before printing rename warnings

Subject: [PATCH] diff.c: flush stdout before printing rename warnings

## tl;dr

3 messages between Jan 16, 2018 and Jan 17, 2018. Diffs are folded; open one to read it.

replies: 2people: 2as markdown or json

Nguyễn Thái Ngọc Duy· Jan 16, 2018, 09:23 UTC · lore

The diff output is buffered in a FILE object and could still be partially buffered when we print these warnings (directly to fd 2). The output is messed up like this

 worktree.c                                   |   138 +-
 worktree.h        warning: inexact rename detection was skipped due to too many files.
                           |    12 +-
 wrapper.c                                    |    83 +-

It gets worse if the warning is printed after color codes for the graph part are already printed. You'll get a warning in green or red.

Flush stdout first, so we can get something like this instead:

 xdiff/xutils.c                               |    42 +-
 xdiff/xutils.h                               |     4 +-
 1033 files changed, 150824 insertions(+), 69395 deletions(-)
warning: inexact rename detection was skipped due to too many files.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 diff.c | 1 +
 1 file changed, 1 insertion(+)

Show changes to diff.c +1 −0

diff --git a/diff.c b/diff.c
index fb22b19f09..5545c25640 100644
--- a/diff.c
+++ b/diff.c
@@ -5454,6 +5454,7 @@ N_("you may want to set your %s variable to at least "
 
 void diff_warn_rename_limit(const char *varname, int needed, int degraded_cc)
 {
+	fflush(stdout);
 	if (degraded_cc)
 		warning(_(degrade_cc_to_c_warning));
 	else if (needed)

-- 
2.15.1.600.g899a5f85c6

Junio C Hamano· Jan 16, 2018, 19:06 UTC · re: Nguyễn Thái Ngọc Duy · lore

Re: [PATCH] diff.c: flush stdout before printing rename warnings

Nguyễn Thái Ngọc Duy  <pclouds@gmail.com> writes:

Show 18 quoted lines

> The diff output is buffered in a FILE object and could still be
> partially buffered when we print these warnings (directly to fd 2).
> The output is messed up like this
>
>  worktree.c                                   |   138 +-
>  worktree.h        warning: inexact rename detection was skipped due to too many files.
>                            |    12 +-
>  wrapper.c                                    |    83 +-
>
> It gets worse if the warning is printed after color codes for the graph
> part are already printed. You'll get a warning in green or red.
>
> Flush stdout first, so we can get something like this instead:
>
>  xdiff/xutils.c                               |    42 +-
>  xdiff/xutils.h                               |     4 +-
>  1033 files changed, 150824 insertions(+), 69395 deletions(-)
> warning: inexact rename detection was skipped due to too many files.

The patch sort-of makes sense, and I am not sure if any of the issues that show rooms for potential improvements I'll mention are worth doing.

 - This matters only when the standard output and the starndard error
   are going to the same place.  It also would be conceptually nicer to
   flush stderr as well even though it is by default not fully
   buffered.

 - Also this function can take two bools and gives a warning that
   potentially cause the issue three out of four combinations, so one
   out of four cases we would be unnecessarily flushing.

Show 17 quoted lines

> Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
> ---
>  diff.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/diff.c b/diff.c
> index fb22b19f09..5545c25640 100644
> --- a/diff.c
> +++ b/diff.c
> @@ -5454,6 +5454,7 @@ N_("you may want to set your %s variable to at least "
>  
>  void diff_warn_rename_limit(const char *varname, int needed, int degraded_cc)
>  {
> +	fflush(stdout);
>  	if (degraded_cc)
>  		warning(_(degrade_cc_to_c_warning));
>  	else if (needed)

Duy Nguyen· Jan 17, 2018, 01:04 UTC · re: Junio C Hamano · lore

Re: [PATCH] diff.c: flush stdout before printing rename warnings

On Wed, Jan 17, 2018 at 2:06 AM, Junio C Hamano <gitster@pobox.com> wrote:

Show 27 quoted lines

> Nguyễn Thái Ngọc Duy  <pclouds@gmail.com> writes:
>
>> The diff output is buffered in a FILE object and could still be
>> partially buffered when we print these warnings (directly to fd 2).
>> The output is messed up like this
>>
>>  worktree.c                                   |   138 +-
>>  worktree.h        warning: inexact rename detection was skipped due to too many files.
>>                            |    12 +-
>>  wrapper.c                                    |    83 +-
>>
>> It gets worse if the warning is printed after color codes for the graph
>> part are already printed. You'll get a warning in green or red.
>>
>> Flush stdout first, so we can get something like this instead:
>>
>>  xdiff/xutils.c                               |    42 +-
>>  xdiff/xutils.h                               |     4 +-
>>  1033 files changed, 150824 insertions(+), 69395 deletions(-)
>> warning: inexact rename detection was skipped due to too many files.
>
> The patch sort-of makes sense, and I am not sure if any of the
> issues that show rooms for potential improvements I'll mention are
> worth doing.
>
>  - This matters only when the standard output and the starndard error
>    are going to the same place.

Which is usually true when pager is involved because we redirect stderr to the pager too 61b80509e3 (sending errors to stdout under $PAGER - 2008-02-16)

>    It also would be conceptually nicer to
>    flush stderr as well even though it is by default not fully
>    buffered.

There's more than that. I briefly considered if the same thing could happen elsewhere, this is not the only place we use buffered stdio. But since it's just a minor annoyance, I stuck to minimal fixes and the "fix as we see" approach instead of scanning the whole code base and preventing similar cases.

-- 
Duy