[FFmpeg-devel] [PATCH V5 1/2] configure: sort decoder/encoder/filter/... names in alphabet order

Alexander Strasser eclipse7 at gmx.net
Mon Jun 3 00:35:18 EEST 2019

Hi all,
hi Avi!

Sorry for the late reply.

On 2019-05-07 12:22 +0000, avih wrote:
> > patch1 (awk) configure: print_in_columns: replace pr with awk version: http://ffmpeg.org/pipermail/ffmpeg-devel/2019-May/243380.html
> > patch2 (shell) configure: sort decoder/encoder/filter/... names in alphabet
> > order (v5 as posted in this thread)
> >
> > - Why do you prefer patch1 over patch2?
> In general, I agree with your reasoning, but not entirely sure about the
> conclusion and its implications.
> Few exceptions first:
> > 1. Statistics
> >     * patch1: 1 file changed, 16 insertions(+), 2 deletions(-)
> >     * patch2: 1 file changed, 24 insertions(+), 2 deletions(-)
> As you said later, and I agree, it's a weak argument, as both versions
> are the same code and complexity, but differ mostly in style and language
> syntax.
> > 2. patch2 uses lots of variables (which is good in itself), but those
> >   should be local and we cannot express that portably in shell.
> >   In turn we have raised potential for clashes now or in the future.
> This is incorrect. A subshell provides identical isolation as an external
> process, it's fully portable, it can be used here, and it's not slower than
> an external process, e.g. `print_in_columns() ( ... )`.

No disagreement regarding those 2 points. Though with all other things
being equal, it's nice to have patches ready and tested as they are.

> > 4. Depending on the input (stdin) unexpected things can happen in the
> >   "eval" and "set" lines. One example is given in the comment.
> "Unexpected" is a bit general. Specifically, the "eval" line does not
> execute arbitrary inputs, and neither does the "set" line.

Regarding eval it seems I misremembered something or it was in
previous iteration. I looked at it again and I can't see how the
latest version can do something unexpected trough the eval.

> The only potential surprise is globbing, as documented, and the technique
> of expanding variables unquoted - like here, is used throughout configure.
> Look no further than 100% of the very places which pipe the input stream
> into print_in_columns, and multiple times earlier than that.
> It's not great of course, but it's not worse than elsewhere, and it's at
> least documented. Some day maybe someone would improve it.
> I generally agree with rest of your comments/arguments, including:
> > 1. Statistics ... (patch2 is more verbose).
> > 3. patch2 uses eval combined with non-trivial quoting, which is hard
> >   to read and hard to grasp quickly. It's not wrong, but it's not as
> >   easy and straight forward as in patch1.
> > 5. patch2 to uses shell for the parts easily expressed in shell and
> >   uses awk for the parts that are non-trivial in shell.
> > 6. The awk implementation should be easier to read and understand for
> >   the majority of readers/developers.
> which basically say that shell is hard, and, pardon the pun, awkward.


> And also I agree with:
> > Shell though is not really suited for implementing all kinds of algorithms.
> > OTOH shell is an excellent choice for starting processes that communicate
> > via stdin/stdout/stderr and plugging them together.
> and with
> > Shell is best when coordinating the execution of other programs.
> And similar assertions, however, let's keep the following facts in mind:
> - We both agree that we should keep to shell except where an external tool
>   provides some meaningful value over shell code.
> - An external tool only helps by being more suitable for the task than
>   shell code (readability, portability, correctness, performance, etc),
>   but it doesn't help with isolation more than a subshell would.
> - Shell is capable in a portable way, including for some tasks which can
>   be considered "programming".
> - configure is currently written in shell, including some non trivial
>   functionality, and doesn't use external script engines for "scriptlets".
> - Being a shell script, it already requires competence and reasonable
>   understanding of shell, including quoting, expansions, etc.
> - Maintaining code in more than one scripting system does carry a cost.
> - Yes, shell is a lousy programming language.
> I really don't care if this specific patch ends up as shell or awk. What
> I do care about is being consistent, and understanding the implications.
> IMHO, because I'm not convinced that the awk version provides a meaningful
> additional value other than not being shell, if we use the awk version then
> it sets a precedence of using awk (or maybe also other languages) for small
> scripts even if they're basically the same complexity and code as they
> would be in shell.
> It's not necessarily a bad precedence (even if I wouldn't do that myself),
> because I do agree that not being shell code does have value, but I think
> it's important to acknowledge that it does set such precedence.
> If there was already such practice of using awk scriptlets where possible
> then I wouldn't have said anything. So I mainly care about the precedence.

Interesting point. We seem to disagree regarding the reach of the
precedence. I don't believe it sets a precedence "of using awk (or
maybe also other languages) for small scripts even if they're basically
the same complexity and code as they would be in shell". For me this
is just an individual instance where we decided to implement the needed
"leaf" functionality in awk. With leaf I mean it is lower level code that
does not need to call into e.g. shell functions we provide on the
same or lower level. Comparable to e.g. calling an external process,
like we do with pr in the current implementation.

I would oppose to implement more upper level configure logic in
any language other than shell.

> I guess it comes down to a judgment call on whether or not this awk script
> can be considered meaningfully more suitable for the task than shell.
> I think not, but clearly it's only my subjective assessment.

I would still prefer to commit the awk version. IMHO it is better
suited for this task. Surely it's not ideal and it's not clearly
better in all ways.

I intent to push the awk version. I will wait at least until
Thursday, so people can further test, comment, or object.


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