[MPlayer-dev-eng] demux_mf: improve format string processing

[Erik Auerswald]

Hello Alexander,

On 28.04.21 08:47, Alexander Strasser wrote:
> [...]
> Not sure when or if it will ever happen, but as a first step in
> that direction I will start with a documentation for the current
> state of mf:

If you do re-implement this functionality, it might be nice to
have a way of "quoting" special characters.  This is just a
suggestion, of course.  I'll add some details below.

If you do re-implement this functionality, and if you do add
some kind of quoting mechanism, I would prefer consistent
quoting (at least) inside the <mf spec>, e.g., a backslash
('\') removes the special meaning from the following character,
including "\\" to get a single backslash ('\').  As opposed
to, e.g., doubling special characters if they shall be taken
literally in one of the sub-specifications.

(I did not check if MPlayer already supports some kind of
quoting, since I have not yet needed something like this.)

> ---8<---
> 
>      mf://[<mf spec>]
> 
> The *mf spec* is a specification from which the input paths should be
> deduced. It can be one of these:
> 
>      <glob pattern>|<printf format>|<comma delimited paths>|@<list file>
> 
> If the *mf spec* is left out, a shell globbing pattern of `*` is used.
> 
> ## list file
> 
> If the first character of after *mf spec* is `@`, the remaining
> characters will be used as a path to open as a file.

If a file name starts with '@' this can probably worked around
by using "./@...", thus quoting does not seem to be essential
here.

> If the file is
> readable, each line will be interpreted as one path for the multi
> file input.
> 
> ## comma delimited paths
> 
> If *mf spec* contains a `,` it is considered to be list of comma
> delimited paths, that will be used as input paths in that order.

This seems a bit more restrictive in that file names that
contain a comma would need some way to prevent this rule
from kicking in ("quoting").

> ## glob pattern
> 
> If it **doesn't** contain a `%` it is assumed to be a glob pattern.

It might be helpful to have a method of allowing '%' in a
glob pattern.

> The glob pattern will be evaluated and the resulting list of files
> if any, will be used as paths for the multi file input.
> 
> If the glob pattern doesn't contain a `*`, it will be appended.
> 
> ## printf format
> 
> If *mf spec* **didn't** yet match any of the cases described above,

So this seems to be keyed on seeing '%' and thus inferring
a printf format string.  I would think it would be nice to
have a method of quoting '%', but not necessarily the printf
method "%%".

> it will be considered to be a printf format string. This string
> will be used to render an input path with a single int argument.
> This argument is a counter starting at zero, incrementing by one
> after each call.
> 
> Should it happen that for, in sum, 5 of any of the rendered paths
> the file status (stat) couldn't be retrieved, the path sequence
> will be considered finished.
> 
> --->8---
> 
> WARNING: I reverse engineered the description from the source
> code, but didn't actually test if all my assertions are correct.

Thanks,
Erik