[FFmpeg-devel] [PATCH v13 2/4] libavformat/avisynth.c: Remove MAX_PATH limit

[Stephen Hutchinson]

On 6/11/22 1:01 PM, nil-admirari at mailo.com wrote:
>> Why not use the AviSynth mechanism that allows to supply a UTF-8 string?
>>
>> https://github.com/AviSynth/AviSynthPlus/blob/c377916aa4146d2f4386852d91dc177d49103c16/avs_core/core/parser/script.cpp#L477-L481
> 
> Was not aware such a mechanism exists.
> 
> Commit dates back to 10 April 2017, first release supporting it is, apparently, Avisynth+ r2487-MT: https://github.com/pinterf/AviSynthPlus/releases/tag/r2489-MT.
> 
> A remark in https://github.com/FFmpeg/FFmpeg/blob/master/libavformat/avisynth.c#L844 says:
> 
> /* On Windows, FFmpeg supports AviSynth interface version 6 or higher.
>   * This includes AviSynth 2.6 RC1 or higher, and AviSynth+ r1718 or higher,
>   * and excludes 2.5 and the 2.6 alphas. */
> 
> Support for plain AviSynth will have to be dropped.

Presumably, the original manifest idea, parsed down to only using it to 
force FFmpeg into UTF-8, would be sufficient for this, right?  As long 
as AviSynth inherits that from FFmpeg, UTF-8 strings would be pervasive 
and both A) the utf8 parameter would not need to be used and B) 2.6 
would work just fine with it, transparently.

The Windows API does have a SetConsoleCP function.  If that accomplishes 
the same effect as the manifest idea, that would be simpler, but it 
probably would need to be located somewhere *other* than the AviSynth 
demuxer.  And while it might work for the fftools themselves, does it 
also work for usage of the libraries directly in applications that may 
not be console apps?

Barring that, if/else checks to ensure that
A) IsWindowsVersionOrGreater is at least 1903
A1) If yes, go to B
A2) If no, use the existing logic

B) If yes, GetACP to check that it's UTF8
B1) If yes, the Import call stays the same as it is now, no utf8 parameter
B2) If no, that's where things get complicated:

C1) Use the no result to tell it to then force UTF-8 mode with 
SetConsoleCP, if that actually works for what we need it to do.  Then 
don't use the utf8 parameter.
C2) Use avs_get_version to detect an incompatible version of AviSynth 
and gracefully exit with a message about upgrading to a supported 
version of AviSynth+, before then using the utf8 parameter for real.
C3) Use avs_get_version, but if it's not a new enough version, just fall 
back to the logic that exists now, where 2.6 may or may not work just 
because the system may or may not be already set to UTF-8.

C2 should really be considered a last resort IMO, because it's an 
artificial limit and doesn't actually have anything to do with the 
AviSynth API.

The reason is that the utf8 parameter being discussed here is not part 
of the AviSynth API, it's an option handed to one of the script-level 
functions that avs_invoke (which is the actual API call there) is using.

> On the other hand, configure checks for https://github.com/FFmpeg/FFmpeg/blob/master/libavformat/avisynth.c#L844
> 
>> die "ERROR: AviSynth+ header version must be >= 3.7.1"
> 
> so probably plain AviSynth and AviSynth+ below r2489-MT are already unsupported.
> 

The header version check there isn't because of old versions of 
AviSynth(+) being unsupported (as far as the demuxer is concerned, 
anyway).  3.7.1 is still API compatible with 2.6 in all the functions 
the demuxer uses that are shared between them. The additional 
Plus-specific functionality is enabled with runtime checks, so if you 
don't use the newer header, it will fail to build, but you can run 2.6 
without problems even when using the newer header to compile the demuxer.