Move the element size check in EbmlCallbacks

[robUx4]

Implements #272

And so we can check the validity of the size before an element is created.

[mbunkus]

Unfortunately these changes cause aborts due to failed assertions when reading data from damaged files:

[0 mosu@sweet-chili (main) ~/prog/video/mkvtoolnix/tests] mkvinfo data/mkv/sample-bug2989.mkv
+ EBML head
|+ EBML version: 1
|+ EBML read version: 1
|+ Maximum EBML ID length: 4
|+ Maximum EBML size length: 8
|+ Document type: matroska
|+ Document type version: 4
|+ (Unknown element: DummyElement; ID: 0x4220 size: 4)
+ Segment: size 129
mkvinfo: lib/libebml/src/EbmlElement.cpp:327: static EbmlElement *libebml::EbmlElement::FindNextElement(IOCallback &, const EbmlSemanticContext &, int &, std::uint64_t, bool, unsigned int): Assertion `Result->ElementSpec().IsSizeValid(SizeFound)' failed.
zsh: IOT instruction (core dumped)  mkvinfo data/mkv/sample-bug2989.mkv

This is the same old topic: reading invalid files must not abort the process (be strict in what you write/produce but lenient in what you read/accept).

[robUx4]

The assert is not there to break on bogus files, but to double check on the coherence of the code. The code that creates Result is probably correct. But it probably has an "infinite" size which is not handled in that test. I'll do a fix.

[robUx4]

And thanks for testing this (and the rest) by the way. I need to finish porting mkvalidator and mkclean as I would probably have caught it earlier.

[robUx4]

@mbunkus can you check with this new version ? Now the size needs to check if this a finite or infinite size value. The code behavior should be the same as before. Finite size elements with a bogus size were not accepted before (created by then discarded).

If it still fails, can you share the file ?

[mbunkus]

A small ask for the future: when you publish PRs in libEBML for which a corresponding change is required in libMatroska, please post in the libEBML PR the number of the libMatroska PR & vice versa. Makes it much easier for me to go through all of those changes. Thanks!

[mbunkus]

After testing your updated changes, reading certain damaged files still aborts with failed assertions:

[0 mosu@sweet-chili (main) ~/prog/video/mkvtoolnix/tests] mkvinfo data/mkv/sample-bug2989.mkv
+ EBML head
|+ EBML version: 1
|+ EBML read version: 1
|+ Maximum EBML ID length: 4
|+ Maximum EBML size length: 8
|+ Document type: matroska
|+ Document type version: 4
|+ (Unknown element: DummyElement; ID: 0x4220 size: 4)
+ Segment: size 129
mkvinfo: lib/libebml/src/EbmlElement.cpp:323: static EbmlElement *libebml::EbmlElement::FindNextElement(IOCallback &, const EbmlSemanticContext &, int &, std::uint64_t, bool, unsigned int): Assertion `Result->ElementSpec().IsSizeValid(SizeFound, SizeFound == SizeUnknown)' failed.

This is the file from above, but I have others.

[mbunkus]

Can you please re-base on current master? I'll give it another try afterwards. Thanks.

[robUx4]

I rebased the code but did not do any further investigation.