Fixed a crash when reading PNG files with an invalid header.

[macmade]

Description

Use of BOOST_ASSERT in png_ptr_read_deleter and png_ptr_write_deleter in ‎⁨include/boost/gil/extension/⁨io/png/⁨detail⁩/reader_backend.hpp will produce a crash with PNG files with an invalid header.

In such a case (bad PNG file), we expect a std::ios_base::failure exception (see below).
Unfortunately, the assertions will prevent the exception to be caught by terminating the program.

This hotfix replaces the assertions with conditional checks, so we can get the expected exception handling.

Steps to reproduce

1. Create a text file with at least 4 characters (like ABCD).
2. Try to read the file as PNG:

using backend_t   = typename boost::gil::get_reader_backend< std::string, boost::gil::png_tag >::type;
backend_t backend = boost::gil::read_image_info( "path/to/invalid/png", boost::gil::png_tag() );

Explanation

A reader_backend object is created: ‎⁨include/boost/gil/extension/⁨io/png/⁨detail⁩/reader_backend.hpp.

The base class' constructor (png_struct_info_wrapper) has a std::shared_ptr member for the png_ptr_wrapper.
The shared pointer is created with a custom deleter (png_ptr_read_deleter and png_ptr_write_deleter).

So inside the backend constructor, the png_ptr_wrapper is non-null.
Then, still from the constructor, the PNG header is read by calling read_header.

    io_error_if( png_sig_cmp( png_bytep(buf)
                            , png_size_t(0)
                            , PNG_BYTES_TO_CHECK
                            ) != 0
               , "png_check_validity: invalid png image"
               );

So for an invalid header, io_error_if will throw an exception.
This is the expected behavior.

But as the exception is thrown, the backend object gets deallocated.
So is the std::shared_ptr. And as the deleter is called:

static void png_ptr_read_deleter( png_ptr_wrapper* png_ptr )
{
    if( png_ptr )
    {
        BOOST_ASSERT(png_ptr->_struct && png_ptr->_info);

png_ptr is valid, but all fields will be null, as we haven't read the PNG file yet.
Assertion will fail, making the program crash, and thus preventing the expected exception to be caught.

References

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[mloskot]

@macmade

Assertion will fail, making the program crash

It should have also stated "only if compiled with -DNDEBUG", no?

Thanks for the report

[mloskot]

LGTM. Thanks for the fix.