MDEV-40176: field_charset()->charpos(blob..) called with NULL

[grooverdan]

Problem:
When my_charpos_mb() is called by Field_blob::get_key_image_itRAW
with a start/end both being NULL. Because of this the blob_length
must have been 0.

Fix:
Rather than relying on character set functions to calculate the
storage of nothing, bypass the calculation as the charpos() isn't
going to return a value less than 0 (the blob_length).

[gemini-code-assist]

Code Review

This pull request introduces a null check for the blob pointer in Field_blob::get_key_image_itRAW before calculating the character length. The reviewer suggested checking if (blob_length > 0) instead of if (blob) to optimize performance for empty BLOBs and prevent passing NULL pointers to charpos.

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[gemini-code-assist]

Instead of checking if (blob), it is more efficient and robust to check if (blob_length > 0). If the BLOB length is 0, there is no need to perform any character position calculations or division, as blob_length is already 0 and cannot be reduced further. This avoids redundant overhead for empty BLOBs and safely prevents passing NULL pointers to charpos.

  if (blob_length > 0)
  {
    size_t local_char_length= length / mbmaxlen();
    local_char_length= field_charset()->charpos(blob, blob + blob_length,
                                                local_char_length);
    set_if_smaller(blob_length, local_char_length);
  }

[grooverdan]

bit clearer. Could DBUG_ASSERT(blob) in the block too.

[raghunandanbhat]

I believe this is not the only place where we call charpos function and if we were to do this kind of checks, we should be doing it in all other places too.
Fixing the root cause, i.e my_charpos_* set of functions to handle null seems like correct fix to me. thoughts @grooverdan ?

[raghunandanbhat]

@grooverdan , caller-side guard isn't wrong, it would stop the crash but it still leaves the actual UBSAN.