MDEV-39559: Assertion b failed in my_strnncoll_xxxx_nopad_ci

[raghunandanbhat]

fixes MDEV-39559

Problem

When verifying a UNIQUE constraint over an empty BLOB/TEXT column, Field_blob::cmp(a_ptr, b_ptr) extracts a NULL data pointer (with length 0) from the record slot and forwards it to the charset-level comparison, which asserts non-NULL pointers (added in MDEV-35717).

A zero-length blob is allowed to have a NULL data pointer in its record slot; Field_blob::val_decimal() already treats (NULL, 0) as an empty value. The comparison paths missed that substitution.

Fix

• In Field_blob::cmp(a_ptr, b_ptr), Field_blob::cmp_prefix() and Field_blob::key_cmp(key_ptr, max_key_length), substitute "" for a NULL data pointer before delegating to the comparison.
• Add a debug assert in each of the above functions to document the invariant that a NULL pointer is only valid alongside zero length and (NULL, length>0) is invalid.

[gemini-code-assist]

Code Review

This pull request addresses MDEV-39559 by adding assertions and null checks for blob pointers in Field_blob::cmp, Field_blob::cmp_prefix, and Field_blob::key_cmp within sql/field.cc, as well as adding corresponding test cases. The review feedback points out a potential out-of-bounds memory read in release builds (where DBUG_ASSERT is disabled) if a blob pointer is null but its length is non-zero. To prevent this, it is recommended to explicitly reset the corresponding length variables to 0 when substituting null pointers with an empty string.

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