MDEV-38899 Fix Assertion !((f->flags & 4096U) && f->vcol_info) upon RBR with NOBLOB and unique blob

[itzanway]

Fixes: Regression introduced by MDEV-36290.

Problem

When using Row-Based Replication (RBR) with binlog_row_image = NOBLOB, operations on a table with a UNIQUE constraint on a BLOB NOT NULL column cause a debug assertion failure in prepare_record() on the replica.

A UNIQUE index on a BLOB creates a hidden virtual column (to store the blob's hash). This hidden virtual column inherits the NOT NULL property and receives the NO_DEFAULT_VALUE_FLAG (4096U). During replication, if the blob isn't included in the row image, prepare_record() evaluates missing columns and hits the overly strict assertion.

Solution

Virtual columns evaluate their own expressions and do not use the default value mechanism. It is completely valid for them (especially automatically generated hidden ones) to have NO_DEFAULT_VALUE_FLAG set.

This PR removes the invalid assertion: DBUG_ASSERT(!((f->flags & NO_DEFAULT_VALUE_FLAG) && f->vcol_info)); and includes an MTR test case to prevent future regressions.

[gkodinov]

Thank you for your contribution! Before I do an actual review please:

• fix the buildbot failures
• squash into a single commit
• add a good commit message to it compliant with CODING_STANDARDS.md
• Make sure there's no extra changes into the PR (there's now removal of the BUILD dir for some reason)
• rebase to 12.3 (as the first affected version): this is a bugfix.

[itzanway]

Hi @gkodinov,

I have addressed all the review comments:

• Restored the accidentally deleted BUILD/ directory.

• Squashed everything into a single commit with a CODING_STANDARDS compliant message.

• Rebased to 12.3.

• Fixed the line endings (CRLF -> LF) and result mismatch in the new MTR test.

My replication tests are now passing! The only remaining failure on the buildbot is main.opt_hints, which appears to be an unrelated flaky test. Could you please re-trigger the buildbot or take another look? Thank you!