Fix Warning 1300 for varbinary columns with bytes invalid as utf8mb4

[ggilder]

Related issue: #1660

Description

From the related issue:

When gh-ost replays a binlog DML event, the go-mysql library returns varbinary column values as a Go string (not []byte). In convertArg, the existing code only converted string → []byte when the column had a non-empty Charset (e.g. utf8mb4 for varchar). varbinary columns have no character set, so Charset is always "", and the string fell through unconverted.

The Go MySQL driver sends string args as MYSQL_TYPE_VAR_STRING with the connection's utf8mb4 charset metadata attached, causing MySQL to validate the bytes. If a varbinary value (e.g. a binary UUID) contains byte sequences that are invalid utf8mb4, MySQL emits Warning 1300. With gh-ost's panic-on-warnings enabled, this aborts the migration.

varbinary columns are commonly used for UUIDs which have arbitrary bytes and can easily trigger this problem.

Fix: add an else-if branch that detects binary storage types by MySQLType (binary, varbinary, *blob) and returns []byte, so the driver sends MYSQL_TYPE_BLOB (binary data) with no charset validation.

MySQLType is used rather than Charset == "" alone because test Column objects built via NewColumnList leave MySQLType unset, which would have changed the return type for all no-charset columns in existing tests. In production, inspect.go always populates MySQLType from information_schema.data_type.

In case this PR introduced Go code changes:

• contributed code is using same conventions as original code
• script/cibuild returns with no formatting errors, build errors or unit test errors.

[Copilot]

Pull request overview

This PR addresses MySQL Warning 1300 during binlog DML replay when varbinary (and other binary-storage) column values are provided as Go strings and end up being sent by the MySQL driver as text with connection charset metadata.

Changes:

• Update Column.convertArg to return []byte for binary-storage types (binary/varbinary/*blob) when Charset is empty.
• Add unit tests ensuring varbinary values returned as string or []uint8 are converted to []byte, preventing charset validation warnings.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
go/sql/types.go	Converts args for binary-storage types to []byte so the driver sends them as binary (avoiding Warning 1300).
go/sql/types_test.go	Adds regression tests for varbinary values containing invalid UTF-8 byte sequences.

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[meiji163]

Just ran into this myself 😄 nice timing!