MDEV-39074 trans_rollback_stmt(THD *): Assertion `! thd->in_sub_stmt' failed.

[midenok]

slave_close_thread_tables() unconditionally called trans_commit_stmt()/trans_rollback_stmt(), which assert !thd->in_sub_stmt. A BINLOG statement with malformed base64 payload executed from an AFTER INSERT trigger hits this: on decode failure, mysql_client_binlog_statement() sets thd->is_error() and calls slave_close_thread_tables() from within the trigger's sub-statement.

Guard it with spcont/in_sub_stmt, deferring cleanup to the enclosing top-level statement. Other callers run only from the top-level SQL slave applier thread, so this doesn't change their behavior.

1. BINLOG statement executed from a trigger, SF or SP is disabled by the patch

Row events (Rows_log_event) open their target table via a one-shot check that only fires at the top of a fresh statement. Inside a trigger, the table is never opened this way. It may be fixed by reusing query_tables, but:

• find_locked_table() matched only by table name -- could return the TABLE instance the enclosing statement was actively writing through, not an idle one. Reusing it would require pre-saving its state: record[0]/bitmaps/handler/etc. (will deprecate ER_CANT_UPDATE_USED_TABLE_IN_SF_OR_TRG)
• set_stmt_row_injection()/set_time() calls mutated thd->lex, which at that point is main_lex -- shared with the enclosing statement, not something safe to touch.

Statement events (Query_log_event) run the embedded query via mysql_parse(): it bundles lex_start(), reset_for_next_command() and parse_sql() as one unit meant for a genuinely new top-level statement, not a one-off nested parse.

Calling parse_sql() directly instead avoids that, but then we own everything mysql_parse() was doing for us: a private LEX and a private Query_arena (or allocations land on main_lex/whatever arena is currently active, shared with the enclosing statement), plus calling mysql_execute_command() ourselves afterwards.

In any case, DML for query_tables cannot be done due to ER_CANT_UPDATE_USED_TABLE_IN_SF_OR_TRG reasons explained above.

2. PS for BINLOG statement still works and needs a leak fix for statement events

A BINLOG statement decoding to a Query_log_event, executed via PREPARE/EXECUTE, leaked its nested query's allocations onto the PS's own persistent arena: thd->stmt_arena pointed at it while mysql_parse() ran the decoded query. Second EXECUTE asserted on ROOT_FLAG_READ_ONLY, since PROTECT_STATEMENT_MEMROOT marks that arena read-only after a successful execution.

The fix redirects thd->stmt_arena to thd itself for the duration of the nested mysql_parse() call, so stmt_arena->is_conventional() reads true and activate_stmt_arena_if_needed() (called e.g. from save_leaf_tables()) never redirects allocations to the PS's arena in the first place.

Harmless for what it's protecting: leaf_tables_exec is normally cached on the persistent arena so a repeatedly-executed statement's SELECT_LEX doesn't rebuild it every time, but our SELECT_LEX is torn down and reparsed fresh (due to mysql_parse() semantics) on every EXECUTE, so there's nothing to cache here regardless of which arena is used.

Initial attempt for more fixes was made by PR #5692 (now closed as too complex for stable branches)

[Copilot]

🟡 Changes recommended

Nested BINLOG events can still mutate the retained format-description cache before rejection.

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Pull request overview

This PR hardens BINLOG execution in stored-program contexts and fixes prepared-statement arena handling.

Changes:

• Defers nested replication cleanup.
• Adds stored-program event checks and PS arena isolation.
• Adds regression coverage.

File summaries

File	Summary
sql/rpl_rli.cc	Defers transaction and table cleanup in sub-statements.
sql/log_event_server.cc	Guards nested event execution and PS parsing.
mysql-test/suite/binlog/t/binlog_base64_flag.test	Adds regression scenarios.
mysql-test/suite/binlog/r/binlog_base64_flag.result	Records expected results.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 1
• Review effort level: Lite

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[Copilot]

🟡 Changes recommended

The rejection occurs after validation has already mutated the retained format-description state.

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Review details

• Files reviewed: 5/5 changed files
• Comments generated: 1
• Review effort level: Balanced

[Copilot]

🔵 Needs a closer look

It changes low-level replication cleanup and prepared-statement memory ownership, requiring final human validation.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 1
• Review effort level: Balanced

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