MDEV-38993 Assertion `trx->undo_no == 1' fails upon ALTER IGNORE

Problem:
========
During ALTER TABLE ... IGNORE, a partial rollback on duplicate key
error resets trx->undo_no to 0. The subsequent insert then enters
the undo rewrite block with undo_no == 0, hitting the assertion
that expected undo_no == 1.

Solution:
=========
Partial rollback which truncates the last insert undo record
via trx_undo_truncate_end(), which rewrites TRX_UNDO_PAGE_FREE
on the page. By checking trx->undo_no as part of the rewrite
predicate, InnoDB correctly skips the rewrite logic after partial
rollback.

trx_undo_report_row_operation(): Pre-compute the full predicate
(clear_ignore) before trx_undo_assign_low(), since old_offset
and top_offset are not modified by that call.

trx_undo_rewrite_ignore(): Extract the rewrite body into a
separate ATTRIBUTE_COLD ATTRIBUTE_NOINLINE static function.

Author: Thirunarayanan

mysql-test/suite/innodb/r/alter_crash.result

@@ -268,3 +268,10 @@ CHECK TABLE t1 EXTENDED;
 Table	Op	Msg_type	Msg_text
 test.t1	check	status	OK
 DROP TABLE t1;
+#
+# MDEV-38993 Assertion `trx->undo_no == 1' fails upon ALTER IGNORE
+#
+CREATE TABLE t (a INT) ENGINE=InnoDB;
+INSERT INTO t VALUES (1),(2),(1),(2);
+ALTER IGNORE TABLE t ADD UNIQUE(a);
+DROP TABLE t;

mysql-test/suite/innodb/t/alter_crash.test

@@ -267,3 +267,11 @@ INSERT INTO t1 VALUES(1), (1);
 ALTER IGNORE TABLE t1 ADD UNIQUE(f1);
 CHECK TABLE t1 EXTENDED;
 DROP TABLE t1;
+
+--echo #
+--echo # MDEV-38993 Assertion `trx->undo_no == 1' fails upon ALTER IGNORE
+--echo #
+CREATE TABLE t (a INT) ENGINE=InnoDB;
+INSERT INTO t VALUES (1),(2),(1),(2);
+ALTER IGNORE TABLE t ADD UNIQUE(a);
+DROP TABLE t;

storage/innobase/trx/trx0rec.cc

@@ -1807,6 +1807,30 @@ static bool trx_has_lock_x(const trx_t &trx, dict_table_t& table)
   return false;
 }
 
+/** For ALTER TABLE...IGNORE ALGORITHM=COPY, rewind the undo log
+to maintain only the latest insert undo record. This allows easy
+rollback of the last inserted row on duplicate key errors.
+@param table  		table being altered
+@param trx		transaction
+@param undo  		insert undo log
+@param undo_block	undo log page
+@param mtr		mini-transaction
+@param m		mod_tables entry to reinitialize */
+static ATTRIBUTE_COLD ATTRIBUTE_NOINLINE
+void trx_undo_rewrite_ignore(dict_table_t *table, trx_t *trx, trx_undo_t *undo,
+                             buf_block_t *undo_block, mtr_t *mtr,
+			     std::pair<trx_mod_tables_t::iterator, bool> &m)
+{
+  ut_ad(trx->undo_no == 1);
+  undo->top_offset= undo->old_offset;
+  undo->top_undo_no= 0;
+  trx->undo_no= 0;
+  trx->mod_tables.clear();
+  m= trx->mod_tables.emplace(index->table, 0);
+  mtr->write<2>(*undo_block, undo_block->page.frame + TRX_UNDO_PAGE_HDR +
+                TRX_UNDO_PAGE_FREE, undo->old_offset);
+}
+
 /***********************************************************************//**
 Writes information to an undo log about an insert, update, or a delete marking
 of a clustered index record. This information is used in a rollback of the
@@ -1912,32 +1936,18 @@ trx_undo_report_row_operation(
 		ut_ad(!trx->read_only);
 		ut_ad(trx->id);
 		pundo = &trx->rsegs.m_redo.undo;
-		const bool empty{!*pundo};
+		const bool clear_ignore{*pundo && trx->undo_no &&
+					(*pundo)->old_offset <=
+					  (*pundo)->top_offset &&
+					index->table->skip_alter_undo ==
+					dict_table_t::IGNORE_UNDO};
 		rseg = trx->rsegs.m_redo.rseg;
 		undo_block = trx_undo_assign_low<false>(trx, rseg, pundo,
 							&mtr, &err);
-		/* For ALTER IGNORE, implement undo log rewriting
-		to maintain only the latest insert undo record.
-		This allows easy rollback of the last inserted row
-		on duplicate key errors. Before writing a new undo
-		record, rewind the undo log to the previous record
-		position, effectively discarding all intermediate
-		undo records and keeping only the most recent one. */
-		if (!empty && (*pundo)->old_offset <= (*pundo)->top_offset &&
-		    index->table->skip_alter_undo ==
-			dict_table_t::IGNORE_UNDO) {
-
+		if (clear_ignore) {
 			ut_ad(!rec);
-			ut_ad(trx->undo_no == 1);
-			(*pundo)->top_offset = (*pundo)->old_offset;
-			(*pundo)->top_undo_no = 0;
-			trx->undo_no = 0;
-			trx->mod_tables.clear();
-			m = trx->mod_tables.emplace(index->table, 0);
-			mtr.write<2>(
-			  *undo_block,
-			  undo_block->page.frame + TRX_UNDO_PAGE_HDR +
-			  TRX_UNDO_PAGE_FREE, (*pundo)->old_offset);
+			trx_undo_rewrite_ignore(index->table, trx, *pundo,
+						undo_block, &mtr, m);
 		}
 	}