MDEV-15914: Restore MLOG_UNDO_INSERT

trx_undof_page_add_undo_rec_log(): Write the MLOG_UNDO_INSERT
record instead of the equivalent MLOG_2BYTES and MLOG_WRITE_STRING.
This essentially reverts commit 9ee8917d.

In MariaDB 10.3, I attempted to simplify the crash recovery code
by making use of lower-level redo log records. It turns out that
we must keep the redo log parsing code in order to allow crash-upgrade
from older MariaDB versions (MDEV-14848).

Now, it further turns out that the InnoDB redo log record format is
suboptimal for logging multiple changes to a single page. This simple
change to the redo logging of undo log significantly affects the
INSERT and UPDATE performance.

Essentially, we wrote
	(space_id,page_number,MLOG_2BYTES,2 bytes)
	(space_id,page_number,MLOG_WRITE_STRING,N+4 bytes)
instead of the previously written
	(space_id,page_number,MLOG_UNDO_INSERT,N+2 bytes)

The added redo log volume caused a single-threaded INSERT
(without innodb_adaptive_hash_index) of
1,000,000 rows to consume 11 seconds instead of 9 seconds,
and a subsequent UPDATE of 30,000,000 rows to consume 64 seconds
instead of 58 seconds. If we omitted all redo logging for the
undo log, the INSERT would consume only 4 seconds.

storage/innobase/include/mtr0types.h

@@ -100,7 +100,7 @@ enum mlog_id_t {
 	/** Create an index page */
 	MLOG_PAGE_CREATE = 19,
 
-	/** insert an undo log record (used in MariaDB 10.2) */
+	/** insert an undo log record */
 	MLOG_UNDO_INSERT = 20,
 
 	/** erase an undo log page end (used in MariaDB 10.2) */

storage/innobase/include/trx0rec.h

@@ -244,7 +244,7 @@ trx_undo_prev_version_build(
 				into this function by purge thread or not.
 				And if we read "after image" of undo log */
 
-/** Parse MLOG_UNDO_INSERT for crash-upgrade from MariaDB 10.2.
+/** Parse MLOG_UNDO_INSERT.
 @param[in]	ptr	log record
 @param[in]	end_ptr	end of log record buffer
 @param[in,out]	page	page or NULL

storage/innobase/trx/trx0rec.cc

@@ -70,12 +70,37 @@ trx_undof_page_add_undo_rec_log(
 	ut_ad(mach_read_from_2(undo_page
 			       + TRX_UNDO_PAGE_HDR + TRX_UNDO_PAGE_FREE)
 	      == new_free);
-	mlog_write_ulint(undo_page + TRX_UNDO_PAGE_HDR + TRX_UNDO_PAGE_FREE,
-			 new_free, MLOG_2BYTES, mtr);
-	mlog_log_string(undo_page + old_free, new_free - old_free, mtr);
+	mtr->set_modified();
+	switch (mtr->get_log_mode()) {
+	case MTR_LOG_NONE:
+	case MTR_LOG_NO_REDO:
+		return;
+	case MTR_LOG_SHORT_INSERTS:
+		ut_ad(0);
+		/* fall through */
+	case MTR_LOG_ALL:
+		break;
+	}
+
+	const uint32_t
+		len = uint32_t(new_free - old_free - 4),
+		reserved = std::min<uint32_t>(11 + 13 + len,
+					      mtr->get_log()->MAX_DATA_SIZE);
+	byte* log_ptr = mtr->get_log()->open(reserved);
+	const byte* log_end = log_ptr + reserved;
+	log_ptr = mlog_write_initial_log_record_fast(
+		undo_page, MLOG_UNDO_INSERT, log_ptr, mtr);
+	mach_write_to_2(log_ptr, len);
+	if (log_ptr + 2 + len <= log_end) {
+		memcpy(log_ptr + 2, undo_page + old_free + 2, len);
+		mlog_close(mtr, log_ptr + 2 + len);
+	} else {
+		mlog_close(mtr, log_ptr + 2);
+		mtr->get_log()->push(undo_page + old_free + 2, len);
+	}
 }
 
-/** Parse MLOG_UNDO_INSERT for crash-upgrade from MariaDB 10.2.
+/** Parse MLOG_UNDO_INSERT.
 @param[in]	ptr	log record
 @param[in]	end_ptr	end of log record buffer
 @param[in,out]	page	page or NULL