MDEV-30671 follow-up: Remove the field TRX_UNDO_NEEDS_PURGE

Because downgrades from 11.0 to older MariaDB server are not possible
due to the removal of the InnoDB change buffer, there is no need to
access the field TRX_UNDO_NEEDS_PURGE anymore.

storage/innobase/include/trx0undo.h

@@ -488,10 +488,10 @@ completely purged and trx_purge_free_segment() has started freeing it */
 /** Transaction end identifier (if the log is in a history list),
 or 0 if the transaction has not been committed */
 #define	TRX_UNDO_TRX_NO		8
-/** Before MariaDB 10.3.1, when purge did not reset DB_TRX_ID of
+/* Before MariaDB 10.3.1, when purge did not reset DB_TRX_ID of
 surviving user records, this used to be called TRX_UNDO_DEL_MARKS.
 
-This field is redundant; it is only being read by some debug assertions.
+This field was removed in MariaDB 11.0.
 
 The value 1 indicates that purge needs to process the undo log segment.
 The value 0 indicates that all of it has been processed, and
@@ -500,7 +500,7 @@ trx_purge_free_segment() has been invoked, so the log is not safe to access.
 Before MariaDB 10.3.1, a log segment may carry the value 0 even before
 trx_purge_free_segment() was called, for those undo log records for
 which purge would not result in removing delete-marked records. */
-#define	TRX_UNDO_NEEDS_PURGE	16
+/*#define	TRX_UNDO_NEEDS_PURGE	16*/
 #define	TRX_UNDO_LOG_START	18	/*!< Offset of the first undo log record
 					of this log on the header page; purge
 					may remove undo log record from the

storage/innobase/trx/trx0purge.cc

@@ -262,7 +262,6 @@ trx_purge_add_undo_to_history(const trx_t* trx, trx_undo_t*& undo, mtr_t* mtr)
 	trx_ulogf_t*	undo_header	= undo_page->page.frame
 		+ undo->hdr_offset;
 
-	ut_ad(mach_read_from_2(undo_header + TRX_UNDO_NEEDS_PURGE) <= 1);
 	ut_ad(rseg->needs_purge > trx->id);
 
 	if (UNIV_UNLIKELY(mach_read_from_4(TRX_RSEG + TRX_RSEG_FORMAT
@@ -352,8 +351,6 @@ trx_purge_add_undo_to_history(const trx_t* trx, trx_undo_t*& undo, mtr_t* mtr)
 	mtr->write<8,mtr_t::MAYBE_NOP>(*undo_page,
 				       undo_header + TRX_UNDO_TRX_NO,
 				       trx->rw_trx_hash_element->no);
-	mtr->write<2,mtr_t::MAYBE_NOP>(*undo_page, undo_header
-				       + TRX_UNDO_NEEDS_PURGE, 1U);
 
 	if (rseg->last_page_no == FIL_NULL) {
 		rseg->last_page_no = undo->hdr_page_no;
@@ -408,13 +405,6 @@ trx_purge_free_segment(mtr_t &mtr, trx_rseg_t* rseg, fil_addr_t hdr_addr)
                                            nullptr, BUF_GET_POSSIBLY_FREED,
                                            &mtr, &err))
   {
-    /* Mark the last undo log totally purged, so that if the system
-    crashes, the tail of the undo log will not get accessed again. The
-    list of pages in the undo log tail gets inconsistent during the
-    freeing of the segment, and therefore purge should not try to
-    access them again. */
-    mtr.write<2,mtr_t::MAYBE_NOP>(*block, block->page.frame +
-                                  hdr_addr.boffset + TRX_UNDO_NEEDS_PURGE, 0U);
     while (!fseg_free_step_not_header(TRX_UNDO_SEG_HDR + TRX_UNDO_FSEG_HEADER +
                                       block->page.frame, &mtr))
     {
@@ -925,12 +915,8 @@ static void trx_purge_rseg_get_next_history_log(
   if (const buf_block_t* undo_page=
       buf_page_get_gen(page_id_t(purge_sys.rseg->space->id, prev_log_addr.page),
                        0, RW_S_LATCH, nullptr, BUF_GET_POSSIBLY_FREED, &mtr))
-  {
-    const byte *log_hdr= undo_page->page.frame + prev_log_addr.boffset;
-
-    trx_no= mach_read_from_8(log_hdr + TRX_UNDO_TRX_NO);
-    ut_ad(mach_read_from_2(log_hdr + TRX_UNDO_NEEDS_PURGE) <= 1);
-  }
+    trx_no= mach_read_from_8(undo_page->page.frame + prev_log_addr.boffset +
+                             TRX_UNDO_TRX_NO);
 
   mtr.commit();
 

storage/innobase/trx/trx0rseg.cc

@@ -522,8 +522,6 @@ static dberr_t trx_rseg_mem_restore(trx_rseg_t *rseg, mtr_t *mtr)
       rseg->needs_purge= id;
 
     rseg->set_last_commit(node_addr.boffset, id);
-    ut_ad(mach_read_from_2(block->page.frame + node_addr.boffset +
-                           TRX_UNDO_NEEDS_PURGE) <= 1);
 
     if (rseg->last_page_no != FIL_NULL)
       /* There is no need to cover this operation by the purge

storage/innobase/trx/trx0undo.cc

@@ -588,13 +588,8 @@ static uint16_t trx_undo_header_create(buf_block_t *undo_page, trx_id_t trx_id,
                                      undo_page->page.frame) != 0))
     mtr->memset(undo_page, free + TRX_UNDO_TRX_NO, 8, 0);
 
-  /* Write TRX_UNDO_NEEDS_PURGE=1 and TRX_UNDO_LOG_START. */
-  mach_write_to_2(buf, 1);
-  memcpy_aligned<2>(buf + 2, start, 2);
-  static_assert(TRX_UNDO_NEEDS_PURGE + 2 == TRX_UNDO_LOG_START,
-                "compatibility");
-  mtr->memcpy<mtr_t::MAYBE_NOP>(*undo_page, free + TRX_UNDO_NEEDS_PURGE +
-                                undo_page->page.frame, buf, 4);
+  mtr->memcpy<mtr_t::MAYBE_NOP>(*undo_page, free + TRX_UNDO_LOG_START +
+                                undo_page->page.frame, start, 2);
   /* Initialize all fields TRX_UNDO_XID_EXISTS to TRX_UNDO_HISTORY_NODE. */
   if (prev_log)
   {