MDEV-30863 Server freeze, all threads in trx_assign_rseg_low()

trx_assign_rseg_low(): Simplify the debug check.

trx_rseg_t::reinit(): Reset the skip_allocation() flag.
This logic was broken in the merge
commit 3e2ad0e9
of commit 0de3be8c
(that is, innodb_undo_log_truncate=ON would never be "completed").

Tested by: Matthias Leich

storage/innobase/include/trx0rseg.h

@@ -129,7 +129,8 @@ struct alignas(CPU_LEVEL1_DCACHE_LINESIZE) trx_rseg_t
 #endif
   }
   /** @return whether the segment is marked for undo truncation */
-  bool skip_allocation() const { return ref_load() & SKIP; }
+  bool skip_allocation() const
+  { return ref.load(std::memory_order_acquire) & SKIP; }
   /** Increment the reference count */
   void acquire()
   { ut_d(auto r=) ref.fetch_add(REF); ut_ad(!(r & SKIP)); }

storage/innobase/trx/trx0rseg.cc

@@ -403,6 +403,7 @@ void trx_rseg_t::reinit(uint32_t page)
   last_commit_and_offset= 0;
   last_page_no= FIL_NULL;
   curr_size= 1;
+  ref.store(0, std::memory_order_release);
 }
 
 /** Read the undo log lists.

storage/innobase/trx/trx0trx.cc

@@ -811,30 +811,20 @@ static void trx_assign_rseg_low(trx_t *trx)
 	static Atomic_counter<unsigned>	rseg_slot;
 	unsigned slot = rseg_slot++ % TRX_SYS_N_RSEGS;
 	ut_d(if (trx_rseg_n_slots_debug) slot = 0);
+	ut_d(const auto start_scan_slot = slot);
 	trx_rseg_t*	rseg;
 
-#ifdef UNIV_DEBUG
-	ulint	start_scan_slot = slot;
-	bool	look_for_rollover = false;
-#endif /* UNIV_DEBUG */
-
 	bool	allocated;
 
 	do {
 		for (;;) {
 			rseg = &trx_sys.rseg_array[slot];
 
-#ifdef UNIV_DEBUG
-			/* Ensure that we are not revisiting the same
-			slot that we have already inspected. */
-			if (look_for_rollover) {
+			do {
+				ut_d(if (!trx_rseg_n_slots_debug) continue);
+				slot = (slot + 1) % TRX_SYS_N_RSEGS;
 				ut_ad(start_scan_slot != slot);
-			}
-			look_for_rollover = true;
-#endif /* UNIV_DEBUG */
-
-			ut_d(if (!trx_rseg_n_slots_debug))
-			slot = (slot + 1) % TRX_SYS_N_RSEGS;
+			} while (0);
 
 			if (!rseg->space) {
 				continue;