Better handling for mediator time jumps in datashard (#2342)

Author: snaury

ydb/core/tx/datashard/datashard.cpp

@@ -2050,68 +2050,74 @@ TRowVersion TDataShard::GetMvccTxVersion(EMvccTxMode mode, TOperation* op) const
         << " ImmediateWriteEdge# " << SnapshotManager.GetImmediateWriteEdge()
         << " ImmediateWriteEdgeReplied# " << SnapshotManager.GetImmediateWriteEdgeReplied());
 
-    TRowVersion edge;
-    TRowVersion readEdge = Max(
-            SnapshotManager.GetCompleteEdge(),
-            SnapshotManager.GetUnprotectedReadEdge());
-    TRowVersion writeEdge = Max(readEdge, SnapshotManager.GetIncompleteEdge());
-    switch (mode) {
-        case EMvccTxMode::ReadOnly:
-            // With read-only transactions we don't need reads to include
-            // changes made at the incomplete edge, as that is a point where
-            // distributed transactions performed some reads, not writes.
-            // Since incomplete transactions are still inflight, the actual
-            // version will stick to the first incomplete transaction is queue,
-            // effectively reading non-repeatable state before that transaction.
-            edge = readEdge;
-            break;
-        case EMvccTxMode::ReadWrite:
-            // With read-write transactions we must choose a point that is
-            // greater than both complete and incomplete edges. The reason
-            // is that incomplete transactions performed some reads at that
-            // point and these snapshot points must be repeatable.
-            // Note that as soon as the first write past the IncompleteEdge
-            // happens it cements all distributed transactions up to that point
-            // as complete, so all future reads and writes are guaranteed to
-            // include that point as well.
-            edge = writeEdge;
-            break;
-    }
+    TRowVersion version = [&]() {
+        TRowVersion edge;
+        TRowVersion readEdge = Max(
+                SnapshotManager.GetCompleteEdge(),
+                SnapshotManager.GetUnprotectedReadEdge());
+        TRowVersion writeEdge = Max(readEdge, SnapshotManager.GetIncompleteEdge());
+        switch (mode) {
+            case EMvccTxMode::ReadOnly:
+                // With read-only transactions we don't need reads to include
+                // changes made at the incomplete edge, as that is a point where
+                // distributed transactions performed some reads, not writes.
+                // Since incomplete transactions are still inflight, the actual
+                // version will stick to the first incomplete transaction is queue,
+                // effectively reading non-repeatable state before that transaction.
+                edge = readEdge;
+                break;
+            case EMvccTxMode::ReadWrite:
+                // With read-write transactions we must choose a point that is
+                // greater than both complete and incomplete edges. The reason
+                // is that incomplete transactions performed some reads at that
+                // point and these snapshot points must be repeatable.
+                // Note that as soon as the first write past the IncompleteEdge
+                // happens it cements all distributed transactions up to that point
+                // as complete, so all future reads and writes are guaranteed to
+                // include that point as well.
+                edge = writeEdge;
+                break;
+        }
 
-    // If there's any planned operation that is above our edge, it would be a
-    // suitable version for a new immediate operation. We effectively try to
-    // execute "before" that point if possible.
-    if (auto nextOp = Pipeline.GetNextPlannedOp(edge.Step, edge.TxId))
-        return TRowVersion(nextOp->GetStep(), nextOp->GetTxId());
+        // If there's any planned operation that is above our edge, it would be a
+        // suitable version for a new immediate operation. We effectively try to
+        // execute "before" that point if possible.
+        if (auto nextOp = Pipeline.GetNextPlannedOp(edge.Step, edge.TxId))
+            return TRowVersion(nextOp->GetStep(), nextOp->GetTxId());
+
+        // Normally we stick transactions to the end of the last known mediator step
+        // Note this calculations only happen when we don't have distributed
+        // transactions left in queue, and we won't have any more transactions
+        // up to the current mediator time. The mediator time itself may be stale,
+        // in which case we may have evidence of its higher value via complete and
+        // incomplete edges above.
+        const ui64 mediatorStep = Max(MediatorTimeCastEntry ? MediatorTimeCastEntry->Get(TabletID()) : 0, writeEdge.Step);
+        TRowVersion mediatorEdge(mediatorStep, ::Max<ui64>());
+
+        switch (mode) {
+            case EMvccTxMode::ReadOnly: {
+                // We read at the end of the current step
+                return mediatorEdge;
+            }
+
+            case EMvccTxMode::ReadWrite: {
+                // We write at the end of the current step, or the start of the next step when that's protected
+                return Max(mediatorEdge, writeEdge.Next());
+            }
+        }
 
-    // Normally we stick transactions to the end of the last known mediator step
-    // Note this calculations only happen when we don't have distributed
-    // transactions left in queue, and we won't have any more transactions
-    // up to the current mediator time. The mediator time itself may be stale,
-    // in which case we may have evidence of its higher value via complete and
-    // incomplete edges above.
-    const ui64 mediatorStep = Max(MediatorTimeCastEntry ? MediatorTimeCastEntry->Get(TabletID()) : 0, writeEdge.Step);
-    TRowVersion mediatorEdge(mediatorStep, ::Max<ui64>());
+        Y_ABORT("unreachable");
+    }();
 
     switch (mode) {
         case EMvccTxMode::ReadOnly: {
-            // We want to include everything that was potentially confirmed to
-            // users, but we don't want to include anything that is not replied
-            // at the start of this read.
-            // Note it's only possible to have ImmediateWriteEdge > mediatorEdge
-            // when ImmediateWriteEdge == mediatorEdge + 1
-            return Max(mediatorEdge, SnapshotManager.GetImmediateWriteEdgeReplied());
+            // We must read all writes we have replied to already
+            return Max(version, SnapshotManager.GetImmediateWriteEdgeReplied());
         }
 
         case EMvccTxMode::ReadWrite: {
-            // We must use at least a previously used immediate write edge
-            // But we must also avoid trumpling over any unprotected mvcc
-            // snapshot reads that have occurred.
-            // Note it's only possible to go past the last known mediator step
-            // when we had an unprotected read, which itself happens at the
-            // last mediator step. So we may only ever have a +1 step, never
-            // anything more.
-            return Max(mediatorEdge, writeEdge.Next(), SnapshotManager.GetImmediateWriteEdge());
+            // We must never go backwards in our single-shard writes
+            return Max(version, SnapshotManager.GetImmediateWriteEdge());
         }
     }
 

ydb/core/tx/time_cast/time_cast.cpp

@@ -30,7 +30,10 @@ void TMediatorTimecastEntry::Update(ui64 step, ui64 *exemption, ui64 exsz) {
     Y_UNUSED(exemption);
     Y_UNUSED(exsz);
 
-    AtomicSet(Step, step);
+    // Mediator time shouldn't go back while shards are running
+    if (Get(0) < step) {
+        AtomicSet(Step, step);
+    }
 }
 
 class TMediatorTimecastProxy : public TActor<TMediatorTimecastProxy> {