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fix(historian): #691.5 a reverse walk proves the rest empty once and carries it page to page

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The last pages of a deep reverse page-through on a forward-only backend
each walked down to 1970 one x4 growth step at a time (~14-16 reads,
~2.5 s live). After two empty segments the walk now asks once per page
for the whole remainder at the page budget (a truncated answer is
discarded and the walk carries on), and the stretch the page's last
complete read proved empty rides the continuation point
(HistoryContinuationState.EmptyFromTicks/EmptyToTicks, session-held, so
no versioning) so the next page steps over it. Fuzz now pages with the
walk's own hints and with true random empty stretches.
This commit is contained in:
Joseph Doherty
2026-10-12 16:32:00 -04:00
parent d33992034d
commit 186b7d5ea5
7 changed files with 222 additions and 29 deletions
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src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/HistoryPaging.cs
+14 -1
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@@ -55,6 +55,13 @@ namespace ZB.MOM.WW.OtOpcUa.OpcUaServer;
/// <see cref="HistoryCursorKind.Raw"/> reverse reads on a forward-only backend only (Gitea #686): the segment width the
/// walk-back ended on, so the next page starts at the observed density instead of re-discovering it. 0 = none.
/// </param>
/// <param name="EmptyFromTicks">
/// Walked reverse reads only (Gitea #691): with <paramref name="EmptyToTicks"/>, a stretch <c>(from, to)</c> the previous
/// page's walk proved holds no row, so the next page steps over it rather than re-walking it (to the 1970 floor, ~14
/// reads, at the end of a deep page-through). 0/0 = none. The cursor lives in the session's in-memory store (never
/// serialized), so a new field with a "none" default is all the versioning it needs.
/// </param>
/// <param name="EmptyToTicks">The exclusive upper end of that stretch.</param>
internal sealed record HistoryContinuationState(
string Tagname,
DateTime NextStartUtc,
@@ -65,7 +72,13 @@ internal sealed record HistoryContinuationState(
long LastModifiedId = 0,
DateTime WindowStartUtc = default,
bool Reverse = false,
long SegmentWidthTicks = 0);
long SegmentWidthTicks = 0,
long EmptyFromTicks = 0,
long EmptyToTicks = 0)
{
/// <summary>The walk-back hint this cursor carries (#686/#689/#691).</summary>
public ReverseWalkHint WalkHint => new(SegmentWidthTicks, EmptyFromTicks, EmptyToTicks);
}
/// <summary>The HistoryRead kind that issued a <see cref="HistoryContinuationState"/>.</summary>
internal enum HistoryCursorKind