Merge master into feat/677-aggregates

# Conflicts: # docs/Historian.md
2026-10-11 11:30:46 -04:00
parent 27ca6cd791 8e6ac4b344
commit 9120182314
66 changed files with 2054 additions and 154 deletions
@@ -54,7 +54,7 @@ public sealed class ModifiedHistoryAtomicityTests(SqlHistorianFixture fx)
    public async Task A_replace_whose_trail_insert_fails_changes_nothing()
    {
        var db = await ReadTestDb.CreateAsync(fx);
-        var u = new SqlServerHistorianDataUpdater(db.Conn, new SqlServerTagCatalog(db.Conn, db.Boot));
+        var u = new SqlServerHistorianDataUpdater(db.Conn, db.Boot, new SqlServerTagCatalog(db.Conn, db.Boot));
        (await u.UpdateDataAsync("t", HistoryUpdateMode.Insert, [V(T1, 1)], Who, CancellationToken.None))
            .ShouldBe([HistorianStatusCodes.GoodEntryInserted]);
        (await CountTrailAsync(db)).ShouldBe(1);
@@ -78,7 +78,7 @@ public sealed class ModifiedHistoryAtomicityTests(SqlHistorianFixture fx)
    public async Task Deletes_whose_trail_insert_fails_delete_nothing()
    {
        var db = await ReadTestDb.CreateAsync(fx);
-        var u = new SqlServerHistorianDataUpdater(db.Conn, new SqlServerTagCatalog(db.Conn, db.Boot));
+        var u = new SqlServerHistorianDataUpdater(db.Conn, db.Boot, new SqlServerTagCatalog(db.Conn, db.Boot));
        await u.UpdateDataAsync("t", HistoryUpdateMode.Insert, [V(T1, 1), V(T2, 2)], Who, CancellationToken.None);

        await BreakTheTrailAsync(db);
@@ -99,7 +99,7 @@ public sealed class ModifiedHistoryAtomicityTests(SqlHistorianFixture fx)
    public async Task An_over_long_principal_is_truncated_never_refused()
    {
        var db = await ReadTestDb.CreateAsync(fx);
-        var u = new SqlServerHistorianDataUpdater(db.Conn, new SqlServerTagCatalog(db.Conn, db.Boot));
+        var u = new SqlServerHistorianDataUpdater(db.Conn, db.Boot, new SqlServerTagCatalog(db.Conn, db.Boot));
        var who = new HistoryUpdateContext(new string('p', 500), new string('k', 500));

        (await u.UpdateDataAsync("t", HistoryUpdateMode.Insert, [V(T1, 1)], who, CancellationToken.None))
@@ -178,6 +178,97 @@ public sealed class RetentionTests(SqlHistorianFixture fx)
        (await LockIsFreeAsync(s)).ShouldBeTrue();
    }

+    [Fact]
+    public async Task Cancelling_mid_command_while_the_delete_is_blocked_still_releases_the_applock()
+    {
+        fx.SkipIfUnavailable();
+        var (c, b, _, s) = Make();
+        await SeedAsync(b, s);
+
+        // A second session holds an exclusive table lock, so the purger's first DELETE blocks server-side.
+        await using var holder = new SqlConnection(fx.ConnectionString);
+        await holder.OpenAsync();
+        await using var tx = (SqlTransaction)await holder.BeginTransactionAsync();
+        await using (var cmd = holder.CreateCommand())
+        {
+            cmd.Transaction = tx;
+            cmd.CommandText = $"SELECT COUNT(*) FROM [{s}].Sample WITH (TABLOCKX, HOLDLOCK)";
+            await cmd.ExecuteScalarAsync();
+        }
+        short holderSpid;
+        await using (var cmd = holder.CreateCommand())
+        {
+            cmd.Transaction = tx;
+            cmd.CommandText = "SELECT CAST(@@SPID AS smallint)";
+            holderSpid = (short)(await cmd.ExecuteScalarAsync())!;
+        }
+
+        using var cts = new CancellationTokenSource();
+        var purger = new SqlServerHistorianRetention(c, b, TimeSpan.FromHours(1), 30, 4000, time: new FixedTime(Now));
+        var purge = purger.PurgeAsync(cts.Token);
+
+        // Cancel only once the server reports a DELETE actually blocked behind the holder — a fixed delay could
+        // cancel during bootstrap/applock acquisition on a slow host and never exercise the mid-command path.
+        var deadline = DateTime.UtcNow.AddSeconds(15);
+        while (await CountAsync(
+                   $"SELECT COUNT(*) FROM sys.dm_exec_requests WHERE blocking_session_id = {holderSpid} AND command = 'DELETE'") == 0)
+        {
+            purge.IsCompleted.ShouldBeFalse("the purge finished without ever blocking on the holder's table lock");
+            DateTime.UtcNow.ShouldBeLessThan(deadline, "the purge's DELETE never blocked behind the holder");
+            await Task.Delay(50);
+        }
+        cts.Cancel();
+
+        // SqlClient surfaces the attention as OperationCanceledException or as a SqlException.
+        var ex = await Should.ThrowAsync<Exception>(() => purge);
+        (ex is OperationCanceledException or SqlException).ShouldBeTrue($"unexpected {ex.GetType().Name}: {ex.Message}");
+
+        await tx.RollbackAsync();
+        (await LockIsFreeAsync(s)).ShouldBeTrue();
+        (await CountAsync($"SELECT COUNT(*) FROM [{s}].Sample")).ShouldBe(21);
+    }
+
+    [Fact]
+    public async Task A_failing_release_falls_back_to_clearing_the_pool_so_the_applock_is_freed()
+    {
+        fx.SkipIfUnavailable();
+        var (c, b, _, s) = Make();
+        await SeedAsync(b, s);
+        var purger = new SqlServerHistorianRetention(c, b, TimeSpan.FromHours(1), 30, 4000, time: new FixedTime(Now))
+        {
+            ReleaseSql = "EXEC sp_this_procedure_does_not_exist;",
+        };
+
+        await purger.PurgeAsync(CancellationToken.None);
+
+        (await LockIsFreeAsync(s)).ShouldBeTrue();
+    }
+
+    [Fact]
+    public async Task Falsification_control_a_failing_release_with_a_no_op_ClearPool_leaves_the_applock_held()
+    {
+        fx.SkipIfUnavailable();
+        var (c, b, _, s) = Make();
+        await SeedAsync(b, s);
+        var purger = new SqlServerHistorianRetention(c, b, TimeSpan.FromHours(1), 30, 4000, time: new FixedTime(Now))
+        {
+            ReleaseSql = "EXEC sp_this_procedure_does_not_exist;",
+            ClearPool = _ => { },
+        };
+
+        await purger.PurgeAsync(CancellationToken.None);
+
+        try
+        {
+            // The pooled session still owns the Session-scoped lock: proves the fallback test above is not vacuous.
+            (await LockIsFreeAsync(s)).ShouldBeFalse();
+        }
+        finally
+        {
+            SqlConnection.ClearAllPools(); // drop the leaked session so the lock does not outlive the test
+        }
+    }
+
    [Fact]
    public async Task A_throwing_purge_releases_the_applock()
    {
@@ -32,7 +32,7 @@ public sealed class SchemaBootstrapIntegrationTests(SqlHistorianFixture fx)
        var s = SqlHistorianFixture.UniqueSchema();
        await Boot(Opts(s)).EnsureAsync(CancellationToken.None);
        await Boot(Opts(s)).EnsureAsync(CancellationToken.None); // fresh instance => re-runs the guarded path
-        (await ScalarAsync($"SELECT COUNT(*) FROM [{s}].SchemaVersion")).ShouldBe(SqlServerSchemaBootstrapper.RequiredVersion);
+        (await ScalarAsync($"SELECT COUNT(*) FROM [{s}].SchemaVersion")).ShouldBe(SqlServerSchemaBootstrapper.ModifiedTrailVersion);
        (await ScalarAsync($"SELECT COUNT(*) FROM sys.indexes WHERE name='CX_Event_Time' AND object_id=OBJECT_ID('[{s}].Event')")).ShouldBe(1);
    }

@@ -51,13 +51,14 @@ public sealed class SchemaBootstrapIntegrationTests(SqlHistorianFixture fx)
            cmd.CommandText = b;
            await cmd.ExecuteNonQueryAsync();
        }
-        (await ScalarAsync($"SELECT COUNT(*) FROM [{s}].SchemaVersion")).ShouldBe(SqlServerSchemaBootstrapper.RequiredVersion);
+        (await ScalarAsync($"SELECT COUNT(*) FROM [{s}].SchemaVersion")).ShouldBe(SqlServerSchemaBootstrapper.ModifiedTrailVersion);
    }

    /// <summary>
    ///     #668 upgrade path: a schema left at version 1 (only <c>001_initial.sql</c> applied, as every pre-#668 install is)
    ///     is upgraded in place by an auto-creating bootstrap — <c>SampleModified</c> appears, both version rows exist, and
-    ///     existing samples are untouched. With <c>AutoCreateSchema=false</c> the same v1 schema is refused.
+    ///     existing samples are untouched. With <c>AutoCreateSchema=false</c> the same v1 schema is accepted with the
+    ///     modification trail switched off and left at v1 (#674).
    /// </summary>
    [Fact]
    public async Task A_version_1_schema_is_upgraded_to_version_2_in_place()
@@ -81,11 +82,15 @@ public sealed class SchemaBootstrapIntegrationTests(SqlHistorianFixture fx)
        }
        (await ScalarAsync($"SELECT COUNT(*) FROM sys.tables WHERE name = 'SampleModified' AND schema_id = SCHEMA_ID('{s}')")).ShouldBe(0);

-        var refused = await Should.ThrowAsync<HistorianSchemaMissingException>(
-            () => Boot(Opts(s, auto: false)).EnsureAsync(CancellationToken.None));
-        refused.Message.ShouldContain("missing version 2 (found 1)");
+        // #674: verify-only accepts v1 — with the modification trail switched off — and does not upgrade it.
+        var verifyOnly = Boot(Opts(s, auto: false));
+        await verifyOnly.EnsureAsync(CancellationToken.None);
+        verifyOnly.SupportsModifiedTrail.ShouldBeFalse();
+        (await ScalarAsync($"SELECT MAX(Version) FROM [{s}].SchemaVersion")).ShouldBe(1);

-        await Boot(Opts(s)).EnsureAsync(CancellationToken.None);
+        var upgrading = Boot(Opts(s));
+        await upgrading.EnsureAsync(CancellationToken.None);
+        upgrading.SupportsModifiedTrail.ShouldBeTrue();

        (await ScalarAsync($"SELECT COUNT(*) FROM [{s}].SchemaVersion")).ShouldBe(2);
        (await ScalarAsync($"SELECT MAX(Version) FROM [{s}].SchemaVersion")).ShouldBe(2);
@@ -101,7 +106,7 @@ public sealed class SchemaBootstrapIntegrationTests(SqlHistorianFixture fx)
        var s = SqlHistorianFixture.UniqueSchema();
        var tasks = Enumerable.Range(0, 4).Select(_ => Task.Run(() => Boot(Opts(s)).EnsureAsync(CancellationToken.None))).ToArray();
        await Task.WhenAll(tasks);
-        (await ScalarAsync($"SELECT COUNT(*) FROM [{s}].SchemaVersion")).ShouldBe(SqlServerSchemaBootstrapper.RequiredVersion);
+        (await ScalarAsync($"SELECT COUNT(*) FROM [{s}].SchemaVersion")).ShouldBe(SqlServerSchemaBootstrapper.ModifiedTrailVersion);
    }

    [Fact]
@@ -110,7 +115,7 @@ public sealed class SchemaBootstrapIntegrationTests(SqlHistorianFixture fx)
        fx.SkipIfUnavailable();
        var ex = await Should.ThrowAsync<HistorianSchemaMissingException>(
            () => Boot(Opts(SqlHistorianFixture.UniqueSchema(), auto: false)).EnsureAsync(CancellationToken.None));
-        ex.Message.ShouldContain("missing version 2");
+        ex.Message.ShouldContain("missing version 1 (found 0)");
    }

    [Fact]
@@ -0,0 +1,181 @@
+using Microsoft.Data.SqlClient;
+using Microsoft.Extensions.Logging;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Core.AlarmHistorian;
+using ZB.MOM.WW.OtOpcUa.Driver.Historian.Sql.Common;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Historian.SqlServer.IntegrationTests;
+
+/// <summary>
+///     Gitea #674: a database left at schema version 1 (no Part 11 <c>SampleModified</c> trail) with
+///     <c>AutoCreateSchema=false</c> keeps historizing. Before #674 the bootstrapper required v2 and refused EVERY call —
+///     live writes, raw reads, alarm events, retention — until script 002 was applied by hand. Now only the features
+///     that need the trail are refused (<c>BadHistoryOperationUnsupported</c>, nothing sent to SQL), and the
+///     bootstrapper logs ONE warning for the whole provider. Built through <see cref="SqlServerHistorianProvider"/>, the
+///     production wiring, so every seam shares the one bootstrapper the way a node does.
+/// </summary>
+[Collection(SqlHistorianCollection.Name)]
+public sealed class SchemaV1PostureTests(SqlHistorianFixture fx)
+{
+    private static CancellationToken Ct => TestContext.Current.CancellationToken;
+    private static readonly DateTime Recent = DateTime.UtcNow.AddHours(-1);
+    private static readonly HistoryUpdateContext Who = new("editor", "k1");
+
+    [Fact]
+    public async Task A_v1_schema_keeps_historizing_and_refuses_only_the_modified_trail_features()
+    {
+        fx.SkipIfUnavailable();
+        var s = SqlHistorianFixture.UniqueSchema();
+        await ApplyV1Async(s);
+        var old = DateTime.UtcNow.AddDays(-40);
+        var loggers = new CapturingLoggerFactory();
+        var provider = new SqlServerHistorianProvider(new SqlServerHistorianOptions
+        {
+            ConnectionString = fx.ConnectionString, Schema = s, AutoCreateSchema = false, RetentionDays = 30,
+        });
+        var sp = new LoggerServices(loggers);
+
+        // Live writes (the continuous-historization drain) and raw reads work.
+        var writer = provider.CreateValueWriter(sp);
+        (await writer.WriteLiveValuesAsync("Plant.Speed",
+            [new HistorizationValue(old, HistorianValue.FromDouble(1), 0), new HistorizationValue(Recent, HistorianValue.FromDouble(2), 0)],
+            Ct)).ShouldBeTrue();
+        var source = provider.CreateDataSource(sp);
+        var raw = await source.ReadRawAsync("Plant.Speed", Recent.AddMinutes(-1), Recent.AddMinutes(1), 10, Ct);
+        raw.Samples.Select(v => v.Value).ShouldBe([2d]);
+
+        // Alarm events are written.
+        var alarms = provider.CreateAlarmWriter(sp);
+        (await alarms.WriteBatchAsync(
+            [new AlarmHistorianEvent("alarm-1", "Plant/Line/Eq", "HiTemp", "LimitAlarm", AlarmSeverity.High, "Activated", "m", "u", null, Recent)],
+            Ct)).ShouldBe([HistorianWriteOutcome.Ack]);
+        (await CountAsync($"SELECT COUNT(*) FROM [{s}].Event")).ShouldBe(1);
+
+        // Modified reads: the interface's "no trail" signal (the node manager reports BadHistoryOperationUnsupported),
+        // not a backend failure — health stays clean.
+        await Should.ThrowAsync<NotSupportedException>(() =>
+            source.ReadModifiedAsync("Plant.Speed", Recent.AddDays(-1), Recent.AddDays(1), 10, null, Ct));
+        source.GetHealthSnapshot().LastError.ShouldBeNull();
+
+        // HistoryUpdate: every item refused, and NOTHING reaches SQL — a sent MERGE/DELETE would either change the
+        // samples or fail on the missing SampleModified target; an Insert on an unknown tag would create its Tag row.
+        var updater = provider.CreateUpdater(sp)!;
+        var tagsBefore = await CountAsync($"SELECT COUNT(*) FROM [{s}].Tag");
+        (await updater.UpdateDataAsync("Plant.Speed", HistoryUpdateMode.Update,
+            [new HistorizationValue(Recent, HistorianValue.FromDouble(99), 0), new HistorizationValue(Recent.AddSeconds(1), HistorianValue.FromDouble(98), 0)],
+            Who, Ct)).ShouldBe([HistorianStatusCodes.BadHistoryOperationUnsupported, HistorianStatusCodes.BadHistoryOperationUnsupported]);
+        (await updater.UpdateDataAsync("Plant.Brand.New", HistoryUpdateMode.Insert,
+            [new HistorizationValue(Recent, HistorianValue.FromDouble(1), 0)], Who, Ct))
+            .ShouldBe([HistorianStatusCodes.BadHistoryOperationUnsupported]);
+        (await updater.DeleteRawAsync("Plant.Speed", Recent.AddMinutes(-1), Recent.AddMinutes(1), Who, Ct))
+            .ShouldBe(HistorianStatusCodes.BadHistoryOperationUnsupported);
+        (await updater.DeleteAtTimeAsync("Plant.Speed", [Recent], Who, Ct))
+            .ShouldBe([HistorianStatusCodes.BadHistoryOperationUnsupported]);
+        (await CountAsync($"SELECT COUNT(*) FROM [{s}].Tag")).ShouldBe(tagsBefore);
+        (await source.ReadRawAsync("Plant.Speed", Recent.AddMinutes(-1), Recent.AddMinutes(1), 10, Ct))
+            .Samples.Select(v => v.Value).ShouldBe([2d]);
+
+        // Retention purges samples and skips the trail it does not have.
+        var purge = await provider.CreateRetention(sp)!.PurgeAsync(Ct);
+        purge.Select(r => r.Table).ShouldBe(["Sample", "Event"]);
+        purge.Single(r => r.Table == "Sample").Rows.ShouldBe(1);
+        (await CountAsync($"SELECT COUNT(*) FROM [{s}].Sample")).ShouldBe(1);
+
+        // One warning for the whole provider (= per boot), however many seams and calls touched the bootstrapper.
+        loggers.Warnings.Count(w => w.Contains("Part 11 modification trail is unavailable")).ShouldBe(1);
+        (await CountAsync($"SELECT MAX(Version) FROM [{s}].SchemaVersion")).ShouldBe(1); // verify-only: never upgraded
+    }
+
+    /// <summary>
+    ///     #674: <c>AutoCreateSchema=true</c> still upgrades a v1 schema to v2 — the modification trail appears and a
+    ///     HistoryUpdate is recorded in it — and logs no v1 warning.
+    /// </summary>
+    [Fact]
+    public async Task Auto_create_still_upgrades_a_v1_schema_and_enables_the_trail()
+    {
+        fx.SkipIfUnavailable();
+        var s = SqlHistorianFixture.UniqueSchema();
+        await ApplyV1Async(s);
+        var loggers = new CapturingLoggerFactory();
+        var provider = new SqlServerHistorianProvider(new SqlServerHistorianOptions
+        {
+            ConnectionString = fx.ConnectionString, Schema = s, AutoCreateSchema = true,
+        });
+        var sp = new LoggerServices(loggers);
+
+        (await provider.CreateUpdater(sp)!.UpdateDataAsync("Plant.Speed", HistoryUpdateMode.Insert,
+            [new HistorizationValue(Recent, HistorianValue.FromDouble(5), 0)], Who, Ct))
+            .ShouldBe([HistorianStatusCodes.GoodEntryInserted]);
+
+        (await CountAsync($"SELECT MAX(Version) FROM [{s}].SchemaVersion")).ShouldBe(SqlServerSchemaBootstrapper.ModifiedTrailVersion);
+        var trail = await provider.CreateDataSource(sp)
+            .ReadModifiedAsync("Plant.Speed", Recent.AddMinutes(-1), Recent.AddMinutes(1), 10, null, Ct);
+        trail.Values.ShouldHaveSingleItem().UpdateType.ShouldBe(HistoryModificationType.Insert);
+        loggers.Warnings.ShouldNotContain(w => w.Contains("Part 11 modification trail is unavailable"));
+    }
+
+    private async Task ApplyV1Async(string schema)
+    {
+        var v1 = new StreamReader(typeof(SqlServerSchemaBootstrapper).Assembly.GetManifestResourceStream("Schema.001_initial.sql")!).ReadToEnd();
+        await using var c = new SqlConnection(fx.ConnectionString);
+        await c.OpenAsync(Ct);
+        foreach (var b in SqlServerSchemaBootstrapper.SplitBatches(v1.Replace("$(Schema)", schema)))
+        {
+            await using var cmd = c.CreateCommand();
+            cmd.CommandText = b;
+            await cmd.ExecuteNonQueryAsync(Ct);
+        }
+    }
+
+    private async Task<long> CountAsync(string sql)
+    {
+        await using var c = new SqlConnection(fx.ConnectionString);
+        await c.OpenAsync(Ct);
+        await using var cmd = c.CreateCommand();
+        cmd.CommandText = sql;
+        return Convert.ToInt64(await cmd.ExecuteScalarAsync(Ct));
+    }
+
+    /// <summary>The only service the provider resolves is <see cref="ILoggerFactory"/>.</summary>
+    private sealed class LoggerServices(ILoggerFactory loggers) : IServiceProvider
+    {
+        public object? GetService(Type serviceType) => serviceType == typeof(ILoggerFactory) ? loggers : null;
+    }
+
+    /// <summary>Captures every Warning-or-above message from every logger it creates.</summary>
+    private sealed class CapturingLoggerFactory : ILoggerFactory
+    {
+        private readonly List<string> _warnings = [];
+
+        public IReadOnlyList<string> Warnings
+        {
+            get { lock (_warnings) return _warnings.ToList(); }
+        }
+
+        public ILogger CreateLogger(string categoryName) => new Capturing(this);
+
+        public void AddProvider(ILoggerProvider provider)
+        {
+        }
+
+        public void Dispose()
+        {
+        }
+
+        private sealed class Capturing(CapturingLoggerFactory owner) : ILogger
+        {
+            public IDisposable? BeginScope<TState>(TState state) where TState : notnull => null;
+
+            public bool IsEnabled(LogLevel logLevel) => true;
+
+            public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Exception? exception,
+                Func<TState, Exception?, string> formatter)
+            {
+                if (logLevel < LogLevel.Warning) return;
+                lock (owner._warnings) owner._warnings.Add(formatter(state, exception));
+            }
+        }
+    }
+}
@@ -60,3 +60,10 @@ public sealed class SqlServerModifiedValueConformance(SqlHistorianFixture fx) :
    /// <inheritdoc/>
    protected override Task<IConformanceBackend> CreateBackendAsync() => SqlServerConformanceBackend.CreateAsync(fx);
 }
+
+[Collection(SqlHistorianCollection.Name)]
+public sealed class SqlServerEventReadConformance(SqlHistorianFixture fx) : EventReadConformance
+{
+    /// <inheritdoc/>
+    protected override Task<IConformanceBackend> CreateBackendAsync() => SqlServerConformanceBackend.CreateAsync(fx);
+}
@@ -34,12 +34,28 @@ internal sealed class SqlServerConformanceBackend : IConformanceBackend

    /// <inheritdoc/>
    public IHistorianDataUpdater CreateUpdater() =>
-        new SqlServerHistorianDataUpdater(_db.Conn, new SqlServerTagCatalog(_db.Conn, _db.Boot));
+        new SqlServerHistorianDataUpdater(_db.Conn, _db.Boot, new SqlServerTagCatalog(_db.Conn, _db.Boot));

    /// <inheritdoc/>
    public async Task SeedAsync(string tag, params (DateTime T, HistorianValue V, uint Status)[] samples) =>
        await _db.SeedAsync(tag, samples);

+    /// <inheritdoc/>
+    public async Task SeedEventsAsync(params (DateTime T, string Id, string Source)[] events)
+    {
+        await using var c = await _db.Conn.OpenAsync(CancellationToken.None);
+        foreach (var (t, id, source) in events)
+        {
+            await using var cmd = _db.Conn.CreateCommand(c,
+                $"INSERT {_db.Conn.Q("Event")} (EventId, SourceName, AlarmId, AlarmName, AlarmTypeName, Severity, EventKind, Message, " +
+                "UserName, Comment, EventTimeUtc, ReceivedTimeUtc) VALUES (@id, @src, 'a', 'n', 'T', 500, 'Activated', NULL, NULL, NULL, @t, @t)");
+            cmd.Parameters.AddWithValue("@id", id);
+            cmd.Parameters.AddWithValue("@src", source);
+            SqlHistorianConnection.AddDateTime2(cmd, "@t", t);
+            await cmd.ExecuteNonQueryAsync();
+        }
+    }
+
    /// <inheritdoc/>
    public async Task<IReadOnlyList<(DateTime T, HistorianValue V, uint Status)>> ReadBackAsync(string tag)
    {