fix(driver-ablegacy): resolve High code-review findings (Driver.AbLegacy-001, Driver.AbLegacy-006)

Driver.AbLegacy-001 — PCCC bit-index range. AbLegacyAddress.TryParse
accepted a bit index of 0..31 for every file type, but a 16-bit
N/B/I/O/S/A word only has bits 0..15. TryParse now range-checks the
bit index against the file's word width (0..15 for 16-bit element
files, 0..31 for the 32-bit L file, no bits on float files), so
addresses like N7:0/20 are rejected at parse time instead of silently
truncating in the (short) cast. WriteBitInWordAsync reads and writes
an L-file parent word as 32-bit Long and masks the RMW arithmetic to
the native width, so a sign-extended 16-bit decode can no longer
corrupt the high bits.

Driver.AbLegacy-006 — shared-runtime concurrency. A per-tag libplctag
Tag handle is cached and reused by both the server read path and the
poll loop, with no synchronisation around Read/GetStatus/DecodeValue.
Added a per-runtime SemaphoreSlim (DeviceState.GetRuntimeLock, keyed
by tag name); ReadAsync and WriteAsync now hold it across the whole
Read -> GetStatus -> Decode / Encode -> Write -> GetStatus sequence so
no two threads touch the same Tag handle concurrently.

Added xUnit + Shouldly regression coverage: AbLegacyBitIndexRangeTests
(per-file bit-range validation + L-file 32-bit RMW + sign-extension
safety) and AbLegacyRuntimeConcurrencyTests (overlap-detecting fake
proving concurrent read/read and read/write are serialised).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

code-reviews/Driver.AbLegacy/findings.md

@@ -7,7 +7,7 @@
 | Review date | 2026-05-22 |
 | Commit reviewed | `76d35d1` |
 | Status | Reviewed |
-| Open findings | 13 |
+| Open findings | 11 |
 
 ## Checklist coverage
 
@@ -36,7 +36,7 @@ a category produced nothing rather than leaving it blank.
 | Severity | High |
 | Category | Correctness & logic bugs |
 | Location | `AbLegacyAddress.cs:54`, `AbLegacyDriver.cs:368-374` |
-| Status | Open |
+| Status | Resolved |
 
 **Description:** `AbLegacyAddress.TryParse` accepts a `BitIndex` of `0..31` for every
 file type. A PCCC N-file word is a signed 16-bit integer, so valid bit indices are
@@ -54,7 +54,10 @@ in `WriteBitInWordAsync` - reject bit > 15 for N/B/I/O/S files and bit > 31 for
 files. For bit-in-word RMW against L files, read the parent as `Long`. Mask the
 read-back value to the word width before applying the bit operation.
 
-**Resolution:** _(open)_
+**Resolution:** Resolved 2026-05-22 — `AbLegacyAddress.TryParse` now range-checks the
+bit index against per-file word width (0..15 for N/B/I/O/S/A, 0..31 for L, no bits on
+F); `WriteBitInWordAsync` reads/writes an L-file parent as 32-bit `Long` and masks the
+RMW arithmetic to the native width so sign-extension can no longer corrupt high bits.
 
 ### Driver.AbLegacy-002
 
@@ -161,7 +164,7 @@ libplctag status per device.
 | Severity | High |
 | Category | Concurrency & thread safety |
 | Location | `AbLegacyDriver.cs:107-158`, `AbLegacyDriver.cs:162-234`, `LibplctagLegacyTagRuntime.cs` |
-| Status | Open |
+| Status | Resolved |
 
 **Description:** A per-tag `IAbLegacyTagRuntime` (wrapping a single libplctag `Tag`)
 is cached in `DeviceState.Runtimes` and reused. `ReadAsync` (called directly by the
@@ -179,7 +182,10 @@ hazard. Only the bit-in-word RMW path is serialised (per-parent `SemaphoreSlim`)
 `SemaphoreSlim`, or a per-device read lock - so no two threads touch the same `Tag`
 handle concurrently.
 
-**Resolution:** _(open)_
+**Resolution:** Resolved 2026-05-22 — added a per-runtime `SemaphoreSlim`
+(`DeviceState.GetRuntimeLock`, keyed by tag name); `ReadAsync` and `WriteAsync` now
+hold it around the whole Read→GetStatus→Decode / Encode→Write→GetStatus sequence so the
+shared libplctag `Tag` handle is never touched by two threads at once.
 
 ### Driver.AbLegacy-007
 

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyAddress.cs

@@ -46,12 +46,14 @@ public sealed record AbLegacyAddress(
         if (string.IsNullOrWhiteSpace(value)) return null;
         var src = value.Trim();
 
-        // BitIndex: trailing /N
+        // BitIndex: trailing /N. The valid range depends on the parent word width, which is
+        // determined by the file letter (16-bit N/B/I/O/S/A → 0..15, 32-bit L → 0..31). Capture
+        // the raw value here and range-check it once the file letter is known (see below).
         int? bitIndex = null;
         var slashIdx = src.IndexOf('/');
         if (slashIdx >= 0)
         {
-            if (!int.TryParse(src[(slashIdx + 1)..], out var bit) || bit < 0 || bit > 31) return null;
+            if (!int.TryParse(src[(slashIdx + 1)..], out var bit) || bit < 0) return null;
             bitIndex = bit;
             src = src[..slashIdx];
         }
@@ -91,9 +93,30 @@ public sealed record AbLegacyAddress(
         // Reject unknown file letters — these cover SLC/ML/PLC-5 canonical families.
         if (!IsKnownFileLetter(letter)) return null;
 
+        // Range-check the bit index against the file's word width. A PCCC N/B/I/O/S/A word is a
+        // 16-bit element, so valid bit indices are 0..15; an L-file element is 32-bit (0..31).
+        // F-files are 32-bit IEEE-754 floats and are not bit-addressable at all.
+        if (bitIndex is int b)
+        {
+            var maxBit = MaxBitIndexFor(letter);
+            if (maxBit < 0 || b > maxBit) return null;
+        }
+
         return new AbLegacyAddress(letter, fileNumber, word, bitIndex, subElement);
     }
 
+    /// <summary>
+    ///     Highest valid bit index for a file letter, or <c>-1</c> if the file type is not
+    ///     bit-addressable. 16-bit element files (N/B/I/O/S/A) permit bits 0..15; the 32-bit
+    ///     L-file permits 0..31.
+    /// </summary>
+    private static int MaxBitIndexFor(string letter) => letter switch
+    {
+        "L" => 31,
+        "N" or "B" or "I" or "O" or "S" or "A" => 15,
+        _ => -1,
+    };
+
     private static bool IsKnownFileLetter(string letter) => letter switch
     {
         "N" or "F" or "B" or "L" or "ST" or "T" or "C" or "R" or "I" or "O" or "S" or "A" => true,

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.cs

@@ -128,9 +128,26 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
             try
             {
                 var runtime = await EnsureTagRuntimeAsync(device, def, cancellationToken).ConfigureAwait(false);
-                await runtime.ReadAsync(cancellationToken).ConfigureAwait(false);
 
-                var status = runtime.GetStatus();
+                int status;
+                object? value;
+                // Serialise the Read → GetStatus → DecodeValue sequence against the shared
+                // runtime — the server read path and the poll loop both call ReadAsync against
+                // the same cached libplctag Tag handle, which is not concurrency-safe.
+                var opLock = device.GetRuntimeLock(def.Name);
+                await opLock.WaitAsync(cancellationToken).ConfigureAwait(false);
+                try
+                {
+                    await runtime.ReadAsync(cancellationToken).ConfigureAwait(false);
+                    status = runtime.GetStatus();
+                    var parsed = AbLegacyAddress.TryParse(def.Address);
+                    value = status == 0 ? runtime.DecodeValue(def.DataType, parsed?.BitIndex) : null;
+                }
+                finally
+                {
+                    opLock.Release();
+                }
+
                 if (status != 0)
                 {
                     results[i] = new DataValueSnapshot(null,
@@ -140,8 +157,6 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
                     continue;
                 }
 
-                var parsed = AbLegacyAddress.TryParse(def.Address);
-                var value = runtime.DecodeValue(def.DataType, parsed?.BitIndex);
                 results[i] = new DataValueSnapshot(value, AbLegacyStatusMapper.Good, now, now);
                 _health = new DriverHealth(DriverState.Healthy, now, null);
             }
@@ -201,10 +216,23 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
                 }
 
                 var runtime = await EnsureTagRuntimeAsync(device, def, cancellationToken).ConfigureAwait(false);
+
+                int status;
+                // Serialise Encode → Write → GetStatus against the shared runtime — the same
+                // cached Tag handle may be in use by a concurrent ReadAsync or poll loop.
+                var opLock = device.GetRuntimeLock(def.Name);
+                await opLock.WaitAsync(cancellationToken).ConfigureAwait(false);
+                try
+                {
                     runtime.EncodeValue(def.DataType, parsed?.BitIndex, w.Value);
                     await runtime.WriteAsync(cancellationToken).ConfigureAwait(false);
+                    status = runtime.GetStatus();
+                }
+                finally
+                {
+                    opLock.Release();
+                }
 
-                var status = runtime.GetStatus();
                 results[i] = new WriteResult(status == 0
                     ? AbLegacyStatusMapper.Good
                     : AbLegacyStatusMapper.MapLibplctagStatus(status));
@@ -345,14 +373,24 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
     }
 
     /// <summary>
-    ///     Read-modify-write one bit within a PCCC N-file word. Strips the /N bit suffix to
-    ///     form the parent-word address (N7:0/3 → N7:0), creates / reuses a parent-word runtime
-    ///     typed as Int16, serialises concurrent bit writers against the same parent via a
-    ///     per-parent <see cref="SemaphoreSlim"/>.
+    ///     Read-modify-write one bit within a PCCC word. Strips the /N bit suffix to form the
+    ///     parent-word address (N7:0/3 → N7:0), creates / reuses a parent-word runtime typed at
+    ///     the parent's native width (Int16 for N/I/O/S/A files, Int32 for the 32-bit L file),
+    ///     and serialises concurrent bit writers against the same parent via a per-parent
+    ///     <see cref="SemaphoreSlim"/>. The parent word is masked to its native width before the
+    ///     bit operation so a sign-extended decode never corrupts the high bits.
     /// </summary>
     private async Task<uint> WriteBitInWordAsync(
         AbLegacyDriver.DeviceState device, AbLegacyAddress bitAddress, int bit, object? value, CancellationToken ct)
     {
+        // The parent word width follows the file letter: an L-file element is 32-bit, every
+        // other bit-addressable PCCC file (N/I/O/S/A) is a 16-bit word. bit is already
+        // range-checked by AbLegacyAddress.TryParse (0..15 for 16-bit files, 0..31 for L), so
+        // 1 << bit can never overflow the chosen width here.
+        var isLong = bitAddress.FileLetter == "L";
+        var parentType = isLong ? AbLegacyDataType.Long : AbLegacyDataType.Int;
+        var widthMask = isLong ? unchecked((int)0xFFFFFFFF) : 0xFFFF;
+
         var parentAddress = bitAddress with { BitIndex = null };
         var parentName = parentAddress.ToLibplctagName();
 
@@ -365,11 +403,18 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
             var readStatus = parentRuntime.GetStatus();
             if (readStatus != 0) return AbLegacyStatusMapper.MapLibplctagStatus(readStatus);
 
-            var current = Convert.ToInt32(parentRuntime.DecodeValue(AbLegacyDataType.Int, bitIndex: null) ?? 0);
+            // Mask the decoded word to its native width: DecodeValue for a 16-bit Int returns a
+            // sign-extended int, so a word with bit 15 set decodes negative. Masking pins the
+            // RMW arithmetic to exactly the parent's bits.
+            var current = Convert.ToInt32(parentRuntime.DecodeValue(parentType, bitIndex: null) ?? 0) & widthMask;
             var updated = Convert.ToBoolean(value)
                 ? current | (1 << bit)
                 : current & ~(1 << bit);
+            updated &= widthMask;
 
+            if (isLong)
+                parentRuntime.EncodeValue(AbLegacyDataType.Long, bitIndex: null, updated);
+            else
                 parentRuntime.EncodeValue(AbLegacyDataType.Int, bitIndex: null, (short)updated);
             await parentRuntime.WriteAsync(ct).ConfigureAwait(false);
             var writeStatus = parentRuntime.GetStatus();
@@ -464,6 +509,20 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
         public SemaphoreSlim GetRmwLock(string parentName) =>
             _rmwLocks.GetOrAdd(parentName, _ => new SemaphoreSlim(1, 1));
 
+        private readonly System.Collections.Concurrent.ConcurrentDictionary<string, SemaphoreSlim> _runtimeLocks = new();
+
+        /// <summary>
+        ///     Per-runtime operation lock. A libplctag <c>Tag</c> handle is not safe for
+        ///     concurrent Read/GetStatus/DecodeValue from multiple threads — the server read
+        ///     path and the poll loop both call <see cref="AbLegacyDriver.ReadAsync"/> against
+        ///     the same cached runtime. Callers hold this lock around the whole
+        ///     Read → GetStatus → Decode (or Encode → Write → GetStatus) sequence so a status
+        ///     or value is never observed mid-update by another thread. Keyed by tag name, which
+        ///     is also the <see cref="Runtimes"/> dictionary key.
+        /// </summary>
+        public SemaphoreSlim GetRuntimeLock(string tagName) =>
+            _runtimeLocks.GetOrAdd(tagName, _ => new SemaphoreSlim(1, 1));
+
         public object ProbeLock { get; } = new();
         public HostState HostState { get; set; } = HostState.Unknown;
         public DateTime HostStateChangedUtc { get; set; } = DateTime.UtcNow;

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyBitIndexRangeTests.cs

@@ -0,0 +1,106 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.AbLegacy;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests;
+
+/// <summary>
+///     Regression coverage for Driver.AbLegacy-001 — a PCCC bit index must be range-checked
+///     against the parent word width: 0..15 for 16-bit element files (N/B/I/O/S/A), 0..31 for
+///     the 32-bit L file. Float files are not bit-addressable.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class AbLegacyBitIndexRangeTests
+{
+    [Theory]
+    [InlineData("N7:0/15")]
+    [InlineData("B3:0/15")]
+    [InlineData("I:0/15")]
+    [InlineData("O:1/15")]
+    [InlineData("S:1/15")]
+    [InlineData("A10:0/15")]
+    public void Bit_index_0_to_15_accepted_on_16bit_files(string input) =>
+        AbLegacyAddress.TryParse(input).ShouldNotBeNull();
+
+    [Theory]
+    [InlineData("N7:0/16")]   // first bit past a 16-bit word
+    [InlineData("N7:0/20")]
+    [InlineData("N7:0/31")]
+    [InlineData("B3:0/16")]
+    [InlineData("I:0/16")]
+    [InlineData("O:1/16")]
+    [InlineData("S:1/16")]
+    [InlineData("A10:0/16")]
+    public void Bit_index_above_15_rejected_on_16bit_files(string input) =>
+        AbLegacyAddress.TryParse(input).ShouldBeNull();
+
+    [Theory]
+    [InlineData("L9:0/0")]
+    [InlineData("L9:0/15")]
+    [InlineData("L9:0/16")]   // L-file words are 32-bit, so 16..31 are valid
+    [InlineData("L9:0/31")]
+    public void Bit_index_0_to_31_accepted_on_L_file(string input) =>
+        AbLegacyAddress.TryParse(input).ShouldNotBeNull();
+
+    [Fact]
+    public void Bit_index_above_31_rejected_on_L_file() =>
+        AbLegacyAddress.TryParse("L9:0/32").ShouldBeNull();
+
+    [Theory]
+    [InlineData("F8:0/0")]    // float files are not bit-addressable at all
+    [InlineData("F8:0/3")]
+    public void Bit_index_rejected_on_float_file(string input) =>
+        AbLegacyAddress.TryParse(input).ShouldBeNull();
+
+    [Fact]
+    public void Negative_bit_index_still_rejected() =>
+        AbLegacyAddress.TryParse("N7:0/-1").ShouldBeNull();
+
+    [Fact]
+    public async Task Bit_in_word_RMW_against_L_file_uses_32bit_parent_and_high_bit()
+    {
+        // L9:0/20 — bit 20 of a 32-bit L-file word. The parent must be read/written as a
+        // 32-bit Long so the high bits are addressable; a 16-bit (short)cast would truncate.
+        var factory = new FakeAbLegacyTagFactory
+        {
+            Customise = p => new FakeAbLegacyTag(p) { Value = 0 },
+        };
+        var drv = new AbLegacyDriver(new AbLegacyDriverOptions
+        {
+            Devices = [new AbLegacyDeviceOptions("ab://10.0.0.5/1,0")],
+            Tags = [new AbLegacyTagDefinition("LBit20", "ab://10.0.0.5/1,0", "L9:0/20", AbLegacyDataType.Bit)],
+            Probe = new AbLegacyProbeOptions { Enabled = false },
+        }, "drv-1", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var results = await drv.WriteAsync([new WriteRequest("LBit20", true)], CancellationToken.None);
+
+        results.Single().StatusCode.ShouldBe(AbLegacyStatusMapper.Good);
+        factory.Tags.ShouldContainKey("L9:0");
+        Convert.ToInt32(factory.Tags["L9:0"].Value).ShouldBe(1 << 20);
+    }
+
+    [Fact]
+    public async Task Bit_in_word_RMW_high_bit_15_does_not_corrupt_via_sign_extension()
+    {
+        // Parent word has bit 15 set (0x8000) — DecodeValue returns a sign-extended negative
+        // int. Setting bit 0 must yield exactly 0x8001, not a sign-extended value.
+        var factory = new FakeAbLegacyTagFactory
+        {
+            Customise = p => new FakeAbLegacyTag(p) { Value = unchecked((short)0x8000) },
+        };
+        var drv = new AbLegacyDriver(new AbLegacyDriverOptions
+        {
+            Devices = [new AbLegacyDeviceOptions("ab://10.0.0.5/1,0")],
+            Tags = [new AbLegacyTagDefinition("Bit0", "ab://10.0.0.5/1,0", "N7:0/0", AbLegacyDataType.Bit)],
+            Probe = new AbLegacyProbeOptions { Enabled = false },
+        }, "drv-1", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await drv.WriteAsync([new WriteRequest("Bit0", true)], CancellationToken.None);
+
+        // (short)0x8001 round-trips through the fake as -32767.
+        Convert.ToInt32(factory.Tags["N7:0"].Value).ShouldBe(unchecked((short)0x8001));
+    }
+}

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyRuntimeConcurrencyTests.cs

@@ -0,0 +1,120 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.AbLegacy;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests;
+
+/// <summary>
+///     Regression coverage for Driver.AbLegacy-006 — a per-tag libplctag runtime (a single
+///     <c>Tag</c> handle) is cached and shared between the server read path and the poll loop.
+///     A <c>Tag</c> is not safe for concurrent operations, so the driver must serialise the
+///     Read → GetStatus → DecodeValue (and Encode → Write → GetStatus) sequence per runtime.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class AbLegacyRuntimeConcurrencyTests
+{
+    /// <summary>
+    ///     A fake runtime that records the maximum number of operations in flight against the
+    ///     <em>same</em> handle. If the driver fails to serialise, two callers overlap inside the
+    ///     Read → GetStatus → Decode window and <see cref="MaxConcurrent"/> exceeds 1.
+    /// </summary>
+    private sealed class OverlapDetectingFake : FakeAbLegacyTag
+    {
+        private int _inFlight;
+        public int MaxConcurrent { get; private set; }
+
+        public OverlapDetectingFake(AbLegacyTagCreateParams p) : base(p) { }
+
+        public override async Task ReadAsync(CancellationToken ct)
+        {
+            EnterOp();
+            try
+            {
+                // Yield + small delay so an unserialised second caller is guaranteed to overlap.
+                await Task.Delay(15, ct).ConfigureAwait(false);
+                await base.ReadAsync(ct).ConfigureAwait(false);
+            }
+            finally { LeaveOp(); }
+        }
+
+        public override async Task WriteAsync(CancellationToken ct)
+        {
+            EnterOp();
+            try
+            {
+                await Task.Delay(15, ct).ConfigureAwait(false);
+                await base.WriteAsync(ct).ConfigureAwait(false);
+            }
+            finally { LeaveOp(); }
+        }
+
+        private void EnterOp()
+        {
+            var n = Interlocked.Increment(ref _inFlight);
+            lock (this) { if (n > MaxConcurrent) MaxConcurrent = n; }
+        }
+
+        private void LeaveOp() => Interlocked.Decrement(ref _inFlight);
+    }
+
+    [Fact]
+    public async Task Concurrent_reads_of_same_tag_are_serialised_against_the_shared_runtime()
+    {
+        OverlapDetectingFake? shared = null;
+        var factory = new FakeAbLegacyTagFactory
+        {
+            Customise = p =>
+            {
+                shared = new OverlapDetectingFake(p) { Value = 7 };
+                return shared;
+            },
+        };
+        var drv = new AbLegacyDriver(new AbLegacyDriverOptions
+        {
+            Devices = [new AbLegacyDeviceOptions("ab://10.0.0.5/1,0")],
+            Tags = [new AbLegacyTagDefinition("X", "ab://10.0.0.5/1,0", "N7:0", AbLegacyDataType.Int)],
+            Probe = new AbLegacyProbeOptions { Enabled = false },
+        }, "drv-1", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        // Eight callers race for the same tag — mimics the server read path + poll loop(s)
+        // hitting one cached runtime at once.
+        var reads = Enumerable.Range(0, 8)
+            .Select(_ => drv.ReadAsync(["X"], CancellationToken.None))
+            .ToArray();
+        await Task.WhenAll(reads);
+
+        shared.ShouldNotBeNull();
+        shared!.MaxConcurrent.ShouldBe(1, "operations on a shared libplctag Tag must not overlap");
+        reads.ShouldAllBe(r => r.Result.Single().Value!.Equals(7));
+    }
+
+    [Fact]
+    public async Task Concurrent_read_and_write_of_same_tag_do_not_overlap()
+    {
+        OverlapDetectingFake? shared = null;
+        var factory = new FakeAbLegacyTagFactory
+        {
+            Customise = p =>
+            {
+                shared = new OverlapDetectingFake(p) { Value = 1 };
+                return shared;
+            },
+        };
+        var drv = new AbLegacyDriver(new AbLegacyDriverOptions
+        {
+            Devices = [new AbLegacyDeviceOptions("ab://10.0.0.5/1,0")],
+            Tags = [new AbLegacyTagDefinition("X", "ab://10.0.0.5/1,0", "N7:0", AbLegacyDataType.Int)],
+            Probe = new AbLegacyProbeOptions { Enabled = false },
+        }, "drv-1", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var readTask = drv.ReadAsync(["X"], CancellationToken.None);
+        var writeTask = drv.WriteAsync([new WriteRequest("X", 99)], CancellationToken.None);
+        await Task.WhenAll(readTask, writeTask);
+
+        shared.ShouldNotBeNull();
+        shared!.MaxConcurrent.ShouldBe(1);
+    }
+}