fix(worker): StaWaitHelper regression tests + full-drain contract notes; consolidate wait P/Invoke

docs/MxAccessWorkerInstanceDesign.md

@@ -295,6 +295,14 @@ cache lock) so a cross-thread producer wakes the waiter immediately; in the live
 worker the update arrives on the STA from inside `pumpStep` itself, and the
 message wake is what carries it.
 
+`MWMO_INPUTAVAILABLE` makes the drain contract load-bearing: the wait wakes on
+input that is merely *present*, including input an earlier `PeekMessage` saw but
+did not remove. A `pumpStep` that drains only part of the queue — or a no-op one
+— therefore leaves a message that satisfies the wake condition forever, and the
+loop spins at 100% CPU until its deadline (deadline and reply shape still hold;
+it is a CPU fault, not a correctness one). Every `pumpStep` must drain to empty,
+as `StaRuntime.PumpPendingMessages` does.
+
 The wait slice is capped at 50 ms so `pumpStep` runs periodically even when
 nothing wakes the wait — a process with no STA message queue (unit tests drive
 these caches from ordinary threads, standing in for the STA by updating the

src/ZB.MOM.WW.MxGateway.Worker.Tests/MxAccess/MxAccessValueCacheTests.cs

@@ -179,27 +179,35 @@ public sealed class MxAccessValueCacheTests
     }
 
     /// <summary>
-    ///     Verifies the wait is message/signal-driven rather than sleep-polled:
+    ///     Verifies the wait slice stays bounded no matter what poll interval the
-    ///     with a poll interval far longer than the deadline, a value set from
+    ///     caller asks for: with a 10 s interval and a 3 s deadline, a value set
-    ///     another thread still wakes the wait and is returned well inside the
+    ///     50 ms in is still returned in time, because every slice is capped at
-    ///     deadline. The sleep-polled loop this replaced would have sat blind for
+    ///     the 50 ms fallback tick. The blind <c>Thread.Sleep</c> this replaced
-    ///     the whole 10 s interval — on the STA that means 10 s with no Windows
+    ///     would have slept the full interval — on the STA, 10 s with no Windows
     ///     messages dispatched, so the OnDataChange being waited for could not
     ///     have arrived at all — and then reported a timeout.
     /// </summary>
+    /// <remarks>
+    ///     Scope: this proves the cadence bound, not the signal path. The clamp
+    ///     alone would satisfy it, so it stays green if the <c>Set</c>-side
+    ///     signal is deleted. <c>StaWaitHelperTests</c> owns the signal path,
+    ///     where a five-second wait with no clamp in play makes the handle the
+    ///     only thing that can end it early.
+    /// </remarks>
+    /// <returns>A task that represents the asynchronous operation.</returns>
     [Fact]
-    public void TryWaitForUpdate_WakesOnCrossThreadSet_DespiteLongPollInterval()
+    public async Task TryWaitForUpdate_CompletesWithinDeadline_WhenPollIntervalExceedsTheFallbackTick()
     {
         MxAccessValueCache cache = new();
         Timestamp sourceTimestamp = Timestamp.FromDateTime(DateTime.UtcNow);
         using ManualResetEventSlim waitEntered = new(false);
 
-        Task setter = Task.Run(() =>
+        Task setter = Task.Run(async () =>
         {
             // Handshake so the value cannot land before the wait starts —
             // otherwise the first check would satisfy it and prove nothing.
             waitEntered.Wait(TimeSpan.FromSeconds(5));
-            Thread.Sleep(50);
+            await Task.Delay(50, CancellationToken.None);
             cache.Set(7, 21, BuildEvent(7, 21, intValue: 8080, quality: 192, sourceTimestamp));
         });
 
@@ -213,13 +221,13 @@ public sealed class MxAccessValueCacheTests
             out MxAccessValueCache.CachedValue value,
             pollIntervalMs: 10_000);
         elapsed.Stop();
-        setter.Wait(TimeSpan.FromSeconds(5));
+        await setter;
 
         Assert.True(found);
         Assert.Equal(8080, value.Value.Int32Value);
         Assert.True(
             elapsed.Elapsed < TimeSpan.FromSeconds(2),
-            $"The wait should have woken on the cached value, not slept out the poll interval; took {elapsed.ElapsedMilliseconds} ms.");
+            $"The wait should have re-checked within the fallback tick, not slept out the poll interval; took {elapsed.ElapsedMilliseconds} ms.");
     }
 
     /// <summary>

src/ZB.MOM.WW.MxGateway.Worker.Tests/MxAccess/MxAccessWriteCompletionCacheTests.cs

@@ -113,25 +113,33 @@ public sealed class MxAccessWriteCompletionCacheTests
     }
 
     /// <summary>
-    ///     Verifies the wait is message/signal-driven rather than sleep-polled:
+    ///     Verifies the wait slice stays bounded no matter what poll interval the
-    ///     with a poll interval far longer than the deadline, a completion
+    ///     caller asks for: with a 10 s interval and a 3 s deadline, a completion
-    ///     recorded from another thread still wakes the wait and is returned well
+    ///     recorded 50 ms in is still returned in time, because every slice is
-    ///     inside the deadline. The sleep-polled loop this replaced would have sat
+    ///     capped at the 50 ms fallback tick. The blind <c>Thread.Sleep</c> this
-    ///     blind for the whole 10 s interval — on the STA that means 10 s with no
+    ///     replaced would have slept the full interval — on the STA, 10 s with no
     ///     Windows messages dispatched — and then reported a timeout.
     /// </summary>
+    /// <remarks>
+    ///     Scope: this proves the cadence bound, not the signal path. The clamp
+    ///     alone would satisfy it, so it stays green if the <c>Record</c>-side
+    ///     <c>Set()</c> is deleted. <c>StaWaitHelperTests</c> owns the signal
+    ///     path, where a five-second wait with no clamp in play makes the handle
+    ///     the only thing that can end it early.
+    /// </remarks>
+    /// <returns>A task that represents the asynchronous operation.</returns>
     [Fact]
-    public void TryWaitForCompletion_WakesOnCrossThreadRecord_DespiteLongPollInterval()
+    public async Task TryWaitForCompletion_CompletesWithinDeadline_WhenPollIntervalExceedsTheFallbackTick()
     {
         MxAccessWriteCompletionCache cache = new();
         using ManualResetEventSlim waitEntered = new(false);
 
-        Task recorder = Task.Run(() =>
+        Task recorder = Task.Run(async () =>
         {
             // Handshake so the completion cannot land before the wait starts —
             // otherwise the first check would satisfy it and prove nothing.
             waitEntered.Wait(TimeSpan.FromSeconds(5));
-            Thread.Sleep(50);
+            await Task.Delay(50, CancellationToken.None);
             cache.Record(7, 21, BuildStatuses(detail: 8080));
         });
 
@@ -145,13 +153,13 @@ public sealed class MxAccessWriteCompletionCacheTests
             out RepeatedField<MxStatusProxy> statuses,
             pollIntervalMs: 10_000);
         elapsed.Stop();
-        recorder.Wait(TimeSpan.FromSeconds(5));
+        await recorder;
 
         Assert.True(found);
         Assert.Equal(8080, Assert.Single(statuses).Detail);
         Assert.True(
             elapsed.Elapsed < TimeSpan.FromSeconds(2),
-            $"The wait should have woken on the recorded completion, not slept out the poll interval; took {elapsed.ElapsedMilliseconds} ms.");
+            $"The wait should have re-checked within the fallback tick, not slept out the poll interval; took {elapsed.ElapsedMilliseconds} ms.");
     }
 
     /// <summary>Verifies that Record stores an independent clone of the caller's rows.</summary>

src/ZB.MOM.WW.MxGateway.Worker.Tests/Sta/StaWaitHelperTests.cs

@@ -0,0 +1,235 @@
+using System;
+using System.Diagnostics;
+using System.Threading;
+using System.Threading.Tasks;
+using ZB.MOM.WW.MxGateway.Worker.Sta;
+
+namespace ZB.MOM.WW.MxGateway.Worker.Tests.Sta;
+
+/// <summary>
+///     Tests for <see cref="StaWaitHelper"/>, the message-aware wait behind the
+///     write-completion and ReadBulk value waits.
+/// </summary>
+/// <remarks>
+///     <para>
+///         These exist because the cache-level tests cannot guard the signal
+///         path. Every cache wait clamps its slice to
+///         <see cref="StaWaitHelper.MaxFallbackTickMilliseconds"/>, so a wait
+///         woken by the handle and a wait that merely timed out and re-checked
+///         are milliseconds apart — no wall-clock assertion up there can tell
+///         them apart, and deleting the caches' <c>Set()</c> calls would leave
+///         them green. Calling the helper directly with a five-second timeout
+///         removes the clamp from the picture: only the handle can end that wait
+///         early, and the test asserts the handle was the thing consumed.
+///     </para>
+///     <para>
+///         Boundary: the <c>WAIT_FAILED</c> branch is not exercised, for the same
+///         reason <see cref="StaMessagePumpTests"/> gives — forcing
+///         <c>MsgWaitForMultipleObjectsEx</c> to fail means handing it a
+///         deliberately invalid native handle, which is unsafe to construct in a
+///         managed test.
+///     </para>
+/// </remarks>
+public sealed class StaWaitHelperTests
+{
+    /// <summary>Verifies that a null wait handle is rejected.</summary>
+    [Fact]
+    public void WaitForSignalOrMessages_NullSignal_ThrowsArgumentNullException()
+    {
+        ArgumentNullException exception = Assert.Throws<ArgumentNullException>(
+            () => StaWaitHelper.WaitForSignalOrMessages(null!, 10));
+
+        Assert.Equal("signal", exception.ParamName);
+    }
+
+    /// <summary>
+    ///     Verifies the wait ends on a cross-thread signal rather than on its
+    ///     timeout: the handle is set ~50 ms into a five-second wait, and the
+    ///     wait must return two orders of magnitude before that timeout. The
+    ///     post-condition that the handle was consumed is what makes this a
+    ///     discriminator — an auto-reset event that our wait did not take would
+    ///     still be signalled afterwards, which would mean something other than
+    ///     the signal ended the wait.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task WaitForSignalOrMessages_SignalSetDuringWait_ReturnsLongBeforeTheTimeout()
+    {
+        using AutoResetEvent signal = new(initialState: false);
+
+        // Drain anything already queued for this thread so the wait cannot be
+        // woken by stale input instead of by the signal.
+        new StaMessagePump().PumpPendingMessages();
+
+        Task setter = Task.Run(async () =>
+        {
+            await Task.Delay(50, CancellationToken.None);
+            signal.Set();
+        });
+
+        Stopwatch elapsed = Stopwatch.StartNew();
+        StaWaitHelper.WaitForSignalOrMessages(signal, 5000);
+        elapsed.Stop();
+        await setter;
+
+        Assert.True(
+            elapsed.Elapsed < TimeSpan.FromMilliseconds(500),
+            $"Wait took {elapsed.ElapsedMilliseconds} ms of its 5000 ms timeout; the signal should have ended it.");
+        Assert.False(
+            signal.WaitOne(0),
+            "The wait did not consume the signal, so something other than the signal ended it.");
+    }
+
+    /// <summary>
+    ///     Verifies an already-signalled handle ends the wait immediately, which
+    ///     is the case that keeps a completion recorded between the caller's
+    ///     check and its wait from being missed.
+    /// </summary>
+    [Fact]
+    public void WaitForSignalOrMessages_PreSignalledHandle_ReturnsImmediately()
+    {
+        using AutoResetEvent signal = new(initialState: true);
+
+        Stopwatch elapsed = Stopwatch.StartNew();
+        StaWaitHelper.WaitForSignalOrMessages(signal, 30_000);
+        elapsed.Stop();
+
+        Assert.True(
+            elapsed.Elapsed < TimeSpan.FromSeconds(5),
+            $"Wait took {elapsed.ElapsedMilliseconds} ms; a pre-signalled handle must return at once.");
+        Assert.False(signal.WaitOne(0), "The wait should have consumed the signal.");
+    }
+
+    /// <summary>
+    ///     Verifies the wait actually blocks for its timeout when nothing signals
+    ///     it. The lower bound is the point: a wait that returned instantly would
+    ///     turn every caller's poll loop into a spin, which is exactly the
+    ///     failure mode the queue must be drained to avoid.
+    /// </summary>
+    [Fact]
+    public void WaitForSignalOrMessages_NeverSignalled_BlocksUntilTheTimeout()
+    {
+        using AutoResetEvent signal = new(initialState: false);
+
+        // The wait wakes on input that is merely present, so drain first —
+        // otherwise a stale message would end the wait early and the lower
+        // bound below would be measuring the wrong thing.
+        new StaMessagePump().PumpPendingMessages();
+
+        Stopwatch elapsed = Stopwatch.StartNew();
+        StaWaitHelper.WaitForSignalOrMessages(signal, 200);
+        elapsed.Stop();
+
+        // Loose lower bound: the OS may return slightly early on a coarse timer
+        // tick, so this proves "it blocked", not "it blocked for exactly 200 ms".
+        Assert.True(
+            elapsed.Elapsed >= TimeSpan.FromMilliseconds(100),
+            $"Wait returned after {elapsed.ElapsedMilliseconds} ms; a 200 ms wait must not return instantly.");
+        Assert.True(
+            elapsed.Elapsed < TimeSpan.FromSeconds(5),
+            $"Wait took {elapsed.ElapsedMilliseconds} ms; a 200 ms timeout must end it.");
+    }
+
+    /// <summary>
+    ///     Verifies a non-positive timeout returns without entering the wait at
+    ///     all — proven by the signal still being set afterwards, since a wait
+    ///     that ran would have consumed it.
+    /// </summary>
+    /// <param name="timeoutMilliseconds">The non-positive timeout under test.</param>
+    [Theory]
+    [InlineData(0)]
+    [InlineData(-1)]
+    public void WaitForSignalOrMessages_NonPositiveTimeout_DoesNotEnterTheWait(int timeoutMilliseconds)
+    {
+        using AutoResetEvent signal = new(initialState: true);
+
+        Stopwatch elapsed = Stopwatch.StartNew();
+        StaWaitHelper.WaitForSignalOrMessages(signal, timeoutMilliseconds);
+        elapsed.Stop();
+
+        Assert.True(
+            elapsed.Elapsed < TimeSpan.FromSeconds(2),
+            $"Wait took {elapsed.ElapsedMilliseconds} ms; a non-positive timeout must not block.");
+        Assert.True(signal.WaitOne(0), "The signal must be untouched when the wait is skipped.");
+    }
+
+    /// <summary>
+    ///     Boundary table for the slice math. Every caller's wait length comes
+    ///     from here, so each clause is pinned: a poll interval below one clamps
+    ///     up to 1 ms, an expired deadline yields 0 (the caller returns without
+    ///     waiting), a deadline nearer than the tick rounds up so the wait never
+    ///     ends before the deadline, and any interval above the fallback ceiling
+    ///     clamps down to it.
+    /// </summary>
+    /// <param name="remainingMilliseconds">Time left before the caller's deadline.</param>
+    /// <param name="pollIntervalMilliseconds">The caller's requested pump cadence.</param>
+    /// <param name="expected">The expected wait slice in milliseconds.</param>
+    [Theory]
+    // pollInterval < 1 clamps up to 1 ms — never a zero-length spin.
+    [InlineData(1000d, 0, 1)]
+    [InlineData(1000d, -5, 1)]
+    // remaining <= 0 yields 0; the caller has already returned on its deadline.
+    [InlineData(0d, 5, 0)]
+    [InlineData(-10d, 5, 0)]
+    // remaining < tick shortens the wait to the deadline.
+    [InlineData(3d, 5, 3)]
+    [InlineData(1d, 50, 1)]
+    // remaining == tick is not "less than", so the tick stands.
+    [InlineData(5d, 5, 5)]
+    // The caller's interval wins while it is under the ceiling.
+    [InlineData(1000d, 5, 5)]
+    [InlineData(1000d, 49, 49)]
+    // Anything at or above the ceiling clamps to it — this is what bounds the
+    // unconditional pumpStep cadence regardless of what a caller asks for.
+    [InlineData(1000d, 50, 50)]
+    [InlineData(1000d, 10_000, 50)]
+    [InlineData(50d, 10_000, 50)]
+    public void ClampWaitMilliseconds_BoundaryTable(
+        double remainingMilliseconds,
+        int pollIntervalMilliseconds,
+        int expected)
+    {
+        int actual = StaWaitHelper.ClampWaitMilliseconds(
+            TimeSpan.FromMilliseconds(remainingMilliseconds),
+            pollIntervalMilliseconds);
+
+        Assert.Equal(expected, actual);
+    }
+
+    /// <summary>
+    ///     Verifies a sub-millisecond remainder rounds up rather than down: a
+    ///     zero-length wait would spin, and truncating would end the wait before
+    ///     the caller's deadline. Built from ticks because
+    ///     <see cref="TimeSpan.FromMilliseconds(double)"/> rounds its argument to
+    ///     whole milliseconds and so cannot express these values at all.
+    /// </summary>
+    [Fact]
+    public void ClampWaitMilliseconds_SubMillisecondRemaining_RoundsUp()
+    {
+        // 10_000 ticks == 1 ms.
+        Assert.Equal(1, StaWaitHelper.ClampWaitMilliseconds(TimeSpan.FromTicks(4_000), 5));
+        Assert.Equal(3, StaWaitHelper.ClampWaitMilliseconds(TimeSpan.FromTicks(25_000), 5));
+        Assert.Equal(1, StaWaitHelper.ClampWaitMilliseconds(TimeSpan.FromTicks(1), 5));
+    }
+
+    /// <summary>
+    ///     Verifies the clamp never exceeds the time actually left, which is what
+    ///     keeps a wait from overrunning its caller's deadline.
+    /// </summary>
+    [Fact]
+    public void ClampWaitMilliseconds_NeverExceedsRemainingOrTheCeiling()
+    {
+        for (int remaining = 1; remaining <= 200; remaining++)
+        {
+            int slice = StaWaitHelper.ClampWaitMilliseconds(
+                TimeSpan.FromMilliseconds(remaining),
+                pollIntervalMilliseconds: 10_000);
+
+            Assert.True(slice >= 1, $"Slice for {remaining} ms remaining was {slice}; a positive deadline must wait.");
+            Assert.True(slice <= remaining, $"Slice {slice} overran the {remaining} ms left before the deadline.");
+            Assert.True(
+                slice <= StaWaitHelper.MaxFallbackTickMilliseconds,
+                $"Slice {slice} exceeded the {StaWaitHelper.MaxFallbackTickMilliseconds} ms fallback ceiling.");
+        }
+    }
+}

src/ZB.MOM.WW.MxGateway.Worker/MxAccess/MxAccessCommandExecutor.cs

@@ -100,6 +100,20 @@ public sealed class MxAccessCommandExecutor : IStaCommandExecutor
         this.variantConverter = variantConverter ?? throw new ArgumentNullException(nameof(variantConverter));
         this.statusProxyConverter = new MxStatusProxyConverter();
         this.alarmCommandHandler = alarmCommandHandler;
+
+        // The no-op fallback is a TEST affordance only — never pass null on a
+        // real STA. The cache wait loops this pump step feeds
+        // (MxAccessValueCache.TryWaitForUpdate,
+        // MxAccessWriteCompletionCache.TryWaitForCompletion) block in
+        // StaWaitHelper.WaitForSignalOrMessages, whose contract is that the pump
+        // step drains the message queue to empty on every call: the wait wakes on
+        // input that is merely present, so a pump step that removes nothing makes
+        // every wait return instantly and turns the loop into a busy spin at 100%
+        // CPU until its deadline (deadline and reply shape still hold — it is a
+        // CPU fault, not a correctness one). It is harmless here only because the
+        // fakes that pass null pre-populate the caches and run on threads with no
+        // windows and no pending input, so there is never a message left behind
+        // for the wait to keep waking on.
         this.pumpStep = pumpStep ?? (static () => { });
         this.writeCompletionTimeout = writeCompletionTimeout ?? DefaultWriteCompletionTimeout;
     }

src/ZB.MOM.WW.MxGateway.Worker/MxAccess/MxAccessValueCache.cs

@@ -32,11 +32,26 @@ public sealed class MxAccessValueCache
     // the value is recorded from inside the waiter's own pumpStep (COM
     // dispatches OnDataChange on the STA), so the message wake is what does the
     // work there; this handle covers the cross-thread producers — the tests, and
-    // any future off-STA writer. Not IDisposable on purpose: the cache is
+    // any future off-STA writer.
-    // co-owned by MxAccessBaseEventSink and MxAccessSession with no single
+    //
-    // teardown owner, and a late COM callback must never Set a disposed handle.
+    // Single-waiter invariant: an AutoResetEvent releases exactly one waiter per
-    // One kernel event per session (one session per worker process), released by
+    // Set. Today the sole waiter is the STA (one worker per session, and ReadBulk
-    // the handle's finalizer.
+    // holds the apartment while it waits), so no wake is ever lost. A second
+    // concurrent waiter would not hang — the loser still re-checks the entry
+    // every fallback tick and still honors its deadline — it would just fall
+    // back to 50 ms latency for that iteration.
+    //
+    // NOT IDisposable, and that is one invariant with the wait's use of
+    // DangerousGetHandle: the cache is co-owned by MxAccessBaseEventSink and
+    // MxAccessSession with no single teardown owner, so a late COM callback
+    // could otherwise Set a disposed handle. StaWaitHelper deliberately does not
+    // DangerousAddRef the handle; GC.KeepAlive there closes the *finalization*
+    // race but not a dispose race — if this field were ever disposed while the
+    // STA sat inside the wait, the OS could recycle that handle value and the
+    // wait would silently attach to an unrelated object. Making these caches
+    // IDisposable is therefore not a free refactor: it must add DangerousAddRef/
+    // DangerousRelease around the wait in the same change. One kernel event per
+    // session (one session per worker process), released by the finalizer.
     private readonly AutoResetEvent updateSignal = new(false);
 
     /// <summary>Records a fresh OnDataChange payload for the given handle pair.</summary>

src/ZB.MOM.WW.MxGateway.Worker/MxAccess/MxAccessWriteCompletionCache.cs

@@ -31,11 +31,26 @@ public sealed class MxAccessWriteCompletionCache
     // worker the completion is recorded from inside the waiter's own pumpStep
     // (COM dispatches OnWriteComplete on the STA), so the message wake is what
     // does the work there; this handle covers the cross-thread producers — the
-    // tests, and any future off-STA recorder. Not IDisposable on purpose: the
+    // tests, and any future off-STA recorder.
-    // cache is co-owned by MxAccessBaseEventSink and MxAccessSession with no
+    //
-    // single teardown owner, and a late COM callback must never Set a disposed
+    // Single-waiter invariant: an AutoResetEvent releases exactly one waiter per
-    // handle. One kernel event per session (one session per worker process),
+    // Set. Today the sole waiter is the STA (one worker per session, and the
-    // released by the handle's finalizer.
+    // write path holds the apartment while it waits), so no wake is ever lost. A
+    // second concurrent waiter would not hang — the loser still re-checks the
+    // entry every fallback tick and still honors its deadline — it would just
+    // fall back to 50 ms latency for that iteration.
+    //
+    // NOT IDisposable, and that is one invariant with the wait's use of
+    // DangerousGetHandle: the cache is co-owned by MxAccessBaseEventSink and
+    // MxAccessSession with no single teardown owner, so a late COM callback
+    // could otherwise Set a disposed handle. StaWaitHelper deliberately does not
+    // DangerousAddRef the handle; GC.KeepAlive there closes the *finalization*
+    // race but not a dispose race — if this field were ever disposed while the
+    // STA sat inside the wait, the OS could recycle that handle value and the
+    // wait would silently attach to an unrelated object. Making these caches
+    // IDisposable is therefore not a free refactor: it must add DangerousAddRef/
+    // DangerousRelease around the wait in the same change. One kernel event per
+    // session (one session per worker process), released by the finalizer.
     private readonly AutoResetEvent completionSignal = new(false);
 
     /// <summary>Records the status rows of a fresh OnWriteComplete callback for the given handle pair.</summary>

src/ZB.MOM.WW.MxGateway.Worker/Sta/StaMessagePump.cs

@@ -9,10 +9,7 @@ namespace ZB.MOM.WW.MxGateway.Worker.Sta;
 public sealed class StaMessagePump
 {
     private const uint Infinite = 0xFFFFFFFF;
-    private const uint MsgWaitFailed = 0xFFFFFFFF;
-    private const uint MwmoInputAvailable = 0x0004;
     private const uint PmRemove = 0x0001;
-    private const uint QsAllInput = 0x04FF;
 
     /// <summary>Waits for a command wake event or Windows messages, pumping any pending messages.</summary>
     /// <param name="commandWakeEvent">Event to signal when work is available.</param>
@@ -28,14 +25,14 @@ public sealed class StaMessagePump
 
         SafeWaitHandle safeHandle = commandWakeEvent.SafeWaitHandle;
         IntPtr[] handles = [safeHandle.DangerousGetHandle()];
-        uint result = MsgWaitForMultipleObjectsEx(
+        uint result = StaNativeMethods.MsgWaitForMultipleObjectsEx(
             (uint)handles.Length,
             handles,
             timeoutMilliseconds,
-            QsAllInput,
+            StaNativeMethods.QsAllInput,
-            MwmoInputAvailable);
+            StaNativeMethods.MwmoInputAvailable);
 
-        if (result == MsgWaitFailed)
+        if (result == StaNativeMethods.MsgWaitFailed)
         {
             throw new InvalidOperationException(
                 "The worker STA message pump failed while waiting for command work or Windows messages.");
@@ -75,14 +72,6 @@ public sealed class StaMessagePump
             : (uint)Math.Ceiling(timeout.TotalMilliseconds);
     }
 
-    [DllImport("user32.dll", SetLastError = true)]
-    private static extern uint MsgWaitForMultipleObjectsEx(
-        uint count,
-        IntPtr[] handles,
-        uint milliseconds,
-        uint wakeMask,
-        uint flags);
-
     [DllImport("user32.dll", SetLastError = true)]
     private static extern bool PeekMessage(
         out NativeMessage message,

src/ZB.MOM.WW.MxGateway.Worker/Sta/StaNativeMethods.cs

@@ -0,0 +1,58 @@
+using System;
+using System.Runtime.InteropServices;
+
+namespace ZB.MOM.WW.MxGateway.Worker.Sta;
+
+/// <summary>
+///     The single declaration of the Win32 wait the worker's STA is built on.
+///     Both waiters use it: <see cref="StaMessagePump"/> for the runtime's outer
+///     idle wait, and <see cref="StaWaitHelper"/> for the inner completion waits
+///     a command performs while it already holds the apartment.
+/// </summary>
+/// <remarks>
+///     Deliberately one copy. This is the subtlest Win32 contract in the worker —
+///     the wake mask decides which events can wake the STA, and
+///     <see cref="MwmoInputAvailable"/> decides whether a message already seen by
+///     a <c>PeekMessage</c> still counts as a wake. Two copies of it could drift
+///     apart silently, and a divergence would show up only as COM events
+///     arriving late on a live provider.
+/// </remarks>
+internal static class StaNativeMethods
+{
+    /// <summary>Return value of a failed wait (<c>WAIT_FAILED</c>).</summary>
+    internal const uint MsgWaitFailed = 0xFFFFFFFF;
+
+    /// <summary>
+    ///     Wake even when the pending input has already been seen (but not
+    ///     removed) by an earlier <c>PeekMessage</c>. Both callers peek before
+    ///     they wait, so without this flag a message that arrived during the peek
+    ///     would not wake the wait that follows it.
+    /// </summary>
+    internal const uint MwmoInputAvailable = 0x0004;
+
+    /// <summary>Wake on any queued input, including sent and posted messages.</summary>
+    internal const uint QsAllInput = 0x04FF;
+
+    /// <summary>
+    ///     Waits for the given handles, for queued input matching
+    ///     <paramref name="wakeMask"/>, or for the timeout — whichever comes
+    ///     first.
+    /// </summary>
+    /// <param name="count">Number of handles in <paramref name="handles"/>.</param>
+    /// <param name="handles">Handles to wait on; marshaled as a pointer to the array.</param>
+    /// <param name="milliseconds">Timeout in milliseconds; <c>0xFFFFFFFF</c> means INFINITE.</param>
+    /// <param name="wakeMask">Input types that may wake the wait (<see cref="QsAllInput"/>).</param>
+    /// <param name="flags">Wait flags (<see cref="MwmoInputAvailable"/>).</param>
+    /// <returns>
+    ///     <c>WAIT_OBJECT_0 + i</c> for a signaled handle, <c>WAIT_OBJECT_0 + count</c>
+    ///     for available input, <c>WAIT_TIMEOUT</c> (258), or
+    ///     <see cref="MsgWaitFailed"/>.
+    /// </returns>
+    [DllImport("user32.dll", SetLastError = true)]
+    internal static extern uint MsgWaitForMultipleObjectsEx(
+        uint count,
+        IntPtr[] handles,
+        uint milliseconds,
+        uint wakeMask,
+        uint flags);
+}

src/ZB.MOM.WW.MxGateway.Worker/Sta/StaWaitHelper.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Diagnostics;
 using System.Runtime.InteropServices;
 using System.Threading;
 using Microsoft.Win32.SafeHandles;
@@ -17,23 +18,22 @@ namespace ZB.MOM.WW.MxGateway.Worker.Sta;
 ///     Deliberately separate from <see cref="StaMessagePump"/> rather than a new
 ///     method on it: the two caches that use this are plain per-session objects
 ///     with no pump dependency (they take the pump step as an injected delegate
-///     so tests can drive them off the STA). Keeping the helper in this folder
+///     so tests can drive them off the STA). The wait itself is not duplicated —
-///     keeps every <c>user32</c> wait signature in the worker reviewable in one
+///     both this and the pump call the single declaration in
-///     place; the declaration below mirrors the pump's exactly.
+///     <see cref="StaNativeMethods"/>.
 /// </remarks>
 internal static class StaWaitHelper
 {
     /// <summary>
-    ///     Ceiling on the blind fallback tick. A caller's poll interval is
+    ///     Ceiling on the fallback tick — the longest a wait may block when
-    ///     clamped to this so no wait can go longer than 50 ms without calling
+    ///     nothing wakes it. A caller's poll interval is clamped to this, so a
-    ///     its pump step, even when the process has no message queue to wake it
+    ///     caller's pump step keeps running at least this often no matter what
-    ///     (unit-test threads) and the wait handle is never signaled.
+    ///     interval it asked for, even on a thread that never receives a message
+    ///     and whose signal is never set.
     /// </summary>
     internal const int MaxFallbackTickMilliseconds = 50;
 
-    private const uint MsgWaitFailed = 0xFFFFFFFF;
+    private static int waitFailureReported;
-    private const uint MwmoInputAvailable = 0x0004;
-    private const uint QsAllInput = 0x04FF;
 
     /// <summary>
     ///     Blocks the calling thread until <paramref name="signal"/> is set, a
@@ -42,6 +42,29 @@ internal static class StaWaitHelper
     ///     never blinds the thread to inbound COM event messages: the wait
     ///     returns the instant one arrives, so the caller can dispatch it.
     /// </summary>
+    /// <remarks>
+    ///     <para>
+    ///         <em>Caller contract: the pump step must drain the message queue
+    ///         to empty on every call.</em> The wait uses
+    ///         <see cref="StaNativeMethods.MwmoInputAvailable"/>, which reports
+    ///         input that is merely <em>present</em> — including input an earlier
+    ///         <c>PeekMessage</c> has already seen but not removed. A caller
+    ///         whose pump step drains only part of the queue, or does nothing at
+    ///         all, therefore leaves a message that satisfies the wake condition
+    ///         forever: every wait returns immediately, and the caller's poll
+    ///         loop degenerates into a busy loop that spins the thread at 100%
+    ///         until its deadline. That is a CPU fault, not a correctness one —
+    ///         the loop still honors its deadline and return shape — but it must
+    ///         not ship. Drain with <c>PeekMessage(..., PM_REMOVE)</c> in a loop
+    ///         until it returns false, exactly as
+    ///         <see cref="StaMessagePump.PumpPendingMessages"/> does.
+    ///     </para>
+    ///     <para>
+    ///         The wait does not <c>DangerousAddRef</c> the handle. That is
+    ///         sound only because the callers never dispose their wait handles —
+    ///         see the coupled invariant documented on the caches' signal fields.
+    ///     </para>
+    /// </remarks>
     /// <param name="signal">Wait handle set by whoever produces the awaited result.</param>
     /// <param name="timeoutMilliseconds">Maximum wait; values at or below zero return immediately.</param>
     internal static void WaitForSignalOrMessages(
@@ -60,20 +83,28 @@ internal static class StaWaitHelper
 
         SafeWaitHandle safeHandle = signal.SafeWaitHandle;
         IntPtr[] handles = [safeHandle.DangerousGetHandle()];
-        uint result = MsgWaitForMultipleObjectsEx(
+        uint result = StaNativeMethods.MsgWaitForMultipleObjectsEx(
             (uint)handles.Length,
             handles,
             (uint)timeoutMilliseconds,
-            QsAllInput,
+            StaNativeMethods.QsAllInput,
-            MwmoInputAvailable);
+            StaNativeMethods.MwmoInputAvailable);
+
+        // Read before anything else can run another P/Invoke and overwrite the
+        // thread's last-error slot.
+        int lastError = result == StaNativeMethods.MsgWaitFailed
+            ? Marshal.GetLastWin32Error()
+            : 0;
 
         // The handle is a field of a live caller object, so it cannot be
         // collected mid-call; KeepAlive states that dependency explicitly
         // rather than relying on it.
         GC.KeepAlive(signal);
 
-        if (result == MsgWaitFailed)
+        if (result == StaNativeMethods.MsgWaitFailed)
         {
+            ReportWaitFailureOnce(lastError);
+
             // Degrade to the pre-existing blind sleep instead of throwing or
             // hot-spinning: the caller is a bounded, deadline-driven poll loop,
             // and a failed wait must not turn a timed-out write into a fault.
@@ -108,11 +139,30 @@ internal static class StaWaitHelper
             : tick;
     }
 
-    [DllImport("user32.dll", SetLastError = true)]
+    /// <summary>
-    private static extern uint MsgWaitForMultipleObjectsEx(
+    ///     Traces the first wait failure of the process and stays silent after
-        uint count,
+    ///     that.
-        IntPtr[] handles,
+    /// </summary>
-        uint milliseconds,
+    /// <remarks>
-        uint wakeMask,
+    ///     The degrade path is silent by design — a failed wait still honors the
-        uint flags);
+    ///     caller's deadline — but a <em>permanently</em> failing wait means every
+    ///     completion wait has quietly reverted to the sleep-polled behavior this
+    ///     class exists to replace, and nothing else would ever say so. One trace
+    ///     makes that discoverable; latching it keeps a repeating failure from
+    ///     flooding the trace listener from inside a poll loop.
+    /// </remarks>
+    /// <param name="lastError">The Win32 error captured immediately after the failed wait.</param>
+    private static void ReportWaitFailureOnce(int lastError)
+    {
+        if (Interlocked.Exchange(ref waitFailureReported, 1) != 0)
+        {
+            return;
+        }
+
+        Trace.TraceWarning(
+            "MsgWaitForMultipleObjectsEx failed in the worker STA completion wait (Win32 error {0}). "
+            + "The wait has degraded to blind sleeping: deadlines are still honored, but inbound "
+            + "MXAccess COM events will only dispatch at poll-tick granularity. Reported once per process.",
+            lastError);
+    }
 }