[vs18.0] Exit DrainPacketQueue thread when build completes. (#12765)

Backport of #12561 to vs18.0

Work item (Internal use):
[devdiv#2613541](https://devdiv.visualstudio.com/DevDiv/_workitems/edit/2613541)

### Summary

MSBuild communications threads were incorrectly not exiting when builds
completed.

### Customer Impact

Memory leak that gets worse with each build or design-time build
operation.

### Regression?

Yes, from 17.14.

### Testing

Unit tests, manual testing, already inserted into 18.3 builds.

### Risk

Low.

---------

Co-authored-by: Eric Arndt <[email protected]>
Co-authored-by: Rainer Sigwald <[email protected]>

Author: 3 people

eng/Versions.props

@@ -3,7 +3,7 @@
 <Project>
   <Import Project="Version.Details.props" Condition="Exists('Version.Details.props')" />
   <PropertyGroup>
-    <VersionPrefix>18.0.3</VersionPrefix><DotNetFinalVersionKind>release</DotNetFinalVersionKind>
+    <VersionPrefix>18.0.4</VersionPrefix><DotNetFinalVersionKind>release</DotNetFinalVersionKind>
     <PackageValidationBaselineVersion>17.14.8</PackageValidationBaselineVersion>
     <AssemblyVersion>15.1.0.0</AssemblyVersion>
     <PreReleaseVersionLabel>servicing</PreReleaseVersionLabel>

src/Build/BackEnd/Components/Communications/NodeProviderOutOfProcBase.cs

@@ -683,6 +683,7 @@ public NodeContext(
                 // We select a thread size empirically - for debug builds the minimum possible stack size was too small.
                 // The current size is reported to not have the issue.
                 _drainPacketQueueThread = new Thread(DrainPacketQueue, 0x30000);
+                _drainPacketQueueThread.Name = "DrainPacketQueueThread";
                 _drainPacketQueueThread.IsBackground = true;
                 _drainPacketQueueThread.Start(this);
             }
@@ -850,10 +851,13 @@ private void DrainPacketQueue(object state)
                                 serverToClientStream.Write(writeStreamBuffer, i, lengthToWrite);
                             }
 
-                            if (IsExitPacket(packet))
+                            if (packet is NodeBuildComplete)
                             {
-                                context._exitPacketState = ExitPacketState.ExitPacketSent;
-                                context._packetQueueDrainDelayCancellation.Cancel();
+                                if (IsExitPacket(packet))
+                                {
+                                    context._exitPacketState = ExitPacketState.ExitPacketSent;
+                                    context._packetQueueDrainDelayCancellation.Cancel();
+                                }
 
                                 return;
                             }