Allow to attach to out of process collector (#664)

Allow to attach to out of process collector

Author: MarcoRossignoli

Documentation/Troubleshooting.md

@@ -235,4 +235,12 @@ You'll get this message during test run
 dotnet test  -p:Include="[test_coverage.]"   -p:Exclude="[*.Test.*]*"   -p:CollectCoverage=true   -p:CoverletOutputFormat=cobertura   -p:CoverletOutput=coverage.cobertura.xml
 Coverlet msbuild instrumentation task debugging is enabled. Please attach debugger to process to continue
 Process Id: 29228 Name: dotnet
-```
+```
+
+## Enable collector instrumentation debugging
+
+You can live attach and debug collectors with `COVERLET_DATACOLLECTOR_OUTOFPROC_DEBUG` env variable
+```
+ set COVERLET_DATACOLLECTOR_OUTOFPROC_DEBUG=1
+```
+You will be asket to attach a debugger through UI popup.

src/coverlet.collector/DataCollection/CoverletCoverageCollector.cs

@@ -37,6 +37,15 @@ internal CoverletCoverageCollector(TestPlatformEqtTrace eqtTrace, ICoverageWrapp
             _countDownEventFactory = countDownEventFactory;
         }
 
+        private void AttachDebugger()
+        {
+            if (int.TryParse(Environment.GetEnvironmentVariable("COVERLET_DATACOLLECTOR_OUTOFPROC_DEBUG"), out int result) && result == 1)
+            {
+                Debugger.Launch();
+                Debugger.Break();
+            }
+        }
+
         /// <summary>
         /// Initializes data collector
         /// </summary>
@@ -52,6 +61,9 @@ public override void Initialize(
             DataCollectionLogger logger,
             DataCollectionEnvironmentContext environmentContext)
         {
+
+            AttachDebugger();
+
             if (_eqtTrace.IsInfoEnabled)
             {
                 _eqtTrace.Info("Initializing {0} with configuration: '{1}'", CoverletConstants.DataCollectorName, configurationElement?.OuterXml);