Fix Activity.ParentId trace flags part (#49162)

Author: tarekgh

src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Activity.cs

@@ -75,6 +75,7 @@ public partial class Activity : IDisposable
         private string? _spanId;
 
         private byte _w3CIdFlags;
+        private byte _parentTraceFlags;
 
         private TagsLinkedList? _tags;
         private BaggageLinkedList? _baggage;
@@ -213,14 +214,19 @@ public string? Id
         /// </summary>
         public string? ParentId
         {
+#if ALLOW_PARTIALLY_TRUSTED_CALLERS
+            [System.Security.SecuritySafeCriticalAttribute]
+#endif
             get
             {
                 // if we represented it as a traceId-spanId, convert it to a string.
                 if (_parentId == null)
                 {
                     if (_parentSpanId != null)
                     {
-                        string parentId = "00-" + _traceId + "-" + _parentSpanId + "-00";
+                        Span<char> flagsChars = stackalloc char[2];
+                        HexConverter.ToCharsBuffer((byte)((~ActivityTraceFlagsIsSet) & _parentTraceFlags), flagsChars, 0, HexConverter.Casing.Lower);
+                        string parentId = "00-" + _traceId + "-" + _parentSpanId + "-" + flagsChars.ToString();
                         Interlocked.CompareExchange(ref _parentId, parentId, null);
                     }
                     else if (Parent != null)
@@ -551,6 +557,7 @@ public Activity SetParentId(ActivityTraceId traceId, ActivitySpanId spanId, Acti
                 _traceId = traceId.ToHexString();     // The child will share the parent's traceId.
                 _parentSpanId = spanId.ToHexString();
                 ActivityTraceFlags = activityTraceFlags;
+                _parentTraceFlags = (byte) activityTraceFlags;
             }
             return this;
         }
@@ -1072,6 +1079,7 @@ internal static Activity Create(ActivitySource source, string name, ActivityKind
                 }
 
                 activity.ActivityTraceFlags = parentContext.TraceFlags;
+                activity._parentTraceFlags = (byte) parentContext.TraceFlags;
                 activity._traceState = parentContext.TraceState;
             }
 

src/libraries/System.Diagnostics.DiagnosticSource/tests/ActivityTests.cs

@@ -1694,6 +1694,32 @@ public void StructEnumerator_TagsLinkedList()
             Assert.True(method.ReturnType.IsValueType);
         }
 
+        [Fact]
+        public void TestParentTraceFlags()
+        {
+            Activity a = new Activity("ParentFlagsA");
+            a.SetIdFormat(ActivityIdFormat.W3C);
+            a.SetParentId(ActivityTraceId.CreateFromString("0123456789abcdef0123456789abcdef".AsSpan()), ActivitySpanId.CreateFromString("0123456789abcdef".AsSpan()), ActivityTraceFlags.Recorded);
+            Assert.Equal("00-0123456789abcdef0123456789abcdef-0123456789abcdef-01", a.ParentId);
+
+            Activity b = new Activity("ParentFlagsB");
+            b.SetIdFormat(ActivityIdFormat.W3C);
+            b.SetParentId(ActivityTraceId.CreateFromString("0123456789abcdef0123456789abcdef".AsSpan()), ActivitySpanId.CreateFromString("0123456789abcdef".AsSpan()), ActivityTraceFlags.None);
+            b.ActivityTraceFlags = ActivityTraceFlags.Recorded; // Setting ActivityTraceFlags shouldn't affect the parent
+            Assert.Equal("00-0123456789abcdef0123456789abcdef-0123456789abcdef-00", b.ParentId);
+
+            using ActivitySource aSource = new ActivitySource("CheckParentTraceFlags");
+            using ActivityListener listener = new ActivityListener();
+            listener.ShouldListenTo = (activitySource) => object.ReferenceEquals(aSource, activitySource);
+            listener.Sample = (ref ActivityCreationOptions<ActivityContext> activityOptions) => ActivitySamplingResult.AllDataAndRecorded;
+            ActivitySource.AddActivityListener(listener);
+
+            ActivityContext parentContext = new ActivityContext(ActivityTraceId.CreateRandom(), ActivitySpanId.CreateRandom(), ActivityTraceFlags.Recorded);
+            a = aSource.CreateActivity("WithContext", ActivityKind.Internal, parentContext, default, default, ActivityIdFormat.W3C);
+            Assert.NotNull(a);
+            Assert.Equal("00-" + parentContext.TraceId + "-" + parentContext.SpanId + "-01", a.ParentId);
+        }
+
         [Fact]
         public void TestStatus()
         {