fix(tracestate): drop right-most member in tracestate

CHANGELOG.md

@@ -8,6 +8,10 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 ## [Unreleased]
 
+### Changed
+
+- For tracestate's members, prepend the new element and remove the oldest one, which is over capacity (#2592)
+
 ## [1.4.0] - 2022-02-11
 
 ### Added

trace/tracestate.go

@@ -171,25 +171,18 @@ func (ts TraceState) Get(key string) string {
 // specification an error is returned with the original TraceState.
 //
 // If adding a new list-member means the TraceState would have more members
-// than is allowed an error is returned instead with the original TraceState.
+// then it will drop the right-most member and return an nil error when capacity is exceeded.
 func (ts TraceState) Insert(key, value string) (TraceState, error) {
 	m, err := newMember(key, value)
 	if err != nil {
 		return ts, err
 	}
 
 	cTS := ts.Delete(key)
-	if cTS.Len()+1 > maxListMembers {
-		// TODO (MrAlias): When the second version of the Trace Context
-		// specification is published this needs to not return an error.
-		// Instead it should drop the "right-most" member and insert the new
-		// member at the front.
-		//
-		// https://github.com/w3c/trace-context/pull/448
-		return ts, fmt.Errorf("failed to insert: %w", errMemberNumber)
+	if cTS.Len()+1 <= maxListMembers {
+		cTS.list = append(cTS.list, member{})
 	}
-
-	cTS.list = append(cTS.list, member{})
+	// When the number of members exceeds capacity, drop the "right-most".
 	copy(cTS.list[1:], cTS.list)
 	cTS.list[0] = m
 

trace/tracestate_test.go

@@ -482,14 +482,20 @@ func TestTraceStateInsert(t *testing.T) {
 			err:        errInvalidKey,
 		},
 		{
-			name:       "too many entries",
+			name:       "drop the right-most member(oldest) in queue",
 			tracestate: maxMembers,
 			key:        "keyx",
 			value:      "valx",
-			expected:   maxMembers,
-			err:        errMemberNumber,
-		},
-	}
+			expected: func() TraceState {
+				// Prepend the new element and remove the oldest one, which is over capacity.
+				return TraceState{
+					list: append(
+						[]member{{Key: "keyx", Value: "valx"}},
+						maxMembers.list[:len(maxMembers.list)-1]...,
+					),
+				}
+			}(),
+		}}
 
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {