[perf] Cache validated W3C tracestate per-thread

src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/W3CPropagator.cs

@@ -152,13 +152,44 @@ internal static bool TryExtractBaggage(string? baggageString, out IEnumerable<Ke
         // value       = 0*255(chr) nblk-chr
         // nblk-chr    = %x21-2B / %x2D-3C / %x3E-7E
         // chr         = %x20 / nblk-chr
+        // A single incoming request commonly fans out to several outgoing calls that
+        // all inject the same (unmodified) Activity.TraceStateString (see InjectTraceState),
+        // typically all from the same thread for a given request. Cache the last
+        // (input, output) pair per thread to skip re-parsing and rebuilding it every time.
+        [ThreadStatic]
+        private static string? t_lastRawTraceState;
+        [ThreadStatic]
+        private static string? t_lastValidatedTraceState;
+
         internal static string? ValidateTraceState(string? traceState)
         {
             if (string.IsNullOrEmpty(traceState))
             {
                 return null;
             }
 
+            // ValidateTraceState also runs on the raw carrier value from an incoming (untrusted)
+            // request, via ExtractTraceIdAndState. Don't let an arbitrarily large or malformed
+            // value get rooted in per-thread state for the thread's lifetime just because it was
+            // seen once - only cache inputs already within a valid tracestate's own size limit.
+            if (traceState.Length > MaxTraceStateEncodedLength)
+            {
+                return ValidateTraceStateCore(traceState);
+            }
+
+            if (ReferenceEquals(traceState, t_lastRawTraceState))
+            {
+                return t_lastValidatedTraceState;
+            }
+
+            string? validated = ValidateTraceStateCore(traceState);
+            t_lastRawTraceState = traceState;
+            t_lastValidatedTraceState = validated;
+            return validated;
+        }
+
+        private static string? ValidateTraceStateCore(string traceState)
+        {
             int entries = 0;
             using ValueStringBuilder vsb = new ValueStringBuilder(stackalloc char[Math.Min(traceState.Length, MaxTraceStateEncodedLength)]);