Use finally for buffer cleanup.

Author: adityamandaleeka

src/Servers/Kestrel/Core/src/Internal/Http/Http1Connection.cs

@@ -844,6 +844,7 @@ protected override bool BeginRead(out ValueTask<ReadResult> awaitable)
     protected override bool TryParseRequest(ReadResult result, out bool endConnection)
     {
         var reader = new SequenceReader<byte>(result.Buffer);
+        var isConsumed = false;
 
         // Use non-throwing parser path for performance.
         // Note: The parser itself doesn't throw, but handler callbacks (OnStartLine, OnHeader)
@@ -853,14 +854,28 @@ protected override bool TryParseRequest(ReadResult result, out bool endConnectio
         try
         {
             parseResult = TryParseRequestNoThrow(ref reader);
+
+            // Handle parse errors without exceptions
+            if (parseResult.HasError)
+            {
+                // Create exception and call OnBadRequest BEFORE finally runs AdvanceTo,
+                // as that may invalidate the buffer
+                var ex = CreateBadRequestException(parseResult, result.Buffer);
+                OnBadRequest(result.Buffer, ex);
+
+                SetBadRequestState(ex);
+                endConnection = true;
+                return true;
+            }
+
+            isConsumed = parseResult.IsComplete;
         }
 #pragma warning disable CS0618 // Type or member is obsolete
         catch (BadHttpRequestException ex)
         {
+            // OnBadRequest must be called before finally runs AdvanceTo
             OnBadRequest(result.Buffer, ex);
 
-            Input.AdvanceTo(reader.Position, result.Buffer.End);
-
             SetBadRequestState(ex);
             endConnection = true;
             return true;
@@ -870,30 +885,13 @@ protected override bool TryParseRequest(ReadResult result, out bool endConnectio
         {
             // Record OtherError for unexpected exceptions that escape the parser
             KestrelMetrics.AddConnectionEndReason(MetricsContext, ConnectionEndReason.OtherError);
-
-            // Must advance buffer before re-throwing (matches original finally block behavior)
-            Input.AdvanceTo(reader.Position, result.Buffer.End);
             throw;
         }
-
-        var isConsumed = parseResult.IsComplete;
-
-        // Handle parse errors without exceptions
-        if (parseResult.HasError)
+        finally
         {
-            // Create exception and call OnBadRequest BEFORE AdvanceTo, as that may invalidate the buffer
-            var ex = CreateBadRequestException(parseResult, result.Buffer);
-            OnBadRequest(result.Buffer, ex);
-
-            Input.AdvanceTo(reader.Position, result.Buffer.End);
-
-            SetBadRequestState(ex);
-            endConnection = true;
-            return true;
+            Input.AdvanceTo(reader.Position, isConsumed ? reader.Position : result.Buffer.End);
         }
 
-        Input.AdvanceTo(reader.Position, isConsumed ? reader.Position : result.Buffer.End);
-
         if (result.IsCompleted)
         {
             switch (_requestProcessingStatus)