Perf: More efficient range header checks.

Author: Tratcher

src/Microsoft.AspNetCore.StaticFiles/StaticFileContext.cs

@@ -15,6 +15,7 @@
 using Microsoft.AspNetCore.StaticFiles.Infrastructure;
 using Microsoft.Extensions.FileProviders;
 using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Primitives;
 using Microsoft.Net.Http.Headers;
 
 namespace Microsoft.AspNetCore.StaticFiles
@@ -228,21 +229,32 @@ private void ComputeRange()
                 return;
             }
 
-            var rangeHeader = _requestHeaders.Range;
-            if (rangeHeader == null)
+            var rawRangeHeader = _request.Headers[HeaderNames.Range];
+            if (StringValues.IsNullOrEmpty(rawRangeHeader))
             {
                 return;
             }
 
-            if (rangeHeader.Ranges.Count > 1)
+            // Perf: Check for a single entry before parsing it
+            if (rawRangeHeader.Count > 1 || rawRangeHeader[0].IndexOf(',') >= 0)
             {
                 // The spec allows for multiple ranges but we choose not to support them because the client may request
                 // very strange ranges (e.g. each byte separately, overlapping ranges, etc.) that could negatively
                 // impact the server. Ignore the header and serve the response normally.
-                _logger.LogMultipleFileRanges(rangeHeader.ToString());
+                _logger.LogMultipleFileRanges(rawRangeHeader.ToString());
                 return;
             }
 
+            var rangeHeader = _requestHeaders.Range;
+            if (rangeHeader == null)
+            {
+                // Invalid
+                return;
+            }
+
+            // Already verified above
+            Debug.Assert(rangeHeader.Ranges.Count == 1);
+
             // 14.27 If-Range
             var ifRangeHeader = _requestHeaders.IfRange;
             if (ifRangeHeader != null)