(GH-116) Skip blank lines when detecting boundary

Author: Jericho

Source/HttpMultipartParser/StreamingBinaryMultipartFormDataParser.cs

@@ -254,7 +254,16 @@ private static string DetectBoundary(RebufferableBinaryReader reader)
 		{
 			// Presumably the boundary is --|||||||||||||| where -- is the stuff added on to
 			// the front as per the protocol and ||||||||||||| is the part we care about.
-			var line = reader.ReadLine();
+
+			// The following loop ignores blank lines that may be present before the first line of the form.
+			// It's highly unusual to find blank lines at the start of the data but it's a possible scenario described in GH-116.
+			// Please note that we intentionally do NOT check for "string.IsNullOrEmpty(line)" because NULL does
+			// not indicate a blank line. It indicates that we have reached the end of the stream.
+			var line = string.Empty;
+			while (line == string.Empty)
+			{
+				line = reader.ReadLine();
+			}
 
 			// The line must not be empty and must starts with "--".
 			if (string.IsNullOrEmpty(line)) throw new MultipartParseException("Unable to determine boundary: either the stream is empty or we reached the end of the stream");
@@ -296,7 +305,16 @@ private static async Task<string> DetectBoundaryAsync(RebufferableBinaryReader r
 		{
 			// Presumably the boundary is --|||||||||||||| where -- is the stuff added on to
 			// the front as per the protocol and ||||||||||||| is the part we care about.
-			var line = await reader.ReadLineAsync(cancellationToken).ConfigureAwait(false);
+
+			// The following loop ignores blank lines that may be present before the first line of the form.
+			// It's highly unusual to find blank lines at the start of the data but it's a possible scenario described in GH-116.
+			// Please note that we intentionally do NOT check for "string.IsNullOrEmpty(line)" because NULL does
+			// not indicate a blank line. It indicates that we have reached the end of the stream.
+			var line = string.Empty;
+			while (line == string.Empty)
+			{
+				line = await reader.ReadLineAsync(cancellationToken).ConfigureAwait(false);
+			}
 
 			// The line must not be empty and must starts with "--".
 			if (string.IsNullOrEmpty(line)) throw new MultipartParseException("Unable to determine boundary: either the stream is empty or we reached the end of the stream");