Optimization: Use currentBoundary state to skip over boundary markers

Instead of having MimeReader.SkipLine/Async() loop over the entire line
containing what we know is a boundary marker, just skip over the known
length of the currentBoundary - this way we only need to loop over the
trailing whitespace characters.

Also, there's no need to call ReadAhead because we know we already have
the full line.

Author: jstedfast

MimeKit/AsyncMimeReader.cs

@@ -291,19 +291,13 @@ async Task StepHeadersAsync (CancellationToken cancellationToken)
 			await OnHeadersEndAsync (headerBlockBegin, headersBeginLineNumber, headerBlockEnd, lineNumber, cancellationToken).ConfigureAwait (false);
 		}
 
-		async Task<bool> SkipLineAsync (bool consumeNewLine, CancellationToken cancellationToken)
+		bool SkipBoundaryMarker (bool endBoundary)
 		{
-			do {
-				unsafe {
-					fixed (byte* inbuf = input) {
-						if (InnerSkipLine (inbuf, consumeNewLine))
-							return true;
-					}
+			unsafe {
+				fixed (byte* inbuf = input) {
+					return SkipBoundaryMarker (inbuf, endBoundary);
 				}
-
-				if (await ReadAheadAsync (ReadAheadSize, 1, cancellationToken).ConfigureAwait (false) <= 0)
-					return false;
-			} while (true);
+			}
 		}
 
 		async Task<MimeParserState> StepAsync (CancellationToken cancellationToken)
@@ -507,7 +501,7 @@ async Task MultipartScanSubpartsAsync (ContentType multipartContentType, int dep
 
 			do {
 				// skip over the boundary marker
-				if (!await SkipLineAsync (true, cancellationToken).ConfigureAwait (false)) {
+				if (!SkipBoundaryMarker (endBoundary: false)) {
 					await OnMultipartBoundaryAsync (multipartContentType.Boundary, boundaryOffset, GetOffset (inputIndex), lineNumber, cancellationToken).ConfigureAwait (false);
 					boundary = BoundaryType.Eos;
 					return;
@@ -609,7 +603,7 @@ async Task<int> ConstructMultipartAsync (ContentType contentType, int depth, Can
 				var boundaryOffset = GetOffset (inputIndex);
 				var boundaryLineNumber = lineNumber;
 
-				await SkipLineAsync (false, cancellationToken).ConfigureAwait (false);
+				SkipBoundaryMarker (endBoundary: true);
 
 				await OnMultipartEndBoundaryAsync (marker, boundaryOffset, GetOffset (inputIndex), boundaryLineNumber, cancellationToken).ConfigureAwait (false);
 

MimeKit/MimeReader.cs

@@ -1756,14 +1756,22 @@ unsafe void StepHeaders (byte* inbuf, CancellationToken cancellationToken)
 			OnHeadersEnd (headerBlockBegin, headersBeginLineNumber, headerBlockEnd, lineNumber, cancellationToken);
 		}
 
-		unsafe bool InnerSkipLine (byte* inbuf, bool consumeNewLine)
+		unsafe bool SkipBoundaryMarker (byte* inbuf, bool endBoundary)
 		{
+			// Don't consume the newline sequence for end boundaries (those are part of the epilogue).
+			bool consumeNewLine = !endBoundary;
 			byte* inptr = inbuf + inputIndex;
 			byte* inend = inbuf + inputEnd;
 
 			*inend = (byte) '\n';
 
-			// TODO: unroll this loop?
+			// skip over the boundary marker
+			if (endBoundary)
+				inptr += currentBoundary.FinalLength;
+			else
+				inptr += currentBoundary.Length;
+
+			// skip over any trailing whitespace
 			while (*inptr != (byte) '\n')
 				inptr++;
 
@@ -1785,17 +1793,6 @@ unsafe bool InnerSkipLine (byte* inbuf, bool consumeNewLine)
 			return false;
 		}
 
-		unsafe bool SkipLine (byte* inbuf, bool consumeNewLine, CancellationToken cancellationToken)
-		{
-			do {
-				if (InnerSkipLine (inbuf, consumeNewLine))
-					return true;
-
-				if (ReadAhead (ReadAheadSize, 1, cancellationToken) <= 0)
-					return false;
-			} while (true);
-		}
-
 		unsafe MimeParserState Step (byte* inbuf, CancellationToken cancellationToken)
 		{
 			switch (state) {
@@ -2233,7 +2230,7 @@ unsafe void MultipartScanSubparts (ContentType multipartContentType, byte* inbuf
 
 			do {
 				// skip over the boundary marker
-				if (!SkipLine (inbuf, true, cancellationToken)) {
+				if (!SkipBoundaryMarker (inbuf, endBoundary: false)) {
 					OnMultipartBoundary (multipartContentType.Boundary, boundaryOffset, GetOffset (inputIndex), lineNumber, cancellationToken);
 					boundary = BoundaryType.Eos;
 					return;
@@ -2359,7 +2356,7 @@ unsafe int ConstructMultipart (ContentType contentType, byte* inbuf, int depth,
 				var boundaryOffset = GetOffset (inputIndex);
 				var boundaryLineNumber = lineNumber;
 
-				SkipLine (inbuf, false, cancellationToken);
+				SkipBoundaryMarker (inbuf, endBoundary: true);
 
 				OnMultipartEndBoundary (marker, boundaryOffset, GetOffset (inputIndex), boundaryLineNumber, cancellationToken);