Copied buffering split from DataBufferUtils to StringDecoder

Due to the imminent removal of DataBufferUtils.split, this commit copies
over the buffering split algortihm from DataBufferUtils, as it is still
sutable for the StringDecoder

spring-core/src/main/java/org/springframework/core/codec/StringDecoder.java

@@ -16,6 +16,9 @@
 
 package org.springframework.core.codec;
 
+import java.io.InputStream;
+import java.io.OutputStream;
+import java.nio.ByteBuffer;
 import java.nio.CharBuffer;
 import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
@@ -25,13 +28,17 @@ import java.util.List;
 import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
+import java.util.function.IntPredicate;
 
 import org.reactivestreams.Publisher;
 import reactor.core.publisher.Flux;
 
 import org.springframework.core.ResolvableType;
 import org.springframework.core.io.buffer.DataBuffer;
+import org.springframework.core.io.buffer.DataBufferFactory;
 import org.springframework.core.io.buffer.DataBufferUtils;
+import org.springframework.core.io.buffer.DefaultDataBufferFactory;
+import org.springframework.core.io.buffer.PooledDataBuffer;
 import org.springframework.core.log.LogFormatUtils;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
@@ -88,8 +95,15 @@ public final class StringDecoder extends AbstractDataBufferDecoder<String> {
 
 		byte[][] delimiterBytes = getDelimiterBytes(mimeType);
 
-		Flux<DataBuffer> inputFlux =
-				DataBufferUtils.split(input, delimiterBytes, this.stripDelimiter);
+		Flux<DataBuffer> inputFlux = Flux.defer(() -> {
+			DataBufferUtils.Matcher matcher = DataBufferUtils.matcher(delimiterBytes);
+			return Flux.from(input)
+					.concatMapIterable(buffer -> endFrameAfterDelimiter(buffer, matcher))
+					.bufferUntil(buffer -> buffer instanceof EndFrameBuffer)
+					.map(buffers -> joinAndStrip(buffers, this.stripDelimiter))
+					.doOnDiscard(PooledDataBuffer.class, DataBufferUtils::release);
+
+		});
 
 		return super.decode(inputFlux, elementType, mimeType, hints);
 	}
@@ -128,6 +142,69 @@ public final class StringDecoder extends AbstractDataBufferDecoder<String> {
 		}
 	}
 
+	/**
+	 * Finds the first match and longest delimiter, {@link EndFrameBuffer} just after it.
+	 *
+	 * @param dataBuffer the buffer to find delimiters in
+	 * @param matcher used to find the first delimiters
+	 * @return a flux of buffers, containing {@link EndFrameBuffer} after each delimiter that was
+	 * found in {@code dataBuffer}. Returns  Flux, because returning List (w/ flatMapIterable)
+	 * results in memory leaks due to pre-fetching.
+	 */
+	private static List<DataBuffer> endFrameAfterDelimiter(DataBuffer dataBuffer, DataBufferUtils.Matcher matcher) {
+		List<DataBuffer> result = new ArrayList<>();
+		do {
+			int endIdx = matcher.match(dataBuffer);
+			if (endIdx != -1) {
+				int readPosition = dataBuffer.readPosition();
+				int length = endIdx - readPosition + 1;
+				result.add(dataBuffer.retainedSlice(readPosition, length));
+				result.add(new EndFrameBuffer(matcher.delimiter()));
+				dataBuffer.readPosition(endIdx + 1);
+			}
+			else {
+				result.add(DataBufferUtils.retain(dataBuffer));
+				break;
+			}
+		}
+		while (dataBuffer.readableByteCount() > 0);
+
+		DataBufferUtils.release(dataBuffer);
+		return result;
+	}
+
+	/**
+	 * Joins the given list of buffers. If the list ends with a {@link EndFrameBuffer}, it is
+	 * removed. If {@code stripDelimiter} is {@code true} and the resulting buffer ends with
+	 * a delimiter, it is removed.
+	 * @param dataBuffers the data buffers to join
+	 * @param stripDelimiter whether to strip the delimiter
+	 * @return the joined buffer
+	 */
+	private static DataBuffer joinAndStrip(List<DataBuffer> dataBuffers,
+			boolean stripDelimiter) {
+
+		Assert.state(!dataBuffers.isEmpty(), "DataBuffers should not be empty");
+
+		byte[] matchingDelimiter = null;
+
+		int lastIdx = dataBuffers.size() - 1;
+		DataBuffer lastBuffer = dataBuffers.get(lastIdx);
+		if (lastBuffer instanceof EndFrameBuffer) {
+			matchingDelimiter = ((EndFrameBuffer) lastBuffer).delimiter();
+			dataBuffers.remove(lastIdx);
+		}
+
+		DataBuffer result = dataBuffers.get(0).factory().join(dataBuffers);
+
+		if (stripDelimiter && matchingDelimiter != null) {
+			result.writePosition(result.writePosition() - matchingDelimiter.length);
+		}
+		return result;
+	}
+
+
+
 
 	/**
 	 * Create a {@code StringDecoder} for {@code "text/plain"}.
@@ -186,4 +263,167 @@ public final class StringDecoder extends AbstractDataBufferDecoder<String> {
 				new MimeType("text", "plain", DEFAULT_CHARSET), MimeTypeUtils.ALL);
 	}
 
+
+	private static class EndFrameBuffer implements DataBuffer {
+
+		private static final DataBuffer BUFFER = new DefaultDataBufferFactory().wrap(new byte[0]);
+
+		private byte[] delimiter;
+
+
+		public EndFrameBuffer(byte[] delimiter) {
+			this.delimiter = delimiter;
+		}
+
+		public byte[] delimiter() {
+			return this.delimiter;
+		}
+
+		@Override
+		public DataBufferFactory factory() {
+			return BUFFER.factory();
+		}
+
+		@Override
+		public int indexOf(IntPredicate predicate, int fromIndex) {
+			return BUFFER.indexOf(predicate, fromIndex);
+		}
+
+		@Override
+		public int lastIndexOf(IntPredicate predicate, int fromIndex) {
+			return BUFFER.lastIndexOf(predicate, fromIndex);
+		}
+
+		@Override
+		public int readableByteCount() {
+			return BUFFER.readableByteCount();
+		}
+
+		@Override
+		public int writableByteCount() {
+			return BUFFER.writableByteCount();
+		}
+
+		@Override
+		public int capacity() {
+			return BUFFER.capacity();
+		}
+
+		@Override
+		public DataBuffer capacity(int capacity) {
+			return BUFFER.capacity(capacity);
+		}
+
+		@Override
+		public DataBuffer ensureCapacity(int capacity) {
+			return BUFFER.ensureCapacity(capacity);
+		}
+
+		@Override
+		public int readPosition() {
+			return BUFFER.readPosition();
+		}
+
+		@Override
+		public DataBuffer readPosition(int readPosition) {
+			return BUFFER.readPosition(readPosition);
+		}
+
+		@Override
+		public int writePosition() {
+			return BUFFER.writePosition();
+		}
+
+		@Override
+		public DataBuffer writePosition(int writePosition) {
+			return BUFFER.writePosition(writePosition);
+		}
+
+		@Override
+		public byte getByte(int index) {
+			return BUFFER.getByte(index);
+		}
+
+		@Override
+		public byte read() {
+			return BUFFER.read();
+		}
+
+		@Override
+		public DataBuffer read(byte[] destination) {
+			return BUFFER.read(destination);
+		}
+
+		@Override
+		public DataBuffer read(byte[] destination, int offset, int length) {
+			return BUFFER.read(destination, offset, length);
+		}
+
+		@Override
+		public DataBuffer write(byte b) {
+			return BUFFER.write(b);
+		}
+
+		@Override
+		public DataBuffer write(byte[] source) {
+			return BUFFER.write(source);
+		}
+
+		@Override
+		public DataBuffer write(byte[] source, int offset, int length) {
+			return BUFFER.write(source, offset, length);
+		}
+
+		@Override
+		public DataBuffer write(DataBuffer... buffers) {
+			return BUFFER.write(buffers);
+		}
+
+		@Override
+		public DataBuffer write(ByteBuffer... buffers) {
+			return BUFFER.write(buffers);
+		}
+
+		@Override
+		public DataBuffer write(CharSequence charSequence, Charset charset) {
+			return BUFFER.write(charSequence, charset);
+		}
+
+		@Override
+		public DataBuffer slice(int index, int length) {
+			return BUFFER.slice(index, length);
+		}
+
+		@Override
+		public DataBuffer retainedSlice(int index, int length) {
+			return BUFFER.retainedSlice(index, length);
+		}
+
+		@Override
+		public ByteBuffer asByteBuffer() {
+			return BUFFER.asByteBuffer();
+		}
+
+		@Override
+		public ByteBuffer asByteBuffer(int index, int length) {
+			return BUFFER.asByteBuffer(index, length);
+		}
+
+		@Override
+		public InputStream asInputStream() {
+			return BUFFER.asInputStream();
+		}
+
+		@Override
+		public InputStream asInputStream(boolean releaseOnClose) {
+			return BUFFER.asInputStream(releaseOnClose);
+		}
+
+		@Override
+		public OutputStream asOutputStream() {
+			return BUFFER.asOutputStream();
+		}
+	}
+
+
 }