Force flush of nio buffer when threshold is reached

Update unit tests to verify the FormattedByteChannel with the
Netty ChunkedNioStream.

Closes instaclustr#83

common/src/main/java/com/zegelin/prometheus/exposition/FormattedByteChannel.java

@@ -1,32 +1,50 @@
 package com.zegelin.prometheus.exposition;
 
+import com.google.common.annotations.VisibleForTesting;
+
 import java.nio.ByteBuffer;
 import java.nio.channels.ReadableByteChannel;
 
 public class FormattedByteChannel implements ReadableByteChannel {
-    public static final int MIN_CHUNK_SIZE = 1024 * 1024;
-    public static final int MAX_CHUNK_SIZE = MIN_CHUNK_SIZE * 5;
+    public static final int DEFAULT_CHUNK_THRESHOLD = 1024 * 1024;
+    public static final int MAX_CHUNK_SIZE = DEFAULT_CHUNK_THRESHOLD * 5;
 
     private final FormattedExposition formattedExposition;
+    private final int chunkThreshold;
+
+    public FormattedByteChannel(final FormattedExposition formattedExposition) {
+        this(formattedExposition, DEFAULT_CHUNK_THRESHOLD);
+    }
 
-    public FormattedByteChannel(FormattedExposition formattedExposition) {
+    @VisibleForTesting
+    FormattedByteChannel(final FormattedExposition formattedExposition, final int chunkThreshold) {
         this.formattedExposition = formattedExposition;
+        this.chunkThreshold = chunkThreshold;
     }
 
     @Override
-    public int read(ByteBuffer dst) {
+    public int read(final ByteBuffer dst) {
         if (!isOpen()) {
             return -1;
         }
 
+        // Forcing the calling ChunkedNioStream to flush the buffer
+        if (hasBufferReachedChunkThreshold(dst)) {
+            return -1;
+        }
+
         final NioExpositionSink sink = new NioExpositionSink(dst);
-        while (sink.getIngestedByteCount() < MIN_CHUNK_SIZE && isOpen()) {
+        while (!hasBufferReachedChunkThreshold(dst) && isOpen()) {
             formattedExposition.nextSlice(sink);
         }
 
         return sink.getIngestedByteCount();
     }
 
+    private boolean hasBufferReachedChunkThreshold(final ByteBuffer dst) {
+        return dst.position() >= chunkThreshold;
+    }
+
     @Override
     public boolean isOpen() {
         return !formattedExposition.isEndOfInput();

common/src/test/java/com/zegelin/prometheus/exposition/TestFormattedByteChannel.java

@@ -1,5 +1,9 @@
 package com.zegelin.prometheus.exposition;
 
+import io.netty.buffer.ByteBuf;
+import io.netty.buffer.UnpooledByteBufAllocator;
+import io.netty.channel.ChannelHandlerContext;
+import io.netty.handler.stream.ChunkedNioStream;
 import org.mockito.Mock;
 import org.mockito.MockitoAnnotations;
 import org.testng.annotations.BeforeMethod;
@@ -8,49 +12,102 @@
 import java.nio.ByteBuffer;
 
 import static org.assertj.core.api.Assertions.assertThat;
-import static org.mockito.ArgumentMatchers.any;
-import static org.mockito.Mockito.doAnswer;
 import static org.mockito.Mockito.when;
 
 public class TestFormattedByteChannel {
     @Mock
-    private FormattedExposition formattedExposition;
+    private ChannelHandlerContext ctx;
+
+    private ChunkedNioStream chunkedNioStream;
+
+    private TenSliceExposition formattedExposition;
 
     private ByteBuffer buffer;
     private FormattedByteChannel channel;
 
     @BeforeMethod
     public void before() {
         MockitoAnnotations.initMocks(this);
+
         buffer = ByteBuffer.allocate(128);
-        channel = new FormattedByteChannel(formattedExposition);
+        formattedExposition = new TenSliceExposition();
+        channel = new FormattedByteChannel(formattedExposition, 64);
+
+        when(ctx.alloc()).thenReturn(UnpooledByteBufAllocator.DEFAULT);
+        chunkedNioStream = new ChunkedNioStream(channel, 128);
     }
 
     @Test
     public void testClosed() {
-        when(formattedExposition.isEndOfInput()).thenReturn(true);
+        formattedExposition.setSlices(0);
 
         assertThat(channel.read(buffer)).isEqualTo(-1);
         assertThat(channel.isOpen()).isEqualTo(false);
     }
 
     @Test
     public void testOpen() {
-        when(formattedExposition.isEndOfInput()).thenReturn(false);
+        formattedExposition.setSlices(1);
 
         assertThat(channel.isOpen()).isEqualTo(true);
     }
 
     @Test
-    public void testOneChunk() {
-        when(formattedExposition.isEndOfInput()).thenReturn(false).thenReturn(false).thenReturn(true);
-        doAnswer(invocation -> {
-            NioExpositionSink sink = invocation.getArgument(0);
+    public void testOneSlice() throws Exception {
+        formattedExposition.setSlices(1);
+        ByteBuf byteBuf;
+
+        byteBuf = chunkedNioStream.readChunk(ctx);
+        assertThat(byteBuf.readableBytes()).isEqualTo(10);
+        assertThat(chunkedNioStream.isEndOfInput()).isEqualTo(true);
+    }
+
+    @Test
+    public void testTwoSlices() throws Exception {
+        formattedExposition.setSlices(2);
+        ByteBuf byteBuf;
+
+        byteBuf = chunkedNioStream.readChunk(ctx);
+        assertThat(byteBuf.readableBytes()).isEqualTo(20);
+        assertThat(chunkedNioStream.isEndOfInput()).isEqualTo(true);
+    }
+
+    @Test
+    public void testTwoChunks() throws Exception {
+        formattedExposition.setSlices(10);
+        ByteBuf byteBuf;
+
+        byteBuf = chunkedNioStream.readChunk(ctx);
+        assertThat(byteBuf.readableBytes()).isEqualTo(70);
+        assertThat(chunkedNioStream.isEndOfInput()).isEqualTo(false);
+
+        byteBuf = chunkedNioStream.readChunk(ctx);
+        assertThat(byteBuf.readableBytes()).isEqualTo(30);
+        assertThat(chunkedNioStream.isEndOfInput()).isEqualTo(true);
+    }
+
+    // A dummy Exposition implementation that will generate a specific number of slices of size 10.
+    private static class TenSliceExposition implements FormattedExposition {
+        private int slices = 0;
+        private int currentSlice = 0;
+
+        private void setSlices(final int chunks) {
+            this.slices = chunks;
+        }
+
+        @Override
+        public void nextSlice(final ExpositionSink<?> sink) {
+            if (isEndOfInput()) {
+                return;
+            }
+
+            currentSlice++;
             sink.writeAscii("abcdefghij");
-            return null;
-        }).when(formattedExposition).nextSlice(any(NioExpositionSink.class));
+        }
 
-        assertThat(channel.read(buffer)).isEqualTo(10);
-        assertThat(channel.isOpen()).isEqualTo(false);
+        @Override
+        public boolean isEndOfInput() {
+            return currentSlice >= slices;
+        }
     }
 }