diff --git a/netty/src/main/java/io/grpc/netty/NettyAdaptiveCumulator.java b/netty/src/main/java/io/grpc/netty/NettyAdaptiveCumulator.java
index e72ecbb7d4e0..f493e18d1983 100644
--- a/netty/src/main/java/io/grpc/netty/NettyAdaptiveCumulator.java
+++ b/netty/src/main/java/io/grpc/netty/NettyAdaptiveCumulator.java
@@ -106,8 +106,10 @@ static boolean shouldCompose(CompositeByteBuf composite, ByteBuf in, int compose
     if (composite.numComponents() == 0) {
       return true;
     }
-    int tailSize = composite.capacity() - composite.toByteIndex(componentCount - 1);
-    return tailSize + in.readableBytes() >= composeMinSize;
+    int inputSize = in.readableBytes();
+    int tailStart = composite.toByteIndex(componentCount - 1);
+    int tailSize = composite.writerIndex() - tailStart;
+    return tailSize + inputSize >= composeMinSize;
   }
 
   /**
@@ -127,23 +129,20 @@ static boolean shouldCompose(CompositeByteBuf composite, ByteBuf in, int compose
    * is verified in unit tests for this method.
    */
   @VisibleForTesting
-  static void mergeWithCompositeTail(ByteBufAllocator alloc, CompositeByteBuf composite,
-      ByteBuf in) {
-
-    int newBytes = in.readableBytes();
-    int tailIndex = composite.numComponents() - 1;
-    int tailStart = composite.toByteIndex(tailIndex);
-    int tailBytes = composite.capacity() - tailStart;
-    int totalBytes = newBytes + tailBytes;
-
-    ByteBuf tail = composite.component(tailIndex);
-    ByteBuf merged = null;
-
+  static void mergeWithCompositeTail(
+      ByteBufAllocator alloc, CompositeByteBuf composite, ByteBuf in) {
+    int inputSize = in.readableBytes();
+    int tailComponentIndex = composite.numComponents() - 1;
+    int tailStart = composite.toByteIndex(tailComponentIndex);
+    int tailSize = composite.writerIndex() - tailStart;
+    int newTailSize = inputSize + tailSize;
+    ByteBuf tail = composite.component(tailComponentIndex);
+    ByteBuf newTail = null;
     try {
-      if (tail.refCnt() == 1 && !tail.isReadOnly() && totalBytes <= tail.maxCapacity()) {
+      if (tail.refCnt() == 1 && !tail.isReadOnly() && newTailSize <= tail.maxCapacity()) {
         // Ideal case: the tail isn't shared, and can be expanded to the required capacity.
         // Take ownership of the tail.
-        merged = tail.retain();
+        newTail = tail.retain();
         /*
          * The tail is a readable non-composite buffer, so writeBytes() handles everything for us.
          *
@@ -156,33 +155,37 @@ static void mergeWithCompositeTail(ByteBufAllocator alloc, CompositeByteBuf comp
          *   Unwrapping buffers is unsafe, and potential benefit of fast writes may not be
          *   as pronounced because the capacity is doubled with each reallocation.
          */
-        merged.writeBytes(in);
+        newTail.writeBytes(in);
       } else {
         // The tail is shared, or not expandable. Replace it with a new buffer of desired capacity.
-        merged = alloc.buffer(alloc.calculateNewCapacity(totalBytes, Integer.MAX_VALUE));
-        merged.setBytes(0, composite, tailStart, tailBytes)
-            .setBytes(tailBytes, in, in.readerIndex(), newBytes)
-            .writerIndex(totalBytes);
+        newTail = alloc.buffer(alloc.calculateNewCapacity(newTailSize, Integer.MAX_VALUE));
+        newTail.setBytes(0, composite, tailStart, tailSize)
+            .setBytes(tailSize, in, in.readerIndex(), inputSize)
+            .writerIndex(newTailSize);
         in.readerIndex(in.writerIndex());
       }
       // Store readerIndex to avoid out of bounds writerIndex during component replacement.
       int prevReader = composite.readerIndex();
-      // Remove the tail, reset writer index, add merged component.
-      composite.removeComponent(tailIndex).setIndex(0, tailStart)
-          .addFlattenedComponents(true, merged);
-      merged = null;
+      // Remove the old tail, reset writer index.
+      composite.removeComponent(tailComponentIndex).setIndex(0, tailStart);
+      // Add back the new tail.
+      composite.addFlattenedComponents(true, newTail);
+      // New tail's ownership transferred to the composite buf.
+      newTail = null;
       in.release();
       in = null;
-      // Restore the reader.
+      // Restore the reader. In case it fails we restore the reader after releasing/forgetting
+      // the input and the new tail so that finally block can handles them properly.
       composite.readerIndex(prevReader);
     } finally {
       // Input buffer was merged with the tail.
       if (in != null) {
         in.release();
       }
-      // If merge's ownership isn't transferred to the composite buf, release it to prevent a leak.
-      if (merged != null) {
-        merged.release();
+      // If new tail's ownership isn't transferred to the composite buf.
+      // Release it to prevent a leak.
+      if (newTail != null) {
+        newTail.release();
       }
     }
   }
diff --git a/netty/src/test/java/io/grpc/netty/NettyAdaptiveCumulatorTest.java b/netty/src/test/java/io/grpc/netty/NettyAdaptiveCumulatorTest.java
index 09cc88ff1f34..34ef9244a418 100644
--- a/netty/src/test/java/io/grpc/netty/NettyAdaptiveCumulatorTest.java
+++ b/netty/src/test/java/io/grpc/netty/NettyAdaptiveCumulatorTest.java
@@ -374,7 +374,7 @@ public void mergeWithCompositeTail_tailExpandable_fastWrite() {
     public void mergeWithCompositeTail_tailExpandable_reallocateInMemory() {
       int tailFastCapacity = tail.writerIndex() + tail.maxFastWritableBytes();
       String inSuffixOverFastBytes = Strings.repeat("a", tailFastCapacity + 1);
-      int totalBytes =  tail.readableBytes() + inSuffixOverFastBytes.length();
+      int newTailSize =  tail.readableBytes() + inSuffixOverFastBytes.length();
       composite.addFlattenedComponents(true, tail);
 
       // Make input larger than tailFastCapacity
@@ -384,7 +384,7 @@ public void mergeWithCompositeTail_tailExpandable_reallocateInMemory() {
       assertThat(in.readableBytes()).isAtMost(tail.maxWritableBytes());
 
       // Confirm the assumption that new capacity is produced by alloc.calculateNewCapacity().
-      int expectedTailCapacity = alloc.calculateNewCapacity(totalBytes, Integer.MAX_VALUE);
+      int expectedTailCapacity = alloc.calculateNewCapacity(newTailSize, Integer.MAX_VALUE);
       assertTailExpanded(EXPECTED_TAIL_DATA.concat(inSuffixOverFastBytes), expectedTailCapacity);
     }
 
@@ -566,10 +566,10 @@ public CompositeByteBuf addFlattenedComponents(boolean increaseWriterIndex,
       };
 
       // Return our instance of the new buffer to ensure it's released.
-      int totalBytes = tail.readableBytes() + in.readableBytes();
-      ByteBuf merged = alloc.buffer(alloc.calculateNewCapacity(totalBytes, Integer.MAX_VALUE));
+      int newTailSize = tail.readableBytes() + in.readableBytes();
+      ByteBuf newTail = alloc.buffer(alloc.calculateNewCapacity(newTailSize, Integer.MAX_VALUE));
       ByteBufAllocator mockAlloc = mock(ByteBufAllocator.class);
-      when(mockAlloc.buffer(anyInt())).thenReturn(merged);
+      when(mockAlloc.buffer(anyInt())).thenReturn(newTail);
 
       try {
         NettyAdaptiveCumulator.mergeWithCompositeTail(mockAlloc, compositeRo, in);
@@ -579,7 +579,7 @@ public CompositeByteBuf addFlattenedComponents(boolean increaseWriterIndex,
         // Input must be released unless its ownership has been to the composite cumulation.
         assertEquals(0, in.refCnt());
         // New buffer released
-        assertEquals(0, merged.refCnt());
+        assertEquals(0, newTail.refCnt());
         // Composite cumulation is retained
         assertEquals(1, compositeRo.refCnt());
         // Composite cumulation loses the tail