Not cause a CoW when calling ByteBuffer.discardReadBytes() while it w…

…as fully consumed. (#363)

Motivation:

We should not need to trigger any copy when the ByteBuffer was fully consumed and discardReadBytes() is called.

Modifications:

- If fully consumed just reset reader and writer Index.
- Add unit test

Result:

Less memory copies

Sources/NIO/ByteBuffer-core.swift

@@ -563,6 +563,13 @@ public struct ByteBuffer {
             return false
         }
 
+        if self._readerIndex == self._writerIndex {
+            // If the whole buffer was consumed we can just reset the readerIndex and writerIndex to 0 and move on.
+            self._moveWriterIndex(to: 0)
+            self._moveReaderIndex(to: 0)
+            return true
+        }
+
         if isKnownUniquelyReferenced(&self._storage) {
             self._storage.bytes.advanced(by: Int(self._slice.lowerBound))
                 .copyMemory(from: self._storage.bytes.advanced(by: Int(self._slice.lowerBound + self._readerIndex)),

Tests/NIOTests/ByteBufferTest+XCTest.swift

@@ -113,6 +113,7 @@ extension ByteBufferTest {
                 ("testByteBufferFitsInACoupleOfEnums", testByteBufferFitsInACoupleOfEnums),
                 ("testLargeSliceBegin16MBIsOkayAndDoesNotCopy", testLargeSliceBegin16MBIsOkayAndDoesNotCopy),
                 ("testLargeSliceBeginMoreThan16MBIsOkay", testLargeSliceBeginMoreThan16MBIsOkay),
+                ("testDiscardReadBytesOnConsumedBuffer", testDiscardReadBytesOnConsumedBuffer),
            ]
    }
 }

Tests/NIOTests/ByteBufferTest.swift

@@ -1316,6 +1316,21 @@ class ByteBufferTest: XCTestCase {
         XCTAssertEqual(0xdd, slice.getInteger(at: slice.writerIndex - 1, as: UInt8.self))
     }
 
+    func testDiscardReadBytesOnConsumedBuffer() {
+        var buffer = self.allocator.buffer(capacity: 8)
+        buffer.write(integer: 0xaa, as: UInt8.self)
+        XCTAssertEqual(1, buffer.readableBytes)
+        XCTAssertEqual(0xaa, buffer.readInteger(as: UInt8.self))
+        XCTAssertEqual(0, buffer.readableBytes)
+
+        var buffer2 = buffer
+        XCTAssertTrue(buffer.discardReadBytes())
+        XCTAssertEqual(0, buffer.readerIndex)
+        XCTAssertEqual(0, buffer.writerIndex)
+        // As we fully consumed the buffer we should only have adjusted the indices but not triggered a copy as result of CoW sematics.
+        // So we should still be able to also read the old data.
+        XCTAssertEqual(0xaa, buffer.getInteger(at: 0, as: UInt8.self))
+    }
 }
 
 private enum AllocationExpectationState: Int {