Remove incorrect assertion in MarkedCircularBuffer.popFirst (#1627)

Motivation:

`MarkedCircularBuffer.popFirst()` asserts that the backing buffer should
contain more than zero elements yet `popFirst()` allows `nil` to be
returned when there is no value to return.

Modifications:

- Move the assertion from `popFirst() -> Element?` to `removeFirst() -> Element`
- Test for `popFirst()`

Result:

- We can safely call `popFirst()` on an empty `MarkedCircularBuffer` in
  debug mode.

Sources/NIO/MarkedCircularBuffer.swift

@@ -41,12 +41,12 @@ public struct MarkedCircularBuffer<Element>: CustomStringConvertible {
     /// Removes the first element from the buffer.
     @inlinable
     public mutating func removeFirst() -> Element {
+        assert(self._buffer.count > 0)
         return self.popFirst()!
     }
 
     @inlinable
     public mutating func popFirst() -> Element? {
-        assert(self._buffer.count > 0)
         if let markedIndexOffset = self._markedIndexOffset {
             if markedIndexOffset > 0 {
                 self._markedIndexOffset = markedIndexOffset - 1

Tests/NIOTests/MarkedCircularBufferTests+XCTest.swift

@@ -36,6 +36,7 @@ extension MarkedCircularBufferTests {
                 ("testCount", testCount),
                 ("testSubscript", testSubscript),
                 ("testIsEmpty", testIsEmpty),
+                ("testPopFirst", testPopFirst),
            ]
    }
 }

Tests/NIOTests/MarkedCircularBufferTests.swift

@@ -123,11 +123,24 @@ class MarkedCircularBufferTests: XCTestCase {
             buf.append(i)
         }
         XCTAssertFalse(buf.isEmpty)
-        let _ = buf.removeFirst()
-        let _ = buf.removeFirst()
-        let _ = buf.removeFirst()
-        let _ = buf.removeFirst()
+        XCTAssertEqual(buf.removeFirst(), 1)
+        XCTAssertEqual(buf.removeFirst(), 2)
+        XCTAssertEqual(buf.removeFirst(), 3)
+        XCTAssertEqual(buf.removeFirst(), 4)
+        XCTAssertTrue(buf.isEmpty)
+    }
+
+    func testPopFirst() throws {
+        var buf = MarkedCircularBuffer<Int>(initialCapacity: 4)
+        for i in 1...4 {
+            buf.append(i)
+        }
+        XCTAssertFalse(buf.isEmpty)
+        XCTAssertEqual(buf.popFirst(), 1)
+        XCTAssertEqual(buf.popFirst(), 2)
+        XCTAssertEqual(buf.popFirst(), 3)
+        XCTAssertEqual(buf.popFirst(), 4)
+        XCTAssertNil(buf.popFirst())
         XCTAssertTrue(buf.isEmpty)
     }
-    
 }