util/ForeignFifoBuffer: add method GetAvailable()

src/util/DynamicFifoBuffer.hxx

@@ -54,6 +54,7 @@ public:
 
 	using ForeignFifoBuffer<T>::IsEmpty;
 	using ForeignFifoBuffer<T>::IsFull;
+	using ForeignFifoBuffer<T>::GetAvailable;
 	using ForeignFifoBuffer<T>::Read;
 	using ForeignFifoBuffer<T>::Consume;
 	using ForeignFifoBuffer<T>::Write;
@@ -73,7 +74,7 @@ public:
 			/* we already have enough space */
 			return;
 
-		const size_type in_use = Read().size;
+		const size_type in_use = GetAvailable();
 		const size_type required_capacity = in_use + n;
 		size_type new_capacity = GetCapacity();
 		do {

src/util/ForeignFifoBuffer.hxx

@@ -171,6 +171,10 @@ public:
 		tail += n;
 	}
 
+	constexpr size_type GetAvailable() const {
+		return tail - head;
+	}
+
 	/**
 	 * Return a buffer range which may be read.  The buffer pointer is
 	 * writable, to allow modifications while parsing.