Method clearReserve

fdbserver/DiskQueue.actor.cpp:63–67

		reserved = 0;
	}
	void clearReserve(int size) {
		str = Standalone<StringRef>();
		reserved = size;
		str.contents() = StringRef(new (str.arena()) uint8_t[size], 0);
	}
	void append(StringRef x) { memcpy(append(x.size()), x.begin(), x.size()); }
	void* append(int bytes) {
		ASSERT(str.size() + bytes <= reserved);