Allow zero length strings

flow/Tracing.actor.cpp

@@ -209,13 +209,13 @@ private:
 
 	// Writes the given string to the request as a sequence of bytes. Inserts a
 	// format byte at the beginning of the string according to the its length,
-	// as specified by the msgpack specification. String length must be greater
-	// than 0.
+	// as specified by the msgpack specification.
 	inline void serialize_string(const std::string& str, TraceRequest& request) {
 		int size = str.size();
-		ASSERT(size > 0);
 
 		if (size <= 31) {
+			// A size 0 string is ok. We still need to write a byte
+			// identifiying the item as a string, but can set the size to 0.
 			request.write_byte(static_cast<uint8_t>(size) | 0b10100000);
 		} else if (size <= 255) {
 			request.write_byte(0xd9);