Reciprocity failure, a well-known characteristic of film-based photography, is not a practical problem with digital cameras. The responses of sensors used in digital cameras remain almost perfectly linear throughout their usable range - ie up to the maximum charge-handling capacity of each pixel.
The phenomenon known as dark current noise has little or no bearing upon reciprocity failure. Dark current noise, which is essentially generated by stray electrons in areas of a sensor not exposed to light, accumulates over time in a linear fashion provided the temperature of the sensor remains unchanged. Most digital cameras are therefore designed to subtract this dark contribution to the signal by removing the output obtained from a blank frame created using the same exposure as the image but with the lens shielded from light. Good cameras have sophisticated systems for achieving this adjustment.