Who is assisting the goal, if a player  passes the ball to a teammate , who is missing the ball on purpose and is leaving it for a third player, who scores? For example, who is assisting the fourth goal of Antoine Griezmann from the UEFA Euro 2016 quarter-finals (France 5 - 2 Iceland)?
Before going deep in details I would like to give a demonstrative counter-example from the goal scored in UEFA Euro 2012 qualifiers match Azerbaijan 1 - 0 Turkey, where 2 players jump over the ball and leave the shot for the third player. This specific type of corner kick set play is very famous in Azerbaijan and is called "5 qəpik" (5 cents).
The corner kick taker is #4 Mahir Shukurov, the 1st player who jumps over the ball is #7 Vagif Javadov, and the 2nd player jumping over the ball is #17 Vüqar Nadirov. As you can see from the stats in official match report from UEFA, the corner kick taker #4 Mahir Shukurov is awarded the assist, not one of the jumpers. (Which makes sense to me, because simply you can't distribute the assists to all of the players jumping over the ball: imagine 9 players doing that).
Defining the assistant of the goal is up to specific tournament and organisation that holds this tournament. Mostly, only if number of assists matters, the assist stats are kept officially by the organisers. (e.g FIFA World Cup Golden Boot award is awarded to the top goalscorer, and assists play a role when two or more players are equal on goals scored).
Recording assists is nowhere mentioned in the Laws of the Game, but there are some guidelines by the FIFA Technical Study Group. Last time those guidelines were explicitly stated in the official report of the 1990 FIFA World Cup Italy:
1.The player who was the last to pass to the eventual goalscorer receives an assist point.
2.In the event that the penultimate pass may have decisively influenced the play leading up to the goal, the player who executed this pass may also be allocated an assist point.
3.The player whose shot led to the rebound that enabled the goalscorer to successfully complete the action is also granted an assist point.
4.Where goals resulting from penalties are concerned, the player who is fouled in the area receives an assist point (unless, that is, the player who is fouled subsequently executes the penalty himself.)
5.For goals which are the result of solo efforts (dribbling runs, etc .) no assist points are given.
6.No assist point is allocated in those cases where a goal is scored as the result of a mistake made by the opposing team.
These rules are old and in modern competitions even the rules from 2nd, 3rd and 4th paragraphs mentioned above tend not to be applied. In the official report of the 2014 FIFA World Cup Brasil it's only dryly stated that:
The FIFA Technical Study Group, which was represented at each match, decides whether an assist is to be counted.
As per ESPN (still not an official source),
FW Olivier Giroud, 9 -- Having done everything but score against the Irish, Giroud was rewarded for his perseverance here. The Arsenal target man opened the scoring with a clinical finish and scored his second with an easy header. It was also the 29-year-old who assisted Griezmann for his goal.
The player who passes the ball is supposed to get the assist not the one that lets it go.
In that match it was unsure whether Oliver Giroud got a touch or not and hence was given the benefit of the doubt and given the assist.