Possession in Obstruction

[Shmuelg]

For a fielder to block a runner w/o being penalized by an obstruction call, he must have possession of the ball. Or, to quote one of the many rule books we have:

"Obstruction is the act of:
...
b. A fielder who impedes the progress of a runner or batter-runner who is legally running bases while:
1. not in possession of the ball, or
2. not in the act of fielding a batted ball, or ... "

The possession requirement is the same, AFAIK, in all the rulebooks I've seen (including softball, btw).

OK, sitch:

R1, R2. Ground ball to F6, who flips the ball over to F5. F5 is bobbling the ball right next to the base clearly in the way of R2, clearly not in complete control of the ball, when R2 comes in, and there is a sorta kinda scrum. F5 does get control, finally, and tags R2 just before he (finally) gets around F5 and steps on the base.

How would you rule? Is this an out? After all, F5 was in the act of fielding a ball (albeit thrown, not batted).
Or
Is this obstruction? After all, F5 did not have complete "possession" of the ball when he blocked R2.

Must the fielder have complete possession of the ball, or is it sufficient that he's in the act of fielding a ball to allow him to block a runner?