Merger issue (resolved) and enhancement (to do)

[csullivan]

There was a bug in the multithreaded merging step where mergers would not wait for all work to finish on a given layer of the binary tree merge graph. Now (6edc54b) there is a barrier that prevents them from moving on until all threads have finished.

TO DO: convert the multithreaded merger to a master-worker model where branches of the tree are passed as tasks to the workers.

[slipschutz]

It seems to get stuck now.

With 8 threads it does the following and than hangs seemingly indefinitely

hadd Target file: ./tmp//1_2.root
hadd Target file: ./tmp//1_1.root
hadd Target file: ./tmp//1_0.root
hadd Source file 1: ./tmp//0_4.root
hadd Source file 1: ./tmp//0_2.root
hadd Target file: ./tmp//1_3.root
hadd Source file 1: ./tmp//0_0.root
hadd Source file 1: ./tmp//0_6.root
hadd Source file 2: ./tmp//0_3.root
hadd Target path: ./tmp//1_1.root:/
hadd Source file 2: ./tmp//0_1.root
hadd Target path: ./tmp//1_0.root:/
hadd Source file 2: ./tmp//0_5.root
hadd Target path: ./tmp//1_2.root:/
hadd Source file 2: ./tmp//0_7.root
hadd Target path: ./tmp//1_3.root:/
hadd Target file: ./tmp//2_1.root
hadd Target file: ./tmp//2_0.root
hadd Source file 1: ./tmp//1_0.root
hadd Source file 1: ./tmp//1_2.root
hadd Source file 2: ./tmp//1_1.root
hadd Target path: ./tmp//2_0.root:/
hadd Source file 2: ./tmp//1_3.root
hadd Target path: ./tmp//2_1.root:/