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The function intersection(r1,r2) in statement_restrictions.cc is used when several WHERE restrictions were applied to the same column. For example, for "WHERE b<1 AND b<2" the intersection of the two ranges is calculated to be b<1. As noted in issue #18690, Scylla is inconsistent in where it allows or doesn't allow these intersecting restrictions. But where they are allowed they must be implemented correctly. And it turns out the function intersection() had a bug that caused it to sometimes enter an infinite loop - when the intent was only to call itself once with swapped parameters. This patch includes a test reproducing this bug, and a fix for the bug. The test hangs before the fix, and passes after the fix. While at it, I carefully reviewed the entire code used to implement the intersection() function to try to make sure that the bug we found was the only one. I also added a few more comments where I thought they were needed to understand complicated logic of the code. The bug, the fix and the test were originally discovered by Michał Chojnowski. Fixes #18688 Refs #18690 Signed-off-by: Nadav Har'El <nyh@scylladb.com> Closes scylladb/scylladb#18694