How does the mesh in an inguinal hernia repair solve the problem if the canal has two sides? is it safe to 'barrier' up the canal?

Canal is an opening,: not 2 sides, a passage way. Hernia literally means opening. If something additional to what normally passes through opening, then this situation is called a hernia. Hernia "repair" simply means reducing size/closing of the opening with the goal of blocking passage of additional tissue (fat, organ, etc.). Mesh is sometimes used to reinforce the tissue, if judged too weak, around the opening.

Hernia: What you are doing is closing off the ability of the bowel to herniate into the sac

Can be confusing: The inguinal canal is temporary during infant development. The testes that develop next to the kidneys migrate through this tubular canal & drop into the scrotum before the 32nd fetal week.I closes but retains a regional location label.Hernia means something has pushed though a weakened part of the layers that contain the organs. Mesh reinforces weak layers as you would patch a hole in the wall.