Fix a flaky concurrent test with correct locking
Fix a flaky concurrent test with correct locking

The recent refactoring to clone the bare registry left in an accidental path
where index checkouts could clobber one another. This commit updates the logic
with proper locking and attempt ordering, attempting a full retry of the open
operation after the index locked.

nipunn1313/cargo,rustyhorde/cargo,tsurai/cargo,rust-lang/cargo,rust-lang/cargo,durka/cargo,tshepang/cargo,achanda/cargo,tshepang/cargo,tsurai/cargo,durka/cargo,eminence/cargo,achanda/cargo,nipunn1313/cargo,vojtechkral/cargo,eminence/cargo,eminence/cargo,durka/cargo,achanda/cargo,eminence/cargo,achanda/cargo,tsurai/cargo,nipunn1313/cargo,vojtechkral/cargo,tsurai/cargo,nipunn1313/cargo,tshepang/cargo,rust-lang/cargo,durka/cargo,vojtechkral/cargo,rust-lang/cargo,rustyhorde/cargo,tsurai/cargo,tshepang/cargo,vojtechkral/cargo,rustyhorde/cargo,rustyhorde/cargo