tracked-key: remove the dual write and rename to tracked-hint...
tracked-key: remove the dual write and rename to tracked-hint
The dual-write approach was mostly useless. As explained in the previous version
of the help, the key had to be read twice before we could cache a value.
However this "read twice" limitation actually also apply to any usage of the
key. If some operation wants to rely of the "same value == same tracked set"
property it would need to read the value before, and after running that
operation (or at least, after, in all cases). So it cannot be sure the operation
it did is "valid" until checking the key after the operation. As a resultat such
operation can only be read-only or rollbackable.
This reduce the utility of the "same value == same tracked set" a lot.
So it seems simpler to drop the double write and to update the documentation to
highlight that this file does not garantee race-free operation. As a result the
"key" is demoted to a "hint".
Documentation is updated accordingly.
Differential Revision:
https://phab.mercurial-scm.org/D12201
