It's not strictly necessary to have a list of all xids at all. The normal
"shared read lock" is just "take the write lock, increment the readers
counter, unlock" Anyone who wants to write has to wait (using, eg, a condition
variable) until the readers count goes to 0.
This gets the right semantics but without the debugging info of a list of
lockers. Other than debugging the only advantage I see to having the list of
lockers is for deadlock detection. Is that absolutely mandatory?
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