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Document generation and let it wrap-around as is it safe to do.
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@ -94,9 +94,21 @@ mod index_map {
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/// A map of indexes that are not available for queries, either because they are being deleted
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/// or because they are being closed.
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///
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/// If they are being deleted, the UUID point to `None`.
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/// If they are being deleted, the UUID points to `None`.
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unavailable: BTreeMap<Uuid, Option<ClosingIndex>>,
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/// A monotonically increasing generation number, used to differentiate between multiple successive index closing requests.
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///
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/// Because multiple readers could be waiting on an index to close, the following could theoretically happen:
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///
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/// 1. Multiple readers wait for the index closing to occur.
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/// 2. One of them "wins the race", takes the lock and then removes the index that finished closing from the map.
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/// 3. The index is reopened, but must be closed again (such as being resized again).
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/// 4. One reader that "lost the race" in (2) wakes up and tries to take the lock and remove the index from the map.
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///
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/// In that situation, the index may or may not have finished closing. The `generation` field allows to remember which
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/// closing request was made, so the reader that "lost the race" has the old generation and will need to wait again for the index
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/// to close.
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generation: usize,
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}
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@ -239,8 +251,16 @@ mod index_map {
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Ok(index)
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}
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/// Increases the current generation. See documentation for this field.
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///
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/// In the unlikely event that the 2^64 generations would have been exhausted, we simply wrap-around.
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///
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/// For this to cause an issue, one should be able to stop a reader in time after it got a `ReopenableIndex` and before it takes the lock
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/// to remove it from the unavailable map, and keep the reader in this frozen state for 2^64 closing of other indexes.
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///
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/// This seems overwhelmingly impossible to achieve in practice.
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fn next_generation(&mut self) -> usize {
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self.generation = self.generation.checked_add(1).unwrap();
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self.generation = self.generation.wrapping_add(1);
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self.generation
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}
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