Move index swap error handling from meilisearch-http to index-scheduler

And make index_not_found error asynchronous, since we can't know
whether the index will exist by the time the index swap task is
processed.

Improve the index-swap test to verify that future tasks are not swapped
and to test the new error messages that were introduced.

index-scheduler/src/utils.rs

@@ -1,5 +1,6 @@
 //! Utility functions on the DBs. Mainly getter and setters.
 
+use std::collections::{BTreeSet, HashSet};
 use std::ops::Bound;
 
 use meilisearch_types::heed::types::{DecodeIgnore, OwnedType};
@@ -296,6 +297,33 @@ pub(crate) fn filter_out_references_to_newer_tasks(task: &mut Task) {
     }
 }
 
+pub(crate) fn check_index_swap_validity(task: &Task) -> Result<()> {
+    let swaps =
+        if let KindWithContent::IndexSwap { swaps } = &task.kind { swaps } else { return Ok(()) };
+    let mut all_indexes = HashSet::new();
+    let mut duplicate_indexes = BTreeSet::new();
+    for IndexSwap { indexes: (lhs, rhs) } in swaps {
+        for name in [lhs, rhs] {
+            let is_new = all_indexes.insert(name);
+            if !is_new {
+                duplicate_indexes.insert(name);
+            }
+        }
+    }
+    if !duplicate_indexes.is_empty() {
+        if duplicate_indexes.len() == 1 {
+            return Err(Error::SwapDuplicateIndexFound(
+                duplicate_indexes.into_iter().next().unwrap().clone(),
+            ));
+        } else {
+            return Err(Error::SwapDuplicateIndexesFound(
+                duplicate_indexes.into_iter().cloned().collect(),
+            ));
+        }
+    }
+    Ok(())
+}
+
 #[cfg(test)]
 impl IndexScheduler {
     /// Asserts that the index scheduler's content is internally consistent.