Merge #5158

5158: Indexer edition 2024 fix facet fst r=Kerollmops a=ManyTheFish

# Pull Request
Fix a regression in the new indexer; when several filterable attributes containing strings were set, all the field IDs were shifted, and the last one was overwriting the previous FST.

## What does this PR do?
- Add a test reproducing the bug
- fix the bug

Co-authored-by: ManyTheFish <many@meilisearch.com>

crates/meilisearch/tests/search/facet_search.rs

@@ -57,6 +57,116 @@ async fn simple_facet_search() {
     assert_eq!(response["facetHits"].as_array().unwrap().len(), 1);
 }
 
+#[actix_rt::test]
+async fn simple_facet_search_on_movies() {
+    let server = Server::new().await;
+    let index = server.index("test");
+
+    let documents = json!([
+      {
+        "id": 1,
+        "title": "Carol",
+        "genres": [
+          "Romance",
+          "Drama"
+        ],
+        "color": [
+          "red"
+        ],
+        "platforms": [
+          "MacOS",
+          "Linux",
+          "Windows"
+        ]
+      },
+      {
+        "id": 2,
+        "title": "Wonder Woman",
+        "genres": [
+          "Action",
+          "Adventure"
+        ],
+        "color": [
+          "green"
+        ],
+        "platforms": [
+          "MacOS"
+        ]
+      },
+      {
+        "id": 3,
+        "title": "Life of Pi",
+        "genres": [
+          "Adventure",
+          "Drama"
+        ],
+        "color": [
+          "blue"
+        ],
+        "platforms": [
+          "Windows"
+        ]
+      },
+      {
+        "id": 4,
+        "title": "Mad Max: Fury Road",
+        "genres": [
+          "Adventure",
+          "Science Fiction"
+        ],
+        "color": [
+          "red"
+        ],
+        "platforms": [
+          "MacOS",
+          "Linux"
+        ]
+      },
+      {
+        "id": 5,
+        "title": "Moana",
+        "genres": [
+          "Fantasy",
+          "Action"
+        ],
+        "color": [
+          "red"
+        ],
+        "platforms": [
+          "Windows"
+        ]
+      },
+      {
+        "id": 6,
+        "title": "Philadelphia",
+        "genres": [
+          "Drama"
+        ],
+        "color": [
+          "blue"
+        ],
+        "platforms": [
+          "MacOS",
+          "Linux",
+          "Windows"
+        ]
+      }
+    ]);
+    let (response, code) =
+        index.update_settings_filterable_attributes(json!(["genres", "color"])).await;
+    assert_eq!(202, code, "{:?}", response);
+    index.wait_task(response.uid()).await;
+
+    let (response, _code) = index.add_documents(documents, None).await;
+    index.wait_task(response.uid()).await;
+
+    let (response, code) =
+        index.facet_search(json!({"facetQuery": "", "facetName": "genres", "q": "" })).await;
+
+    assert_eq!(code, 200, "{}", response);
+    snapshot!(response["facetHits"], @r###"[{"value":"Action","count":2},{"value":"Adventure","count":3},{"value":"Drama","count":3},{"value":"Fantasy","count":1},{"value":"Romance","count":1},{"value":"Science Fiction","count":1}]"###);
+}
+
 #[actix_rt::test]
 async fn advanced_facet_search() {
     let server = Server::new().await;

crates/milli/src/update/new/facet_search_builder.rs

@@ -103,6 +103,8 @@ impl<'indexer> FacetSearchBuilder<'indexer> {
 
     #[tracing::instrument(level = "trace", skip_all, target = "indexing::facet_fst")]
     pub fn merge_and_write(self, index: &Index, wtxn: &mut RwTxn, rtxn: &RoTxn) -> Result<()> {
+        tracing::trace!("merge facet strings for facet search: {:?}", self.registered_facets);
+
         let reader = self.normalized_facet_string_docids_sorter.into_reader_cursors()?;
         let mut builder = grenad::MergerBuilder::new(MergeDeladdBtreesetString);
         builder.extend(reader);
@@ -118,12 +120,15 @@ impl<'indexer> FacetSearchBuilder<'indexer> {
                 BEU16StrCodec::bytes_decode(key).map_err(heed::Error::Encoding)?;
 
             if current_field_id != Some(field_id) {
-                if let Some(fst_merger_builder) = fst_merger_builder {
+                if let (Some(current_field_id), Some(fst_merger_builder)) =
+                    (current_field_id, fst_merger_builder)
+                {
                     let mmap = fst_merger_builder.build(&mut callback)?;
-                    index
+                    index.facet_id_string_fst.remap_data_type::<Bytes>().put(
-                        .facet_id_string_fst
+                        wtxn,
-                        .remap_data_type::<Bytes>()
+                        &current_field_id,
-                        .put(wtxn, &field_id, &mmap)?;
+                        &mmap,
+                    )?;
                 }
 
                 fst = index.facet_id_string_fst.get(rtxn, &field_id)?;