hg_mutation_store: make mutation length limit configurable and lower …

…the default

Summary: Very long mutation chains can cause issues with database overload, and in practice we rarely need to fetch that far back.  Make the limit configurable via justknob and reduce the default.

Reviewed By: liubov-dmitrieva

Differential Revision: D54119114

fbshipit-source-id: a47893d8c8ea7e5de88ebaf13151bdac6f2e7a02

eden/mononoke/mercurial/mutation/src/builder.rs

@@ -14,7 +14,7 @@ use sql_ext::SqlConnections;
 
 use crate::store::SqlHgMutationStore;
 
-const DEFAULT_MUTATION_CHAIN_LIMIT: usize = 500;
+const DEFAULT_MUTATION_CHAIN_LIMIT: usize = 20;
 
 #[allow(unused)]
 pub struct SqlHgMutationStoreBuilder {

eden/mononoke/repo_factory/src/lib.rs

@@ -1033,11 +1033,17 @@ impl RepoFactory {
         repo_config: &ArcRepoConfig,
         repo_identity: &ArcRepoIdentity,
     ) -> Result<ArcHgMutationStore> {
-        let hg_mutation_store = self
+        let mut builder = self
             .open_sql::<SqlHgMutationStoreBuilder>(repo_config)
             .await
-            .context(RepoFactoryError::HgMutationStore)?
-            .with_repo_id(repo_identity.id());
+            .context(RepoFactoryError::HgMutationStore)?;
+        if let Ok(mutation_limit) = justknobs::get_as::<usize>(
+            "scm/mononoke:mutation_chain_length_limit",
+            Some(repo_identity.name()),
+        ) {
+            builder = builder.with_mutation_limit(mutation_limit);
+        }
+        let hg_mutation_store = builder.with_repo_id(repo_identity.id());
 
         if let Some(cache_handler_factory) = self.cache_handler_factory("hg_mutation_store")? {
             Ok(Arc::new(CachedHgMutationStore::new(

eden/mononoke/tests/integration/just_knobs.json

@@ -31,6 +31,7 @@
     "scm/mononoke:derivation_worker_sleep_duration_ms": 250,
     "scm/mononoke:commit_graph_prefetch_step_limit": 4,
     "scm/mononoke:megarepo_override_squashing_limit": 0,
+    "scm/mononoke:mutation_chain_length_limit": 20,
     "scm/mononoke:repo_client_gettreepack_buffer_size": 1000,
     "scm/mononoke:repo_client_max_nodes_in_known_method": 100000,
     "scm/mononoke:repo_client_concurrent_blob_uploads": 1000,