refs #4964, add some comments for changing a descriptor

git-svn-id: file:///svn/toku/tokudb@45060 c7de825b-a66e-492c-adef-691d508d4ae1

ft/ft-ops.c

@@ -3071,6 +3071,10 @@ verify_builtin_comparisons_consistent(FT_HANDLE t, u_int32_t flags) {
     return 0;
 }
 
+//
+// See comments in toku_db_change_descriptor to understand invariants 
+// in the system when this function is called
+//
 int
 toku_ft_change_descriptor(
     FT_HANDLE ft_h,

src/ydb_db.c

@@ -466,6 +466,12 @@ int toku_db_pre_acquire_fileops_lock(DB *db, DB_TXN *txn) {
 // This function is used both to set an initial descriptor of a DB and to
 // change a descriptor. (only way to set a descriptor of a DB)
 //
+// Requires:
+//  - The caller must not call put_multiple, del_multiple, or update_multiple concurrently
+//  - The caller must not have a hot index running concurrently on db
+//  - If the caller has passed DB_UPDATE_CMP_DESCRIPTOR as a flag, then he is calling this function
+//     ONLY immediately after creating the dictionary and before doing any actual work on the dictionary.
+//
 static int 
 toku_db_change_descriptor(DB *db, DB_TXN* txn, const DBT* descriptor, u_int32_t flags) {
     HANDLE_PANICKED_DB(db);