* The database needs a trigger to get rid of self-references to

prevent a foreign key constraint violation on the Refs table when
  deleting a path.

src/libstore/local-store.cc

@@ -109,7 +109,11 @@ struct SQLiteStmtUse
     }
     ~SQLiteStmtUse()
     {
+        try {
             stmt.reset();
+        } catch (...) {
+            ignoreException();
+        }
     }
 };
 
@@ -799,6 +803,8 @@ void LocalStore::invalidatePath(const Path & path)
 {
     debug(format("invalidating path `%1%'") % path);
 
+    SQLiteTxn txn(db);
+    
     SQLiteStmtUse use(stmtInvalidatePath);
 
     stmtInvalidatePath.bind(path);
@@ -808,6 +814,8 @@ void LocalStore::invalidatePath(const Path & path)
 
     /* Note that the foreign key constraints on the Refs table take
        care of deleting the references entries for `path'. */
+
+    txn.commit();
 }
 
 

src/libstore/schema.sql

@@ -17,6 +17,16 @@ create table if not exists Refs (
 create index if not exists IndexReferrer on Refs(referrer);
 create index if not exists IndexReference on Refs(reference);
 
+-- Paths can refer to themselves, causing a tuple (N, N) in the Refs
+-- table.  This causes a deletion of the corresponding row in
+-- ValidPaths to cause a foreign key constraint violation (due to `on
+-- delete restrict' on the `reference' column).  Therefore, explicitly
+-- get rid of self-references.
+create trigger DeleteSelfRefs before delete on ValidPaths
+  begin
+    delete from Refs where referrer = old.id and reference = old.id;
+  end;
+
 create table if not exists DerivationOutputs (
     drv  integer not null,
     id   text not null, -- symbolic output id, usually "out"