Reproduce race-condition between close & prepare

Author: akash-akya

c_src/sqlite3.c

@@ -135367,6 +135367,12 @@ static int sqlite3LockAndPrepare(
   if( !sqlite3SafetyCheckOk(db)||zSql==0 ){
     return SQLITE_MISUSE_BKPT;
   }
+  // sleep only for insert stmt
+  if (strncmp(zSql, "insert into debug", 17) == 0) {
+    printf("sqlite3LockAndPrepare(): do not acquire the mutex, wait for sqlite3LeaveMutexAndCloseZombie() execute\n");
+    sqlite3_sleep(1000);
+    printf("sqlite3LockAndPrepare(): done waiting, proceeding with the execution\n");
+  }
   sqlite3_mutex_enter(db->mutex);
   sqlite3BtreeEnterAll(db);
   do{
@@ -172100,6 +172106,8 @@ SQLITE_PRIVATE void sqlite3LeaveMutexAndCloseZombie(sqlite3 *db){
     db->xAutovacDestr(db->pAutovacPagesArg);
   }
   sqlite3_mutex_leave(db->mutex);
+  printf("sqlite3LeaveMutexAndCloseZombie(): critical-section execution is done, the mutex is released. Wait for sqlite3LockAndPrepare() to proceeds before we free db\n");
+  sqlite3_sleep(1500);
   db->eOpenState = SQLITE_STATE_CLOSED;
   sqlite3_mutex_free(db->mutex);
   assert( sqlite3LookasideUsed(db,0)==0 );

test/exqlite/timeout_segfault_test.exs

@@ -1,6 +1,8 @@
 defmodule Exqlite.TimeoutSegfaultTest do
   use ExUnit.Case
 
+  alias Exqlite.Sqlite3
+
   setup do
     {:ok, path} = Temp.path()
     on_exit(fn -> File.rm(path) end)
@@ -36,4 +38,17 @@ defmodule Exqlite.TimeoutSegfaultTest do
     end)
     |> Stream.run()
   end
+
+  test "race condition", %{path: path} do
+    {:ok, conn} = Sqlite3.open(path)
+    :ok = Sqlite3.execute(conn, "create table debug(id integer)")
+
+    spawn_link(fn ->
+      Sqlite3.prepare(conn, "insert into debug(id) values (1)")
+    end)
+
+    # ensure `Sqlite3.prepare()` is executed before we attempt to close the connection
+    Process.sleep(100)
+    :ok = Sqlite3.close(conn)
+  end
 end