Daemonize query cancel scheduler thread

The implementation of `Statement#setQueryTimeout()` added in #247 uses a
background thread to perform the query cancellation on timeout. Because
that thread is not marked as "daemon" one, it can prevent JVM shutdown.

This change makes the scheduler thread "daemon" and additionally exposes
`DuckDBDriver#shutdownQueryCancelScheduler()` method that allows to shut
down the scheduler manually.

Testing: manual test added that checks that queries with timeout set
can be executed (with timeout ineffective) after the scheduler is shut down.

Fixes: #504

Author: staticlibs

src/main/java/org/duckdb/DuckDBDriver.java

@@ -54,7 +54,14 @@ public class DuckDBDriver implements java.sql.Driver {
     static {
         try {
             DriverManager.registerDriver(new DuckDBDriver());
-            ThreadFactory tf = r -> new Thread(r, "duckdb-query-cancel-scheduler-thread");
+            ThreadFactory tf = new ThreadFactory() {
+                @Override
+                public Thread newThread(Runnable r) {
+                    Thread th = new Thread(r, "duckdb-query-cancel-scheduler-thread");
+                    th.setDaemon(true);
+                    return th;
+                }
+            };
             scheduler = new ScheduledThreadPoolExecutor(1, tf);
             scheduler.setRemoveOnCancelPolicy(true);
         } catch (SQLException e) {
@@ -242,6 +249,14 @@ public static boolean releaseDB(String url) throws SQLException {
         }
     }
 
+    public static boolean shutdownQueryCancelScheduler() {
+        if (scheduler.isShutdown()) {
+            return false;
+        }
+        scheduler.shutdown();
+        return true;
+    }
+
     private static DriverPropertyInfo createDriverPropInfo(String name, String value, String description) {
         DriverPropertyInfo dpi = new DriverPropertyInfo(name, value);
         dpi.description = description;

src/main/java/org/duckdb/DuckDBPreparedStatement.java

@@ -38,6 +38,7 @@
 import java.util.ArrayList;
 import java.util.Calendar;
 import java.util.List;
+import java.util.concurrent.RejectedExecutionException;
 import java.util.concurrent.ScheduledFuture;
 import java.util.concurrent.locks.Lock;
 import java.util.concurrent.locks.ReentrantLock;
@@ -187,8 +188,14 @@ public boolean execute() throws SQLException {
 
             if (queryTimeoutSeconds > 0) {
                 cleanupCancelQueryTask();
-                cancelQueryFuture =
-                    DuckDBDriver.scheduler.schedule(new CancelQueryTask(), queryTimeoutSeconds, SECONDS);
+                try {
+                    if (!DuckDBDriver.scheduler.isShutdown()) {
+                        cancelQueryFuture =
+                            DuckDBDriver.scheduler.schedule(new CancelQueryTask(), queryTimeoutSeconds, SECONDS);
+                    }
+                } catch (RejectedExecutionException e) {
+                    // no-op, scheduler was shut down concurrently
+                }
             }
 
             resultRef = DuckDBNative.duckdb_jdbc_execute(stmtRef, params);

src/test/java/org/duckdb/TestClosure.java

@@ -322,4 +322,22 @@ public static void test_stmt_query_timeout() throws Exception {
             assertEquals(DuckDBDriver.scheduler.getQueue().size(), 0);
         }
     }
+
+    public static void manual_test_set_query_timeout_wo_scheduler() throws Exception {
+        assertTrue(DuckDBDriver.shutdownQueryCancelScheduler());
+        assertFalse(DuckDBDriver.shutdownQueryCancelScheduler());
+        try (Connection conn = DriverManager.getConnection(JDBC_URL); Statement stmt = conn.createStatement()) {
+            stmt.setQueryTimeout(1);
+            stmt.execute("CREATE TABLE test_fib1(i bigint, p double, f double)");
+            stmt.execute("INSERT INTO test_fib1 values(1, 0, 1)");
+            long start = System.currentTimeMillis();
+            stmt.executeQuery(
+                "WITH RECURSIVE cte AS ("
+                +
+                "SELECT * from test_fib1 UNION ALL SELECT cte.i + 1, cte.f, cte.p + cte.f from cte WHERE cte.i < 50000) "
+                + "SELECT avg(f) FROM cte");
+            long elapsed = System.currentTimeMillis() - start;
+            assertTrue(elapsed > 1500);
+        }
+    }
 }