Fixed Spigot agent attach

- Iris restores the JVM SIGQUIT handler when the server console replaced it
- The Java agent attaches on Spigot servers with a terminal console
- Tests cover restoring the handler after a console resets it

adapters/bukkit/plugin/src/test/java/art/arcane/iris/platform/agent/AttachSignalTest.java

@@ -0,0 +1,57 @@
+package art.arcane.iris.platform.agent;
+
+import org.junit.Test;
+
+import java.lang.foreign.Arena;
+import java.lang.foreign.FunctionDescriptor;
+import java.lang.foreign.Linker;
+import java.lang.foreign.MemorySegment;
+import java.lang.foreign.ValueLayout;
+import java.lang.invoke.MethodHandle;
+import java.util.Locale;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotEquals;
+import static org.junit.Assert.assertTrue;
+import static org.junit.Assume.assumeFalse;
+
+public class AttachSignalTest {
+    private static final int SIGQUIT = 3;
+    private static final int SIGTERM = 15;
+    private static final long ACTION_BYTES = 256;
+    private static final MethodHandle SIGACTION = Linker.nativeLinker().downcallHandle(
+            Linker.nativeLinker().defaultLookup().find("sigaction").orElseThrow(),
+            FunctionDescriptor.of(ValueLayout.JAVA_INT, ValueLayout.JAVA_INT, ValueLayout.ADDRESS, ValueLayout.ADDRESS));
+
+    @Test
+    public void aConsoleThatResetsQuitGetsTheJvmHandlerBack() throws Throwable {
+        assumeFalse(System.getProperty("os.name").toLowerCase(Locale.ROOT).contains("win"));
+        long jvmHandler = handler(SIGTERM);
+        assertNotEquals(0L, jvmHandler);
+        try (Arena arena = Arena.ofConfined()) {
+            MemorySegment original = arena.allocate(ACTION_BYTES);
+            assertEquals(0, (int) SIGACTION.invokeExact(SIGQUIT, MemorySegment.NULL, original));
+            try {
+                MemorySegment reset = arena.allocate(ACTION_BYTES);
+                assertEquals(0, (int) SIGACTION.invokeExact(SIGQUIT, reset, MemorySegment.NULL));
+                assertEquals(0L, handler(SIGQUIT));
+
+                assertTrue(AttachSignal.restoreJvmHandler());
+
+                assertEquals(jvmHandler, handler(SIGQUIT));
+                assertFalse(AttachSignal.restoreJvmHandler());
+            } finally {
+                assertEquals(0, (int) SIGACTION.invokeExact(SIGQUIT, original, MemorySegment.NULL));
+            }
+        }
+    }
+
+    private static long handler(int signal) throws Throwable {
+        try (Arena arena = Arena.ofConfined()) {
+            MemorySegment action = arena.allocate(ACTION_BYTES);
+            assertEquals(0, (int) SIGACTION.invokeExact(signal, MemorySegment.NULL, action));
+            return action.get(ValueLayout.JAVA_LONG, 0);
+        }
+    }
+}