Java: Add 2 double-checked-locking queries.

java/ql/test/query-tests/DoubleCheckedLocking/A.java

@@ -0,0 +1,75 @@
+public class A {
+  public class B {
+    public int x = 2;
+    public void setX(int x) {
+      this.x = x;
+    }
+  }
+
+  private String s;
+  public String getString() {
+    if (s == null) {
+      synchronized(this) {
+        if (s == null) {
+          s = "string"; // OK, immutable
+        }
+      }
+    }
+    return s;
+  }
+
+  private B b1;
+  public B getter1() {
+    B x = b1;
+    if (x == null) {
+      synchronized(this) {
+        if ((x = b1) == null) {
+          b1 = new B(); // BAD, not volatile
+          x = b1;
+        }
+      }
+    }
+    return x;
+  }
+
+  private volatile B b2;
+  public B getter2() {
+    B x = b2;
+    if (x == null) {
+      synchronized(this) {
+        if ((x = b2) == null) {
+          b2 = new B(); // OK
+          x = b2;
+          System.out.println("OK");
+        }
+      }
+    }
+    return x;
+  }
+
+  private volatile B b3;
+  public B getter3() {
+    if (b3 == null) {
+      synchronized(this) {
+        if (b3 == null) {
+          b3 = new B();
+          b3.x = 7; // BAD, post update init
+        }
+      }
+    }
+    return b3;
+  }
+
+  private volatile B b4;
+  public B getter4() {
+    if (b4 == null) {
+      synchronized(this) {
+        if (b4 == null) {
+          b4 = new B();
+          b4.setX(7); // BAD, post update init
+        }
+      }
+    }
+    return b4;
+  }
+}

java/ql/test/query-tests/DoubleCheckedLocking/DoubleCheckedLocking.expected

@@ -0,0 +1 @@
+| A.java:25:7:30:7 | stmt | Double-checked locking on the non-volatile field $@ is not thread-safe. | A.java:21:13:21:14 | b1 | b1 |

java/ql/test/query-tests/DoubleCheckedLocking/DoubleCheckedLocking.qlref

@@ -0,0 +1 @@
+Likely Bugs/Concurrency/DoubleCheckedLocking.ql

java/ql/test/query-tests/DoubleCheckedLocking/DoubleCheckedLockingWithInitRace.expected

@@ -0,0 +1,2 @@
+| A.java:55:11:55:22 | ...=... | Potential race condition. This assignment to $@ is visible to other threads before the subsequent statements are executed. | A.java:50:22:50:23 | b3 | b3 |
+| A.java:68:11:68:22 | ...=... | Potential race condition. This assignment to $@ is visible to other threads before the subsequent statements are executed. | A.java:63:22:63:23 | b4 | b4 |

java/ql/test/query-tests/DoubleCheckedLocking/DoubleCheckedLockingWithInitRace.qlref

@@ -0,0 +1 @@
+Likely Bugs/Concurrency/DoubleCheckedLockingWithInitRace.ql