simplify api, add OneWayFlag

src/main/java/org/cobbzilla/util/daemon/ZillaRuntime.java

@@ -170,12 +170,12 @@ public class ZillaRuntime {
         throw e;
     }
 
-    public static String shortErrorString(Exception e) { return e.getClass().getName()+": "+e.getMessage(); }
+    public static String shortError(Exception e) { return e.getClass().getName()+": "+e.getMessage(); }
 
     public static String errorString(Exception e) { return errorString(e, 1000); }
 
     public static String errorString(Exception e, int maxlen) {
-        return truncate(shortErrorString(e)+"\n"+ getStackTrace(e), maxlen);
+        return truncate(shortError(e)+"\n"+ getStackTrace(e), maxlen);
     }
 
     public static boolean empty(String s) { return s == null || s.length() == 0; }

src/main/java/org/cobbzilla/util/system/OneWayFlag.java

@@ -0,0 +1,45 @@
+package org.cobbzilla.util.system;
+
+import lombok.extern.slf4j.Slf4j;
+
+import java.util.concurrent.Callable;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+import static org.cobbzilla.util.daemon.ZillaRuntime.shortError;
+
+/**
+ * Wraps an Atomic Boolean that can only go from false to true.
+ * Useful when you have a state where checking the current status is expensive,
+ * but once some condition evaluates to true, it is true forever thereafter.
+ */
+@Slf4j
+public class OneWayFlag extends AtomicBoolean {
+
+    private final String name;
+    private final Callable<Boolean> check;
+
+    public OneWayFlag(String name, Callable<Boolean> check) {
+        this.name = name;
+        this.check = check;
+        try {
+            set(check.call());
+        } catch (Exception e) {
+            log.warn("OneWayFlag("+name+") constructor check failed (evaluates to false): "+shortError(e));
+            set(false);
+        }
+    }
+
+    public boolean check () {
+        if (get()) return true;
+        final Boolean ok;
+        try {
+            ok = check.call();
+        } catch (Exception e) {
+            log.warn("OneWayFlag("+name+") check failed (evaluates to false): "+shortError(e));
+            return false;
+        }
+        if (ok) set(true);
+        return ok;
+    }
+
+}