Refactor exit handler mechanism of Entry

- Rename: whenComplete -> whenTerminate
- Execute the exit handler directly after the onExit hook of all slots

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

sentinel-core/src/main/java/com/alibaba/csp/sentinel/CtEntry.java

@@ -15,7 +15,6 @@
  */
 package com.alibaba.csp.sentinel;
 
-import java.util.Iterator;
 import java.util.LinkedList;
 
 import com.alibaba.csp.sentinel.context.Context;
@@ -42,7 +41,6 @@ class CtEntry extends Entry {
     protected Context context;
     protected LinkedList<BiConsumer<Context, Entry>> exitHandlers;
 
-
     CtEntry(ResourceWrapper resourceWrapper, ProcessorSlot<Object> chain, Context context) {
         super(resourceWrapper);
         this.chain = chain;
@@ -68,14 +66,33 @@ class CtEntry extends Entry {
         trueExit(count, args);
     }
 
+    /**
+     * Note: the exit handlers will be called AFTER onExit of slot chain.
+     */
+    private void callExitHandlersAndCleanUp(Context ctx) {
+        if (exitHandlers != null && !exitHandlers.isEmpty()) {
+            for (BiConsumer<Context, Entry> handler : this.exitHandlers) {
+                try {
+                    handler.accept(ctx, this);
+                } catch (Exception e) {
+                    RecordLog.warn("Error occurred when invoking entry exit handler, current entry: "
+                        + resourceWrapper.getName(), e);
+                }
+            }
+            exitHandlers = null;
+        }
+    }
+
     protected void exitForContext(Context context, int count, Object... args) throws ErrorEntryFreeException {
         if (context != null) {
             // Null context should exit without clean-up.
             if (context instanceof NullContext) {
                 return;
             }
+
             if (context.getCurEntry() != this) {
-                String curEntryNameInContext = context.getCurEntry() == null ? null : context.getCurEntry().getResourceWrapper().getName();
+                String curEntryNameInContext = context.getCurEntry() == null ? null
+                    : context.getCurEntry().getResourceWrapper().getName();
                 // Clean previous call stack.
                 CtEntry e = (CtEntry) context.getCurEntry();
                 while (e != null) {
@@ -83,12 +100,17 @@ class CtEntry extends Entry {
                     e = (CtEntry) e.parent;
                 }
                 String errorMessage = String.format("The order of entry exit can't be paired with the order of entry"
-                    + ", current entry in context: <%s>, but expected: <%s>", curEntryNameInContext, resourceWrapper.getName());
+                        + ", current entry in context: <%s>, but expected: <%s>", curEntryNameInContext,
+                    resourceWrapper.getName());
                 throw new ErrorEntryFreeException(errorMessage);
             } else {
+                // Go through the onExit hook of all slots.
                 if (chain != null) {
                     chain.exit(context, resourceWrapper, count, args);
                 }
+                // Go through the existing terminate handlers (associated to this invocation).
+                callExitHandlersAndCleanUp(context);
+
                 // Restore the call stack.
                 context.setCurEntry(parent);
                 if (parent != null) {
@@ -111,31 +133,17 @@ class CtEntry extends Entry {
     }
 
     @Override
-    public void whenComplete(BiConsumer<Context, Entry> consumer) {
+    public void whenTerminate(BiConsumer<Context, Entry> handler) {
         if (this.exitHandlers == null) {
             this.exitHandlers = new LinkedList<>();
         }
-        this.exitHandlers.add(consumer);
+        this.exitHandlers.add(handler);
     }
 
     @Override
     protected Entry trueExit(int count, Object... args) throws ErrorEntryFreeException {
         exitForContext(context, count, args);
 
-        if (this.exitHandlers != null) {
-            Iterator<BiConsumer<Context, Entry>> it = this.exitHandlers.iterator();
-            BiConsumer<Context, Entry> cur;
-            while (it.hasNext()) {
-                cur = it.next();
-                try {
-                    cur.accept(this.context, this);
-                } catch (Exception e) {
-                    RecordLog.warn("Error invoking exit handler", e);
-                }
-            }
-            this.exitHandlers = null;
-        }
-
         return parent;
     }
 

sentinel-core/src/main/java/com/alibaba/csp/sentinel/Entry.java

@@ -180,12 +180,13 @@ public abstract class Entry implements AutoCloseable {
     }
 
     /**
-     * Like `CompletableFuture` since JDK8 it guarantees specified consumer
-     * is invoked when this entry exited.
+     * Like {@code CompletableFuture} since JDK 8, it guarantees specified handler
+     * is invoked when this entry terminated (exited), no matter it's blocked or permitted.
      * Use it when you did some STATEFUL operations on entries.
      * 
-     * @param consumer
+     * @param handler handler function on the invocation terminates
+     * @since 1.8.0
      */
-    public abstract void whenComplete(BiConsumer<Context, Entry> consumer);
+    public abstract void whenTerminate(BiConsumer<Context, Entry> handler);
     
 }

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/block/degrade/circuitbreaker/AbstractCircuitBreaker.java

@@ -105,7 +105,7 @@ public abstract class AbstractCircuitBreaker implements CircuitBreaker {
         if (currentState.compareAndSet(State.OPEN, State.HALF_OPEN)) {
             notifyObservers(State.OPEN, State.HALF_OPEN, null);
             Entry entry = context.getCurEntry();
-            entry.whenComplete(new BiConsumer<Context, Entry>() {
+            entry.whenTerminate(new BiConsumer<Context, Entry>() {
                 @Override
                 public void accept(Context context, Entry entry) {
                     // Note: This works as a temporary workaround for https://github.com/alibaba/Sentinel/issues/1638

sentinel-core/src/test/java/com/alibaba/csp/sentinel/EntryTest.java

@@ -68,7 +68,7 @@ public class EntryTest {
         }
 
         @Override
-        public void whenComplete(BiConsumer<Context, Entry> consumer) {
+        public void whenTerminate(BiConsumer<Context, Entry> consumer) {
             // do nothing
         }
     }