keep TagRouter consistent with 2.6.x

dubbo-cluster/src/main/java/org/apache/dubbo/rpc/cluster/router/tag/TagRouter.java

@@ -34,6 +34,7 @@
 import org.apache.dubbo.rpc.cluster.router.tag.model.TagRouterRule;
 import org.apache.dubbo.rpc.cluster.router.tag.model.TagRuleParser;
 
+import java.util.ArrayList;
 import java.util.List;
 import java.util.function.Predicate;
 import java.util.stream.Collectors;
@@ -88,7 +89,7 @@ public <T> List<Invoker<T>> route(List<Invoker<T>> invokers, URL url, Invocation
         }
 
         if (tagRouterRule == null || !tagRouterRule.isValid() || !tagRouterRule.isEnabled()) {
-            return invokers;
+            return filterUsingStaticTag(invokers, url, invocation);
         }
 
         List<Invoker<T>> result = invokers;
@@ -112,7 +113,7 @@ public <T> List<Invoker<T>> route(List<Invoker<T>> invokers, URL url, Invocation
             }
             // If there's no tagged providers that can match the current tagged request. force.tag is set by default
             // to false, which means it will invoke any providers without a tag unless it's explicitly disallowed.
-            if (CollectionUtils.isNotEmpty(result) || isForceUse(invocation)) {
+            if (CollectionUtils.isNotEmpty(result) || isForceUseTag(invocation)) {
                 return result;
             }
             // FAILOVER: return all Providers without any tags.
@@ -141,6 +142,49 @@ public <T> List<Invoker<T>> route(List<Invoker<T>> invokers, URL url, Invocation
         }
     }
 
+    /**
+     * If there's no dynamic tag rule being set, use static tag in URL.
+     *
+     * A typical scenario is a Consumer using version 2.7.x calls Providers using version 2.6.x or lower,
+     * the Consumer should always respect the tag in provider URL regardless of whether a dynamic tag rule has been set to it or not.
+     *
+     * TODO, to guarantee consistent behavior of interoperability between 2.6- and 2.7+, this method should has the same logic with the TagRouter in 2.6.x.
+     *
+     * @param invokers
+     * @param url
+     * @param invocation
+     * @param <T>
+     * @return
+     */
+    private <T> List<Invoker<T>> filterUsingStaticTag(List<Invoker<T>> invokers, URL url, Invocation invocation) {
+        List<Invoker<T>> result = invokers;
+        // Dynamic param
+        String tag = StringUtils.isEmpty(invocation.getAttachment(TAG_KEY)) ? url.getParameter(TAG_KEY) :
+                invocation.getAttachment(TAG_KEY);
+        // Tag request
+        if (!StringUtils.isEmpty(tag)) {
+            List<Invoker<T>> tagedInvokers = new ArrayList<>();
+            // Select tag invokers first
+            for (Invoker<T> invoker : invokers) {
+                if (tag.equals(invoker.getUrl().getParameter(Constants.TAG_KEY))) {
+                    tagedInvokers.add(invoker);
+                }
+            }
+            result = tagedInvokers;
+        }
+        // If Constants.REQUEST_TAG_KEY unspecified or no invoker be selected, downgrade to normal invokers
+        if (result.isEmpty() && !isForceUseTag(invocation)) {
+            List<Invoker<T>> untagedInvokers = new ArrayList<>();
+            for (Invoker<T> invoker : invokers) {
+                if (StringUtils.isEmpty(invoker.getUrl().getParameter(Constants.TAG_KEY))) {
+                    untagedInvokers.add(invoker);
+                }
+            }
+            result = untagedInvokers;
+        }
+        return result;
+    }
+
     @Override
     public int getPriority() {
         return DEFAULT_PRIORITY;
@@ -157,7 +201,7 @@ public boolean isForce() {
         return tagRouterRule != null && tagRouterRule.isForce();
     }
 
-    private boolean isForceUse(Invocation invocation) {
+    private boolean isForceUseTag(Invocation invocation) {
         return Boolean.valueOf(invocation.getAttachment(FORCE_USE_TAG, url.getParameter(FORCE_USE_TAG, "false")));
     }