[fix](catalog) remove backend in black list from candidate backends f…

…or external table (apache#38984) (apache#39451)

When select backends for external table's query,
skip the backends in black list

fe/fe-core/src/main/java/org/apache/doris/system/BeSelectionPolicy.java

@@ -17,6 +17,8 @@
 
 package org.apache.doris.system;
 
+import org.apache.doris.common.Config;
+import org.apache.doris.qe.SimpleScheduler;
 import org.apache.doris.resource.Tag;
 import org.apache.doris.thrift.TStorageMedium;
 
@@ -209,6 +211,10 @@ public List<Backend> getCandidateBackends(Collection<Backend> backends) {
         } else {
             candidates.addAll(filterBackends);
         }
+        // filter out backends in black list
+        if (!Config.disable_backend_black_list) {
+            candidates = candidates.stream().filter(b -> SimpleScheduler.isAvailable(b)).collect(Collectors.toList());
+        }
         Collections.shuffle(candidates);
         return candidates;
     }

fe/fe-core/src/test/java/org/apache/doris/system/SystemInfoServiceTest.java

@@ -21,6 +21,7 @@
 import org.apache.doris.catalog.Env;
 import org.apache.doris.catalog.ReplicaAllocation;
 import org.apache.doris.common.AnalysisException;
+import org.apache.doris.common.Config;
 import org.apache.doris.common.FeMetaVersion;
 import org.apache.doris.common.Pair;
 import org.apache.doris.meta.MetaContext;
@@ -133,6 +134,7 @@ public void testBackendHbResponseSerialization() throws IOException {
 
     @Test
     public void testSelectBackendIdsByPolicy() throws Exception {
+        Config.disable_backend_black_list = true;
         // 1. no backend
         BeSelectionPolicy policy = new BeSelectionPolicy.Builder().needLoadAvailable().build();
         Assert.assertEquals(0, infoService.selectBackendIdsByPolicy(policy, 1).size());