[jvm-packages] Add XGBOOST_RABIT_TRACKER_IP_FOR_TEST to set rabit tracker IP. (#6869)

* Add `XGBOOST_RABIT_TRACKER_IP_FOR_TEST` to set rabit tracker IP

* change spark and rabit tracker IP to 127.0.0.1on GitHub Action.

Co-authored-by: fis <jm.yuan@outlook.com>

.github/workflows/main.yml

@@ -210,6 +210,8 @@ jobs:
       if: matrix.os == 'ubuntu-latest'  # Distributed training doesn't work on Windows
       env:
         RABIT_MOCK: ON
+        SPARK_LOCAL_IP: 127.0.0.1
+        XGBOOST_RABIT_TRACKER_IP_FOR_TEST: 127.0.0.1
 
   lint:
     runs-on: ubuntu-latest

doc/jvm/xgboost4j_spark_tutorial.rst

@@ -586,3 +586,11 @@ An equivalent way is to pass in parameters in XGBoostClassifier's constructor:
       setLabelCol("classIndex")
 
 If the training failed during these 100 rounds, the next run of training would start by reading the latest checkpoint file in ``/checkpoints_path`` and start from the iteration when the checkpoint was built until to next failure or the specified 100 rounds.
+
+
+Developer Notes
+===============
+
+There's an environment variable called ``XGBOOST_RABIT_TRACKER_IP_FOR_TEST`` used to
+specify the tracker IP, which can be used in combination with ``SPARK_LOCAL_IP``.  It's
+only used for testing and is not maintained as a part of the interface.

jvm-packages/xgboost4j/src/main/java/ml/dmlc/xgboost4j/java/TrackerProperties.java

@@ -51,6 +51,8 @@ public class TrackerProperties {
   }
 
   public String getHostIp(){
-    return this.properties.getProperty(HOST_IP);
+    // mostly for testing
+    String hostIp = System.getenv("XGBOOST_RABIT_TRACKER_IP_FOR_TEST");
+    return hostIp != null ? hostIp : this.properties.getProperty(HOST_IP);
   }
 }