Avoid Unidoc build when Hadoop 2.6 is explicitly set.

Author: HyukjinKwon

dev/run-tests.py

@@ -365,8 +365,16 @@ def build_spark_assembly_sbt(hadoop_version):
     print("[info] Building Spark assembly (w/Hive 1.2.1) using SBT with these arguments: ",
           " ".join(profiles_and_goals))
     exec_sbt(profiles_and_goals)
-    # Make sure that Java and Scala API documentation can be generated
-    build_spark_unidoc_sbt(hadoop_version)
+
+    # Note that we skip Unidoc build only if Hadoop 2.6 is explicitly set in this SBT build.
+    # Due to a different dependency resolution in SBT & Unidoc by an unknown reason, the
+    # documentation build fails on a specific machine & environment in Jenkins but it was unable
+    # to reproduce. Please see SPARK-20343. This is a band-aid fix that should be removed in
+    # the future.
+    is_hadoop_version_2_6 = os.environ.get("AMPLAB_JENKINS_BUILD_PROFILE") == "hadoop2.6"
+    if not is_hadoop_version_2_6:
+        # Make sure that Java and Scala API documentation can be generated
+        build_spark_unidoc_sbt(hadoop_version)
 
 
 def build_apache_spark(build_tool, hadoop_version):