feat: Document the class path / classloader issue with the shuffle ma…

…nager (#256)

README.md

@@ -127,6 +127,14 @@ Comet shuffle feature is disabled by default. To enable it, please add related c
 Above configs enable Comet native shuffle which only supports hash partiting and single partition.
 Comet native shuffle doesn't support complext types yet.
 
+Comet doesn't have official release yet so currently the only way to test it is to build jar and include it in your Spark application. Depending on your deployment mode you may also need to set the driver & executor class path(s) to explicitly contain Comet otherwise Spark may use a different class-loader for the Comet components than its internal components which will then fail at runtime. For example:
+
+```
+--driver-class-path spark/target/comet-spark-spark3.4_2.12-0.1.0-SNAPSHOT.jar
+```
+
+Some cluster managers may require additional configuration, see https://spark.apache.org/docs/latest/cluster-overview.html
+
 To enable columnar shuffle which supports all partitioning and basic complex types, one more config is required:
 ```
 --conf spark.comet.columnar.shuffle.enabled=true

spark/src/main/java/org/apache/spark/shuffle/sort/CometShuffleExternalSorter.java

@@ -431,7 +431,14 @@ class SpillSorter extends SpillWriter {
       // As we cannot access the address of the internal array in the sorter, so we need to
       // allocate the array manually and expand the pointer array in the sorter.
       // We don't want in-memory sorter to allocate memory but the initial size cannot be zero.
-      this.inMemSorter = new ShuffleInMemorySorter(allocator, 1, true);
+      try {
+        this.inMemSorter = new ShuffleInMemorySorter(allocator, 1, true);
+      } catch (java.lang.IllegalAccessError e) {
+        throw new java.lang.RuntimeException(
+            "Error loading in-memory sorter check class path -- see "
+                + "https://github.com/apache/arrow-datafusion-comet?tab=readme-ov-file#enable-comet-shuffle",
+            e);
+      }
       sorterArray = allocator.allocateArray(initialSize);
       this.inMemSorter.expandPointerArray(sorterArray);