fix: describe CometInMemoryTableScan by the scan it replaces in EXPLAIN

spark/src/main/scala/org/apache/spark/sql/comet/CometInMemoryTableScanExec.scala

@@ -73,6 +73,12 @@ case class CometInMemoryTableScanExec(
   // for instance -- then reads the wrong column.
   override def output: Seq[Attribute] = originalPlan.output
 
+  // Described by the Spark scan it replaces: the table's name when it has one, the attributes it
+  // reads and any pruning predicates. The default would print every constructor field, among them
+  // the CachedRDDBuilder with the whole cached plan, physical and logical, inline and with its raw
+  // newlines, which breaks the tree of every plan that reads the cache.
+  override def stringArgs: Iterator[Any] = Iterator(originalPlan)
+
   // `originalPlan` is a plan-typed field rather than a child, so QueryPlan's canonicalization
   // walks straight past it: its attributes and predicates keep the expression IDs of whichever
   // occurrence of the cached relation produced them. Two scans of one cache then compare unequal,

spark/src/test/scala/org/apache/comet/exec/CometInMemoryCacheSuite.scala

@@ -324,6 +324,43 @@ class CometInMemoryCacheSuite extends CometTestBase {
     }
   }
 
+  test("CometInMemoryTableScan is described by the Spark scan it replaces") {
+    // With every constructor field printed, the node dumped its CachedRDDBuilder, the whole cached
+    // plan both physical and logical, into the middle of every plan that read the cache, raw
+    // newlines and all, in the tree string and in EXPLAIN FORMATTED alike.
+    withSQLConf(
+      SQLConf.ADAPTIVE_EXECUTION_ENABLED.key -> "false",
+      CometConf.COMET_EXEC_IN_MEMORY_CACHE_ENABLED.key -> "true") {
+      spark.catalog.clearCache()
+      spark
+        .range(1000)
+        .selectExpr("id AS key", "id % 8 AS value")
+        .createOrReplaceTempView("explain_cache")
+      spark.catalog.cacheTable("explain_cache")
+      try {
+        val df = spark.sql("SELECT key FROM explain_cache WHERE value = 3")
+        df.collect()
+        val plan = df.queryExecution.executedPlan
+        val scans = collect(plan) { case s: CometInMemoryTableScanExec => s }
+        assert(scans.size == 1)
+        val line = scans.head.simpleString(SQLConf.get.maxToStringFields)
+        assert(
+          line.startsWith("CometInMemoryTableScan Scan In-memory table explain_cache ["),
+          line)
+        assert(line.contains("= 3)"), s"the pruning predicates should be shown: $line")
+        Seq(
+          plan.treeString,
+          df.queryExecution.explainString(org.apache.spark.sql.execution.FormattedMode))
+          .foreach { text =>
+            assert(!text.contains("CachedRDDBuilder"), text)
+            assert(!text.contains(classOf[ArrowCachedBatchSerializer].getName), text)
+          }
+      } finally {
+        spark.catalog.clearCache()
+      }
+    }
+  }
+
   test("Comet in-memory cache disabled keeps SparkToColumnar fallback path") {
     withSQLConf(
       SQLConf.ADAPTIVE_EXECUTION_ENABLED.key -> "false",