Improve the diagnostics for 'conv' fallback explain (NVIDIA#11076)

* Improve the diagnostics for 'conv' fallback explain

Signed-off-by: Jihoon Son <ghoonson@gmail.com>

* don't use nil

Signed-off-by: Jihoon Son <ghoonson@gmail.com>

* the bases should not be an empty string in the error message when the user input is not

Signed-off-by: Jihoon Son <ghoonson@gmail.com>

* more user-friendly message

* Update sql-plugin/src/main/scala/org/apache/spark/sql/rapids/stringFunctions.scala

Co-authored-by: Gera Shegalov <gshegalov@nvidia.com>

---------

Signed-off-by: Jihoon Son <ghoonson@gmail.com>
Co-authored-by: Gera Shegalov <gshegalov@nvidia.com>

integration_tests/src/main/python/string_test.py

@@ -820,6 +820,22 @@ def test_conv_dec_to_from_hex(from_base, to_base, pattern):
         conf={'spark.rapids.sql.expression.Conv': True}
     )
 
+@pytest.mark.parametrize('from_base,to_base,expected_err_msg_prefix',
+                         [
+                             pytest.param(10, 15, '15 is not a supported target radix', id='to_base_unsupported'),
+                             pytest.param(11, 16, '11 is not a supported source radix', id='from_base_unsupported'),
+                             pytest.param(9, 17, 'both 9 and 17 are not a supported radix', id='both_base_unsupported')
+                         ])
+def test_conv_unsupported_base(from_base, to_base, expected_err_msg_prefix):
+    def do_conv(spark):
+        gen = StringGen()
+        df = unary_op_df(spark, gen).select('a', f.conv(f.col('a'), from_base, to_base))
+        explain_str = spark.sparkContext._jvm.com.nvidia.spark.rapids.ExplainPlan.explainPotentialGpuPlan(df._jdf, "ALL")
+        unsupported_base_str = f'{expected_err_msg_prefix}, only literal 10 or 16 are supported for source and target radixes'
+        assert unsupported_base_str in explain_str
+
+    with_cpu_session(do_conv)
+
 format_number_gens = integral_gens + [DecimalGen(precision=7, scale=7), DecimalGen(precision=18, scale=0),
                                       DecimalGen(precision=18, scale=3), DecimalGen(precision=36, scale=5),
                                       DecimalGen(precision=36, scale=-5), DecimalGen(precision=38, scale=10),

sql-plugin/src/main/scala/org/apache/spark/sql/rapids/stringFunctions.scala

@@ -2084,11 +2084,25 @@ class GpuConvMeta(
   override def tagExprForGpu(): Unit = {
     val fromBaseLit = GpuOverrides.extractLit(expr.fromBaseExpr)
     val toBaseLit = GpuOverrides.extractLit(expr.toBaseExpr)
+    val errorPostfix = "only literal 10 or 16 are supported for source and target radixes"
     (fromBaseLit, toBaseLit) match {
-      case (Some(Literal(fromBaseVal, IntegerType)), Some(Literal(toBaseVal, IntegerType)))
-        if Set(fromBaseVal, toBaseVal).subsetOf(Set(10, 16)) => ()
+      case (Some(Literal(fromBaseVal, IntegerType)), Some(Literal(toBaseVal, IntegerType))) =>
+        def isBaseSupported(base: Any): Boolean = base == 10 || base == 16
+        if (!isBaseSupported(fromBaseVal) && !isBaseSupported(toBaseVal)) {
+          willNotWorkOnGpu(because = s"both ${fromBaseVal} and ${toBaseVal} are not " +
+            s"a supported radix, ${errorPostfix}")
+        } else if (!isBaseSupported(fromBaseVal)) {
+          willNotWorkOnGpu(because = s"${fromBaseVal} is not a supported source radix, " +
+            s"${errorPostfix}")
+        } else if (!isBaseSupported(toBaseVal)) {
+          willNotWorkOnGpu(because = s"${toBaseVal} is not a supported target radix, " +
+            s"${errorPostfix}")
+        }
       case _ =>
-        willNotWorkOnGpu(because = "only literal 10 or 16 for from_base and to_base are supported")
+        // This will never happen in production as the function signature enforces
+        // integer types for the bases, but nice to have an edge case handling.
+        willNotWorkOnGpu(because = "either source radix or target radix is not an integer " +
+          "literal, " + errorPostfix)
     }
   }