[CORE] Address @yaooqinn review: defensive check on customMetrics array lengths

#12114 (comment)

If a future native-side producer ever ships mismatched key/value arrays,
the AIOOBE inside the synchronized block in `getCustomMetrics()` would
leave the volatile cache field unassigned — every subsequent call would
re-enter the synchronized block and re-throw, an unhelpful failure mode.

Adds an explicit length-match check (before the cache field is assigned
to a partial map) that throws `IllegalStateException` mentioning both
array lengths so the producer-side bug is unambiguous.

Adds two new `GlutenSplitResultSuite` cases covering:
- mismatched lengths (3-element keys + 1-element values)
- null values array when keys is non-empty

The second call after a failure must still throw (the cache field stays
null on the failure path); both new tests assert this.

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Author: luis4a0

gluten-arrow/src/main/java/org/apache/gluten/vectorized/GlutenSplitResult.java

@@ -158,6 +158,19 @@ public Map<String, Long> getCustomMetrics() {
       if (customMetricsKeys == null || customMetricsKeys.length == 0) {
         customMetricsCache = Collections.emptyMap();
       } else {
+        // Defensive check: if a future native-side producer ever ships
+        // mismatched arrays, fail loudly here (before the cache field is
+        // assigned to a partial result). Without this, the AIOOBE inside the
+        // loop would leave `customMetricsCache` null and every subsequent
+        // `getCustomMetrics()` call would re-enter the synchronized block
+        // and re-throw — a confusing failure mode.
+        if (customMetricsValues == null || customMetricsValues.length != customMetricsKeys.length) {
+          throw new IllegalStateException(
+              "customMetricsKeys / customMetricsValues length mismatch: "
+                  + customMetricsKeys.length
+                  + " vs "
+                  + (customMetricsValues == null ? "null" : customMetricsValues.length));
+        }
         LinkedHashMap<String, Long> map = new LinkedHashMap<>(customMetricsKeys.length);
         for (int i = 0; i < customMetricsKeys.length; i++) {
           map.put(customMetricsKeys[i], customMetricsValues[i]);

gluten-arrow/src/test/scala/org/apache/gluten/vectorized/GlutenSplitResultSuite.scala

@@ -94,4 +94,23 @@ class GlutenSplitResultSuite extends SparkFunSuite {
     assert(empty.isEmpty)
     assert(!populated.isEmpty)
   }
+
+  test("getCustomMetrics fails loudly on mismatched key/value array lengths") {
+    // A future native-side producer that ships mismatched arrays must not
+    // silently corrupt the metrics map (and must not leave the lazy cache
+    // field unassigned so subsequent calls re-enter and re-throw). Assert
+    // that the first call throws IllegalStateException with both lengths
+    // mentioned, and that a second call still throws (the cache field is
+    // never assigned to a partial map).
+    val r = newResult(Array("a", "b"), Array(1L))
+    val ex = intercept[IllegalStateException](r.getCustomMetrics)
+    assert(ex.getMessage.contains("2") && ex.getMessage.contains("1"))
+    intercept[IllegalStateException](r.getCustomMetrics)
+  }
+
+  test("getCustomMetrics fails loudly when values array is null but keys is non-empty") {
+    val r = newResult(Array("a"), null)
+    val ex = intercept[IllegalStateException](r.getCustomMetrics)
+    assert(ex.getMessage.contains("null"))
+  }
 }