GH-1125: Preserve reads before value count finalization

Author: bodduv

vector/src/main/java/org/apache/arrow/vector/complex/impl/UnionLargeListReader.java

@@ -54,7 +54,9 @@ public boolean isSet() {
   @Override
   public void setPosition(int index) {
     int valueCount = vector.getValueCount();
-    if (valueCount == 0 && index == 0) {
+    if (valueCount == 0
+        && index == 0
+        && vector.getOffsetBuffer().capacity() < 2L * OFFSET_WIDTH) {
       setEmptyPosition(index);
       return;
     }

vector/src/main/java/org/apache/arrow/vector/complex/impl/UnionLargeListViewReader.java

@@ -56,7 +56,10 @@ public boolean isSet() {
   @Override
   public void setPosition(int index) {
     int valueCount = vector.getValueCount();
-    if (valueCount == 0 && index == 0) {
+    if (valueCount == 0
+        && index == 0
+        && (vector.getOffsetBuffer().capacity() < LargeListViewVector.OFFSET_WIDTH
+            || vector.getSizeBuffer().capacity() < LargeListViewVector.SIZE_WIDTH)) {
       setEmptyPosition(index);
       return;
     }

vector/src/main/java/org/apache/arrow/vector/complex/impl/UnionListReader.java

@@ -53,7 +53,9 @@ public boolean isSet() {
   @Override
   public void setPosition(int index) {
     int valueCount = vector.getValueCount();
-    if (valueCount == 0 && index == 0) {
+    if (valueCount == 0
+        && index == 0
+        && vector.getOffsetBuffer().capacity() < 2L * OFFSET_WIDTH) {
       setEmptyPosition(index);
       return;
     }

vector/src/main/java/org/apache/arrow/vector/complex/impl/UnionListReaderPositionValidator.java

@@ -30,7 +30,9 @@ private UnionListReaderPositionValidator() {}
 
   /** Check if the given index is within the current value count of the vector. */
   static void checkIndex(int index, int valueCount) {
-    if (index < 0 || index >= valueCount) {
+    // Writers may populate the buffers before the caller finalizes valueCount. Preserve the
+    // historical ability to read those buffers while still rejecting negative positions.
+    if (index < 0 || (valueCount > 0 && index >= valueCount)) {
       throw new IndexOutOfBoundsException(
           String.format("index: %s, expected range [0, %s)", index, valueCount));
     }

vector/src/main/java/org/apache/arrow/vector/complex/impl/UnionListViewReader.java

@@ -54,7 +54,10 @@ public boolean isSet() {
   @Override
   public void setPosition(int index) {
     int valueCount = vector.getValueCount();
-    if (valueCount == 0 && index == 0) {
+    if (valueCount == 0
+        && index == 0
+        && (vector.getOffsetBuffer().capacity() < ListViewVector.OFFSET_WIDTH
+            || vector.getSizeBuffer().capacity() < ListViewVector.SIZE_WIDTH)) {
       setEmptyPosition(index);
       return;
     }