fix: read back the maintained-index selection a table was given (#4447)

`maintained_indexes` on a MemWAL table has three values: a named list,
`[]` for none, and `null` for every index the table has, including ones
created later. This PR makes every client read all three back as they
were set.

- **Rust remote client** (also used by the Python and Node SDKs) reads
`null` as `None`.
- **Java client** reads `null` as `null`, not as an empty list.
- **Docs** for the setter, the field and the getter describe the three
values the same way in Rust, Python, Node and Java. The generated JS
docs are regenerated to match.

## Testing

- `rust/lancedb`: 73 LSM tests pass, including one case per value. With
the decoder typed as a list, the `null` case fails.
- Java: two cases for `null` and `[]` on the read path, run by the Java
CI job.
- `cargo fmt`, `cargo clippy -D warnings` and `ruff format` are clean.

java/lancedb-core/src/main/java/com/lancedb/LanceDbTableLsm.java

@@ -117,9 +117,13 @@ public class LanceDbTableLsm {
   /**
    * Read the {@link LsmWriteSpec} currently installed on this table.
    *
-   * <p>Empty when the LSM write path is not enabled. The returned spec mirrors what was installed,
-   * except that {@link LsmWriteSpec#maintainedIndexes()} always reports the concrete list resolved
-   * when the spec was set — a null selection never round-trips.
+   * <p>Empty when the LSM write path is not enabled at all — no spec has been set, or one was
+   * removed with {@link #unsetLsmWriteSpec()}. That is a different answer from a present spec
+   * whose {@link LsmWriteSpec#maintainedIndexes()} is null, which selects indexes automatically.
+   *
+   * <p>The spec read back is the one that was installed, selection included: null maintains every
+   * supported index the table has now or gains later, an empty list maintains none, and a
+   * non-empty list maintains exactly those. All three round-trip.
    */
   public Optional<LsmWriteSpec> getLsmWriteSpec() {
     JsonNode response = client.post(route("get_lsm_write_spec"), null);

java/lancedb-core/src/main/java/com/lancedb/LsmWriteSpec.java

@@ -36,6 +36,8 @@ import java.util.Map;
  */
 public class LsmWriteSpec {
 
+  private static final String CONTEXT = "get_lsm_write_spec response";
+
   /** How writes are routed to MemWAL shards. */
   public enum Sharding {
     /** Hash-bucket writes by a scalar column. */
@@ -124,9 +126,8 @@ public class LsmWriteSpec {
   /**
    * Set the indexes the MemWAL keeps up to date as rows are appended.
    *
-   * <p>Pass {@code null} — the default for a fresh spec — to maintain every index the MemWAL can,
-   * resolved when the spec is installed. That is a snapshot: indexes created later are not
-   * maintained until the spec is unset and set again. Pass an empty list to maintain none.
+   * <p>Pass {@code null} — the default for a fresh spec — to maintain every index the table has,
+   * including ones created later. Pass an empty list to maintain none.
    *
    * <p>Note that {@code null} and the empty list mean opposite things here.
    */
@@ -176,8 +177,10 @@ public class LsmWriteSpec {
   }
 
   /**
-   * The indexes the MemWAL maintains, or null to have the server resolve every maintainable index
-   * on install. An empty list means none.
+   * Which indexes the MemWAL maintains.
+   *
+   * <p>Null selects automatically: every supported index the table has now or gains later. An empty
+   * list maintains none. A non-empty list maintains exactly those.
    */
   public List<String> maintainedIndexes() {
     return maintainedIndexes == null ? null : Collections.unmodifiableList(maintainedIndexes);
@@ -210,8 +213,8 @@ public class LsmWriteSpec {
   /**
    * Rebuild a spec from a {@code get_lsm_write_spec} response body.
    *
-   * <p>The server always reports a concrete maintained-index list, so a null selection never
-   * round-trips.
+   * <p>A null {@code maintained_indexes} selects every index the table has; an empty list selects
+   * none.
    */
   static LsmWriteSpec fromJson(JsonNode node) {
     JsonNode shardingNode = node.get("sharding");
@@ -224,10 +227,19 @@ public class LsmWriteSpec {
     Integer numBuckets =
         shardingNode.hasNonNull("num_buckets") ? shardingNode.get("num_buckets").asInt() : null;
 
-    List<String> maintainedIndexes = new ArrayList<String>();
-    JsonNode indexesNode = node.get("maintained_indexes");
-    if (indexesNode != null && indexesNode.isArray()) {
+    // Absent or null is the automatic selection; anything else has to be an
+    // array of names. Reading a string or an object as automatic would turn a
+    // malformed response into "maintain every index", which is a policy, not a
+    // parse failure.
+    JsonNode indexesNode = JsonFields.optionalArray(node, "maintained_indexes", CONTEXT);
+    List<String> maintainedIndexes = null;
+    if (indexesNode != null) {
+      maintainedIndexes = new ArrayList<String>();
       for (JsonNode index : indexesNode) {
+        if (!index.isTextual()) {
+          throw new IllegalStateException(
+              CONTEXT + ": maintained_indexes must hold index names, got " + index);
+        }
         maintainedIndexes.add(index.asText());
       }
     }

java/lancedb-core/src/test/java/com/lancedb/LanceDbTableLsmTest.java

@@ -215,6 +215,61 @@ public class LanceDbTableLsmTest {
     assertEquals("true", spec.get().writerConfigDefaults().get("durable_write"));
   }
 
+  /** A null selection, every index the table has, reads back as null rather than an empty list. */
+  @Test
+  public void testGetLsmWriteSpecRoundTripsEveryIndexSelection() {
+    enqueue(
+        "get_lsm_write_spec",
+        200,
+        "{\"lsm_write_spec\":{\"sharding\":{\"mode\":\"unsharded\"},"
+            + "\"maintained_indexes\":null,\"writer_config_defaults\":{}}}");
+
+    Optional<LsmWriteSpec> spec = lsm.getLsmWriteSpec();
+
+    assertTrue(spec.isPresent());
+    assertNull(spec.get().maintainedIndexes());
+  }
+
+  @Test
+  public void testGetLsmWriteSpecRoundTripsNoIndexSelection() {
+    enqueue(
+        "get_lsm_write_spec",
+        200,
+        "{\"lsm_write_spec\":{\"sharding\":{\"mode\":\"unsharded\"},"
+            + "\"maintained_indexes\":[],\"writer_config_defaults\":{}}}");
+
+    Optional<LsmWriteSpec> spec = lsm.getLsmWriteSpec();
+
+    assertTrue(spec.isPresent());
+    assertEquals(Collections.emptyList(), spec.get().maintainedIndexes());
+  }
+
+  /**
+   * Null is the automatic selection, so anything that is neither null nor an array has to be
+   * rejected: reading a string or an object as null would turn a malformed response into a policy.
+   */
+  @Test
+  public void testGetLsmWriteSpecRejectsANonArrayMaintainedIndexes() {
+    enqueue(
+        "get_lsm_write_spec",
+        200,
+        "{\"lsm_write_spec\":{\"sharding\":{\"mode\":\"unsharded\"},"
+            + "\"maintained_indexes\":\"id_idx\",\"writer_config_defaults\":{}}}");
+
+    assertThrows(IllegalStateException.class, () -> lsm.getLsmWriteSpec());
+  }
+
+  @Test
+  public void testGetLsmWriteSpecRejectsNonStringIndexNames() {
+    enqueue(
+        "get_lsm_write_spec",
+        200,
+        "{\"lsm_write_spec\":{\"sharding\":{\"mode\":\"unsharded\"},"
+            + "\"maintained_indexes\":[7],\"writer_config_defaults\":{}}}");
+
+    assertThrows(IllegalStateException.class, () -> lsm.getLsmWriteSpec());
+  }
+
   @Test
   public void testGetLsmWriteSpecAbsent() {
     enqueue("get_lsm_write_spec", 200, "{\"lsm_write_spec\":null}");