E8 fixup: four correctness fixes from Phase 0 review

1. strlen(\$query_string) → null-safe check
   \$query->getParameter('query') may return null; strlen(null) is
   deprecated in PHP 8.1+. Switch to explicit !== null && !== '' check.

2. ids.values nesting fix in buildSpec()
   'values' => array(\$exclude) wraps an already-array \$exclude in a
   nested array, producing invalid Elasticsearch ids syntax. Cast to
   (array) then re-index with array_values() so both scalar PHID and
   array-of-PHIDs produce a flat list.

3. initIndex() host consistency: pass write host to indexExists()
   initIndex() called indexExists() without a host argument, which
   fell back to the read host, then deleted via getHostForWrite().
   A read/write split lag window could cause false-positive
   "index doesn't exist" results. Pass the already-resolved write host.

4. getIndexStats() unchecked array access
   \$res['indices'][\$this->index] was accessed without verifying the
   key exists. If the index was just deleted or the name drifted, this
   throws an undefined-offset PHP notice. Add an explicit isset() guard
   with a clear exception message naming the index.

🤖 Generated by [robots](https://vyos.io)

2 files changed, 31 insertions, 3 deletions

diff --git a/src/__tests__/VyOSElasticModernFulltextStorageEngineTestCase.php b/src/__tests__/VyOSElasticModernFulltextStorageEngineTestCase.php
index 9f08f8c..8a8b29c 100644
--- a/src/__tests__/VyOSElasticModernFulltextStorageEngineTestCase.php
+++ b/src/__tests__/VyOSElasticModernFulltextStorageEngineTestCase.php
@@ -257,4 +257,24 @@ final class VyOSElasticModernFulltextStorageEngineTestCase
     $this->assertEqual('lastModified', $engine->getTimestampField());
   }
 
+  public function testBuildSpecTreatsZeroQueryAsSearchTerm() {
+    // '0' is falsy in PHP but must be treated as a non-empty search term.
+    // strlen('0') == 1, so the query clause should be present and the
+    // default date-sorted path should NOT fire.
+    $engine = $this->newEngine()->setVersion(7);
+    $query = id(new PhabricatorSavedQuery())
+      ->setParameter('query', '0');
+
+    $method = new ReflectionMethod($engine, 'buildSpec');
+    $method->setAccessible(true);
+    $spec = $method->invoke($engine, $query, array('TASK'));
+
+    $this->assertFalse(isset($spec['sort']));
+    $this->assertEqual(
+      '0',
+      idxv(
+        $spec,
+        array('query', 'bool', 'must', 0, 'simple_query_string', 'query')));
+  }
+
 }
diff --git a/src/engine/VyOSElasticModernFulltextStorageEngine.php b/src/engine/VyOSElasticModernFulltextStorageEngine.php
index c49ff5e..70844f9 100644
--- a/src/engine/VyOSElasticModernFulltextStorageEngine.php
+++ b/src/engine/VyOSElasticModernFulltextStorageEngine.php
@@ -313,7 +313,7 @@ abstract class VyOSElasticModernFulltextStorageEngine
   private function buildSpec(PhabricatorSavedQuery $query, array $types) {
     $q = new PhabricatorElasticsearchQueryBuilder();
     $query_string = $query->getParameter('query');
-    if (strlen($query_string)) {
+    if ($query_string !== null && $query_string !== '') {
       $q->addMustClause(array(
         'simple_query_string' => array(
           'query'  => $query_string,
@@ -343,9 +343,11 @@ abstract class VyOSElasticModernFulltextStorageEngine
     $exclude = $query->getParameter('exclude');
     if ($exclude) {
       // Correct from day one: bool.must_not, not the obsolete 'not' clause.
+      // Cast to array so a single PHID scalar and an already-array of PHIDs
+      // both produce a flat list for the Elasticsearch ids.values field.
       $q->addMustNotClause(array(
         'ids' => array(
-          'values' => array($exclude),
+          'values' => array_values((array)$exclude),
         ),
       ));
     }
@@ -460,7 +462,7 @@ abstract class VyOSElasticModernFulltextStorageEngine
 
   public function initIndex() {
     $host = $this->getHostForWrite();
-    if ($this->indexExists()) {
+    if ($this->indexExists($host)) {
       $this->executeRequest($host, '/', array(), 'DELETE');
     }
     $data = $this->getIndexConfiguration();
@@ -472,6 +474,12 @@ abstract class VyOSElasticModernFulltextStorageEngine
       $host = $this->getHostForRead();
     }
     $res = $this->executeRequest($host, '/_stats/', array());
+    if (!isset($res['indices'][$this->index])) {
+      throw new Exception(
+        pht(
+          'Index "%s" not found in Elasticsearch _stats response.',
+          $this->index));
+    }
     $stats = $res['indices'][$this->index];
     return array(
       pht('Queries') =>