Replace all db calls to file_managed table with Entity APIs

1.     $fids = \Drupal::entityQuery('file')->sort('fid', 'DESC')->range(0, 1)->execute();
   -    return empty($fids) ? 0 : array_values($fids)[0];
   +    return (int) reset($fids);

   We enshure that returns an integer value because it may be used on compare operations.

2.      $fids_query = \Drupal::entityQuery('file')->sort('fid', 'DESC')->range(0, 1);
   -    $max_fid_after = array_values($fids_query->execute())[0];
   +    $max_fid_after = (int) reset($fids_query->execute());
        $this->assertTrue($max_fid_after > $this->maxFidBefore, 'A new file was created.');

   For empty $fids_query results $max_fid_after is NULL (or FALSE) and can't be used on compare operation.

Or use current() instead reset() to prevent errors with reset's argument array &$array.

@mondrake , I'll do a patch.

I understand that specific tests will be correct, but I would like to make uniform functions.

Patch #4

#9 addressed all from #4
Tests pass
+1 for rtbc

+++ b/core/modules/file/src/Tests/FileFieldTestBase.php
@@ -59,7 +59,8 @@ public function getTestFile($type_name, $size = NULL) {
-    return (int) db_query('SELECT MAX(fid) FROM {file_managed}')->fetchField();
+    $fids = \Drupal::entityQuery('file')->sort('fid', 'DESC')->range(0, 1)->execute();
+    return (int) current($fids);

+++ b/core/modules/file/tests/src/Functional/FileFieldTestBase.php
@@ -60,7 +60,8 @@ public function getTestFile($type_name, $size = NULL) {
-    return (int) db_query('SELECT MAX(fid) FROM {file_managed}')->fetchField();
+    $fids = \Drupal::entityQuery('file')->sort('fid', 'DESC')->range(0, 1)->execute();
+    return (int) current($fids);

+++ b/core/modules/file/tests/src/Functional/SaveUploadFormTest.php
@@ -66,7 +66,8 @@ protected function setUp() {
-    $this->maxFidBefore = db_query('SELECT MAX(fid) AS fid FROM {file_managed}')->fetchField();
+    $fids = \Drupal::entityQuery('file')->sort('fid', 'DESC')->range(0, 1)->execute();
+    $this->maxFidBefore = (int) current($fids);

@@ -89,7 +90,8 @@ protected function setUp() {
-    $max_fid_after = db_query('SELECT MAX(fid) AS fid FROM {file_managed}')->fetchField();
+    $fids_query = \Drupal::entityQuery('file')->sort('fid', 'DESC')->range(0, 1);
+    $max_fid_after = (int) current($fids_query->execute());

@@ -107,7 +109,7 @@ public function testNormal() {
-    $max_fid_after = db_query('SELECT MAX(fid) AS fid FROM {file_managed}')->fetchField();
+    $max_fid_after = (int) current($fids_query->execute());

This can use an aggregate query. Ie.
return (int) \Drupal::entityQueryAggregate('file')->aggregate('fid', 'max')->execute()[0]['fid_max']
... and all the other places... it seems the only query this replaces is the max ones so we should be using \Drupal::entityQueryAggregate('file')->aggregate('fid', 'max') so people learn the right way to do that.

Apply #11

+++ b/core/modules/file/tests/src/Functional/SaveUploadFormTest.php
@@ -89,7 +89,8 @@ protected function setUp() {
+    $fids_query = \Drupal::entityQueryAggregate('file')->aggregate('fid', 'max');
+    $max_fid_after = (int) $fids_query->execute()[0]['fid_max'];

+++ b/core/modules/file/tests/src/Functional/SaveUploadTest.php
@@ -82,7 +82,9 @@ protected function setUp() {
+    $fids_query = \Drupal::entityQueryAggregate('file')->aggregate('fid', 'max');
+    $max_fid_after = (int) $fids_query->execute()[0]['fid_max'];

Unfortunately re-executing an aggregate query appears not to work. Let's do (int) \Drupal::entityQueryAggregate('file')->aggregate('fid', 'max')->execute()[0]['fid_max']; each time and open a bug report about this as you would expect it to work (if there is no existing bug report).

Committed c503bb6 and pushed to 8.7.x. Thanks!